Physics is taught, researched and published largely in English, from international textbooks and olympiad problems to lab manuals and journal articles. Knowing the core words for motion and forces, energy, waves, light, electricity and magnetism, heat, modern physics and lab work helps you follow lectures, read problems quickly and explain your reasoning clearly in exams and lab reports.
100 terms
10 topics
59 core terms
201 example sentences
Read, understand, keep going
No account needed. Read the meaning, see where the term is used, then study the examples. Click any word in the lesson for help. You can also select a word and press Alt + Enter.
Measurement and Units
SI units
noun
Meaning
The international system of measurement units used in science, based on seven base units such as the metre, kilogram, second, ampere and kelvin. SI comes from the French 'Système international'.
Where you use it
Teachers insist on SI units in calculations: convert grams to kilograms and centimetres to metres before putting numbers into a formula. Derived units such as the newton, joule and watt are built from the base units. Say 'S-I units', letter by letter.
Examples
Give all your answers in SI units, so convert 250 g to 0.25 kg first.
The joule is a derived SI unit equal to one newton metre.
Usage note
Unit names are written in lowercase even when they come from a person's name (newton, joule, watt, kelvin), but their symbols start with a capital letter (N, J, W, K).
Common phrases
SI base units · derived SI unit · convert to SI units · SI prefix
A quantity that has both a size (magnitude) and a direction, such as displacement, velocity, force or acceleration. It is often drawn as an arrow.
Where you use it
Used constantly when adding forces or velocities. You add, subtract and resolve vectors and find their components and resultant. In print, vectors appear in bold (F) or with an arrow above the letter.
Examples
Velocity is a vector, so you must give a direction as well as a magnitude.
Add the two force vectors tip to tail to find the resultant.
Common phrases
vector quantity · vector sum · add vectors · resultant vector · unit vector
A quantity that has size (magnitude) but no direction, such as mass, time, temperature, energy or speed. Scalars are added like ordinary numbers.
Where you use it
Often used in the phrase 'scalar quantity' and contrasted with 'vector quantity'. A classic first-week question is 'Is work a scalar or a vector?' The answer is scalar, even though force and displacement are both vectors.
Examples
Energy is a scalar, so you can simply add the energies of the two objects.
The mass of a material per unit volume, found by dividing mass by volume. It is measured in kilograms per cubic metre (kg/m³) or grams per cubic centimetre (g/cm³).
Where you use it
Symbol ρ (rho). Pure water has a density of about 1000 kg/m³, which is 1 g/cm³, and objects less dense than water float. In the lab you find density by measuring mass on a balance and volume with a ruler or by displacement of water.
Examples
Ice floats because its density is lower than that of liquid water.
Find the density of the stone by dividing its mass by the volume of water it displaces.
Common phrases
density of water · high density · low density · measure the density · mass per unit volume
The digits in a number that carry real information about its precision, counted from the first non-zero digit. For example, 0.00450 has three significant figures.
Where you use it
Often shortened to 'sig figs' or 's.f.'. As a rule of thumb, give a final answer to the same number of significant figures as the least precise data. Exam questions say 'give your answer to 3 significant figures'.
Examples
Your calculator says 9.81476, but the data only justify three significant figures, so write 9.81.
Round the final answer to two significant figures.
Usage note
English uses a decimal point (9.81), while Turkish uses a decimal comma (9,81). In an English report, always use the point.
Common phrases
to three significant figures · round to significant figures · number of significant figures · sig figs
A way of writing very large or very small numbers as a number between 1 and 10 multiplied by a power of ten, for example 3.0 × 10⁸ instead of 300,000,000.
Where you use it
Called standard form in the UK. Read 3.0 × 10⁸ aloud as 'three point oh times ten to the eight'. Calculators show it with E or EXP, so 6.63E-34 means 6.63 × 10⁻³⁴. It also makes the number of significant figures clear.
Examples
Write the mass of the electron in scientific notation: 9.11 × 10⁻³¹ kg.
Give your answer in scientific notation to three significant figures.
Common phrases
in scientific notation · standard form · power of ten · exponent
The approximate size of a number expressed as the nearest power of ten. Two values that differ by a factor of about ten differ by one order of magnitude.
Where you use it
Used in estimates and in conversation: 'the nucleus is about five orders of magnitude smaller than the atom', 'your answer is out by an order of magnitude'. Order-of-magnitude estimates, also called Fermi problems, check whether a result is reasonable.
Examples
Your answer is two orders of magnitude too big; did you forget to convert centimetres to metres?
Make an order-of-magnitude estimate of the number of air molecules in this room.
Common phrases
orders of magnitude larger · order-of-magnitude estimate · out by an order of magnitude · within an order of magnitude
A method of checking or building an equation by comparing the units or basic dimensions (mass, length, time and so on) on each side. Both sides of a valid equation must match.
Where you use it
Dimensions are written in square brackets: [M], [L], [T]. Dimensional analysis can prove that an equation is wrong but not that it is right, because it cannot detect missing numbers such as ½ or 2π. In class it is often called a 'units check'.
Examples
Use dimensional analysis to check that √(gh) has the units of a speed.
Dimensional analysis suggests that the period of a pendulum depends on its length and on g, but not on its mass.
Common phrases
check the units · base dimensions · dimensionally consistent · homogeneous equation
A push or a pull that can change how an object moves or change its shape. It has both size and direction and is measured in newtons (N).
Where you use it
The central word of mechanics. A force acts on, is applied to or is exerted on an object; forces can be balanced or unbalanced, and you add them as vectors. Teachers often ask students to 'draw all the forces' before solving a problem.
Examples
A force of 20 N acts on the box at an angle of 30° to the horizontal.
If the forces on the car are balanced, it keeps moving at a constant velocity.
Usage note
In everyday English, 'force' can also mean making someone do something. In physics it is always a measurable push or pull, and it is not 'güç': power (güç) is a different quantity.
Common phrases
apply a force · exert a force · a force acts on · balanced forces · resultant force
The amount of matter in an object, which also measures how hard it is to change the object's motion. It is measured in kilograms (kg) and does not depend on location.
Where you use it
Used in almost every mechanics equation, starting with F = ma. You say an object 'has a mass of 2 kg'. Physicists keep mass and weight apart: an astronaut's mass is the same on the Moon, but their weight is only about one sixth.
Examples
The trolley has a mass of 1.5 kg, so we need a bigger force to get the same acceleration.
Measure the mass of each sample on the digital balance and record it to two decimal places.
Usage note
In daily English people say 'I weigh 70 kilos', but in physics 70 kg is a mass. Weight is a force and is measured in newtons.
Common phrases
have a mass of · measure the mass · mass per unit volume · point mass · rest mass
How fast an object moves in a particular direction. It is speed with a direction, so it is a vector, measured in metres per second (m/s).
Where you use it
Used whenever direction matters: initial velocity (u or v₀), final velocity (v), average and instantaneous velocity, constant velocity. If up is positive, a ball thrown upwards has a negative velocity on its way back down.
Examples
The car's velocity changed from 15 m/s east to 15 m/s west, so its speed stayed the same but it still accelerated.
Take upwards as positive, so the initial velocity is +12 m/s.
Usage note
Turkish school physics uses 'sürat' for speed and 'hız' for velocity, but in everyday Turkish 'hız' covers both. When you translate, check whether direction matters.
Common phrases
initial velocity · final velocity · constant velocity · average velocity · angular velocity
The rate at which an object's velocity changes over time. It can mean speeding up, slowing down or changing direction, and it is measured in metres per second squared (m/s²).
Where you use it
Appears in the kinematics equations and in F = ma. Common phrases: constant (uniform) acceleration, acceleration due to gravity (g ≈ 9.8 m/s²), centripetal acceleration. Read m/s² aloud as 'metres per second squared'.
Examples
Near the Earth's surface, the acceleration due to gravity is about 9.8 m/s².
The graph shows constant acceleration for the first four seconds; after that the cyclist moves at a steady speed.
Usage note
Physicists rarely use 'deceleration' in calculations; they say the acceleration is negative or opposite to the velocity. 'Deceleration' is more common in everyday and engineering English.
Common phrases
constant acceleration · acceleration due to gravity · centripetal acceleration · average acceleration · calculate the acceleration
Three basic laws that describe how forces affect motion: an object keeps its state of motion unless a net force acts, net force equals mass times acceleration, and every action has an equal and opposite reaction.
Where you use it
The starting point of classical mechanics. Students refer to them by number: Newton's first law (inertia), second law (F = ma) and third law (action and reaction). In written solutions you 'apply' a law: 'Applying Newton's second law to the block gives...'.
Examples
Newton's laws of motion explain why passengers lurch forward when a bus brakes suddenly.
This week we'll apply Newton's laws of motion to blocks on inclined planes.
Newton's third law of motion says the rocket pushes the gas backwards and the gas pushes the rocket forwards.
Usage note
Action and reaction forces act on different objects, so they never cancel each other out. Mixing this up is one of the most common mistakes in exams.
Common phrases
Newton's first law · Newton's second law · Newton's third law · apply Newton's laws · action and reaction
The attractive force that every mass exerts on every other mass. On Earth it pulls objects towards the centre of the planet and gives them weight.
Where you use it
'Gravity' is common in speech and introductory texts; formal writing often prefers 'gravitation' or 'the gravitational force'. Useful phrases: under gravity, centre of gravity, acceleration due to gravity, gravitational field. Newton's law of gravitation is F = Gm₁m₂/r².
Examples
Ignoring air resistance, the stone falls under gravity alone.
The Moon's gravity is about one sixth as strong as the Earth's.
Common phrases
under gravity · centre of gravity · acceleration due to gravity · gravitational force · law of gravitation
The force with which gravity pulls on an object. It equals mass times the gravitational field strength (W = mg), is measured in newtons and changes from planet to planet.
Where you use it
In physics, weight is always a force acting downwards, towards the centre of the Earth. In free-body diagrams it is drawn from the centre of mass and labelled W or mg. Related phrases: apparent weight, weightlessness, weigh an object.
Examples
A 5 kg bag has a weight of about 49 N on Earth.
In a lift accelerating upwards, the scale shows a larger apparent weight.
Usage note
Kitchen scales show kilograms, but they actually measure the force on them and convert it to a mass. Never give a weight in kg in a physics answer.
Common phrases
weight of an object · apparent weight · weightlessness · weigh an object
A force that acts between two surfaces in contact and opposes their sliding past each other. It usually turns kinetic energy into heat.
Where you use it
Physicists distinguish static friction (before sliding starts) from kinetic or sliding friction (during motion), and describe surfaces with a coefficient of friction (μ). Problems often say 'the surface is frictionless' or 'ignore friction'.
Examples
The coefficient of friction between the tyre and the dry road is about 0.7.
Without friction, the puck would keep sliding across the ice at a constant speed.
How fast an object is moving, found by dividing the distance travelled by the time taken. It has no direction, so it is a scalar, measured in metres per second (m/s).
Where you use it
Used for average speed, instantaneous speed and constant speed. A speedometer shows speed, not velocity. Exam questions often contrast the two: a car going round a bend at constant speed is still accelerating, because the direction of its velocity changes.
Examples
Her average speed over the 400 m race was 8 m/s.
The speed of sound in air is about 340 m/s at room temperature.
Usage note
Some fixed phrases keep 'speed' even in physics: 'speed of light' and 'speed of sound', not 'velocity of light'.
Common phrases
average speed · constant speed · top speed · speed of light · speed of sound
The straight-line distance from an object's starting point to its final position, together with its direction. It is a vector, measured in metres.
Where you use it
Used in kinematics with the symbol s or Δx. Distance is how far you actually travel; displacement only cares about the start and end points. A runner who completes one lap of a 400 m track has a displacement of zero.
Examples
The hiker walked 3 km north and then 4 km east, so her displacement is 5 km at about 53° east of north.
Displacement can be negative if the object ends up behind its starting point.
Usage note
In engineering and shipping, 'displacement' also means engine capacity or the weight of water a ship pushes aside. In kinematics it means change of position only.
The single force that has the same effect as all the forces acting on an object together. It is the vector sum of those forces and decides the object's acceleration.
Where you use it
Also called the resultant force, especially in British textbooks. Newton's second law is really about net force: F_net = ma. If the net force is zero, the object is either at rest or moving at a constant velocity.
Examples
A 30 N push to the right and 10 N of friction give a net force of 20 N to the right.
Since the parachutist falls at a constant speed, the net force on her must be zero.
Common phrases
net force on · resultant force · zero net force · calculate the net force
A state in which all the forces and turning effects on an object balance, so its motion does not change. An object in equilibrium is at rest or moving at a constant velocity.
Where you use it
Used in statics: an object is in equilibrium, reaches equilibrium or is held in equilibrium. Types include static, dynamic, stable and unstable equilibrium. In thermodynamics, thermal equilibrium means two objects have reached the same temperature.
Examples
The sign hangs in equilibrium, so the tensions in the two cables must balance its weight.
A marble at the bottom of a bowl is in stable equilibrium.
Usage note
The plural is 'equilibria'. The stress falls on the third syllable: e-qui-LIB-ri-um.
Common phrases
in equilibrium · stable equilibrium · unstable equilibrium · thermal equilibrium · equilibrium position
The motion of an object that is thrown or launched and then moves only under gravity, following a curved path. Its horizontal and vertical motions are treated separately.
Where you use it
In ideal problems you neglect air resistance, so the horizontal velocity is constant and the vertical acceleration is g downwards. The path is a parabola. Typical questions ask for the range, the maximum height and the time of flight.
Examples
In projectile motion, the horizontal and vertical components are independent of each other.
We filmed the ball and used the video to analyse its projectile motion.
Common phrases
range of a projectile · time of flight · maximum height · launch angle · horizontal projection
The tendency of an object to resist any change in its motion, so it stays at rest or keeps moving with the same velocity. Objects with more mass have more inertia.
Where you use it
Linked to Newton's first law, which is sometimes called the law of inertia. Used when explaining seat belts, the tablecloth trick or why heavy trucks need longer to stop. For rotation, physicists use 'moment of inertia'.
Examples
Because of inertia, your body keeps moving forward when the car stops suddenly.
A loaded truck has more inertia than an empty one, so it is harder to stop.
Usage note
In everyday English, 'inertia' also means a lack of energy or an unwillingness to change, as in 'organisational inertia'.
Common phrases
law of inertia · moment of inertia · inertial frame of reference
The turning effect of a force about a pivot or axis. It equals the force multiplied by the perpendicular distance from the pivot and is measured in newton metres (N·m).
Where you use it
Symbol τ (tau). Used for spanners, doors, seesaws and motors. British school textbooks often call it the 'moment of a force' and talk about the principle of moments. In equilibrium, the clockwise and anticlockwise torques balance.
Examples
A longer spanner gives more torque for the same force.
The net torque on the beam is zero, so it does not rotate.
Usage note
The unit of torque, N·m, looks like the joule (also N·m), but torque is not energy. Write N·m for torque and J for energy.
Common phrases
apply a torque · net torque · clockwise torque · moment of a force · principle of moments
The net force that keeps an object moving in a circle, always pointing towards the centre of the circle. It equals mv²/r.
Where you use it
It is not a separate kind of force: tension, gravity, friction or a normal force provides it. For a satellite it is gravity; for a car on a bend it is friction. The outward 'centrifugal force' only appears when you describe motion from a rotating frame of reference.
Examples
Gravity provides the centripetal force that keeps the Moon in orbit.
On an icy road, friction may be too small to supply the centripetal force, and the car skids.
Usage note
Students often draw an extra outward 'centrifugal force' on free-body diagrams. In an inertial frame there is no such force; the object simply tends to continue in a straight line.
Common phrases
provide the centripetal force · centripetal acceleration · circular motion · centrifugal force
The pulling force in a stretched string, rope, cable or chain. It acts along the string and pulls on the objects attached at both ends.
Where you use it
Symbol T. In simple problems the string is 'light and inextensible', so the tension is the same all along it. Tension appears in lifts, pulleys, pendulums and hanging signs. A string is 'under tension', or it 'goes slack' when the tension drops to zero.
Examples
Find the tension in the cable when the lift accelerates upwards at 2 m/s².
The tension in the string is greatest at the bottom of the pendulum's swing.
Usage note
Tension is a force, not 'gerilim' in the electrical sense. For electrical gerilim, use 'voltage' or 'potential difference'.
Common phrases
tension in the string · under tension · light inextensible string · surface tension
The push from a surface on an object that rests on it or presses against it. It always acts at right angles (normal) to the surface.
Where you use it
Symbol N or R; British books also call it the normal reaction or normal contact force. It is not always equal to the weight: on a slope it is mg cos θ, and in an accelerating lift it changes. Friction depends on it: f = μN.
Examples
On a 30° slope, the normal force on the block is smaller than its weight.
A bathroom scale actually measures the normal force between your feet and the scale.
Usage note
Here 'normal' means 'perpendicular', not 'usual'. This is a common source of confusion for learners.
Common phrases
normal reaction · normal contact force · reaction force · normal to the surface
The constant top speed reached by a falling object when the air resistance on it becomes equal to its weight, so the net force on it is zero.
Where you use it
Used for skydivers, raindrops and ball bearings falling through oil. A skydiver falling belly-down reaches about 200 km/h; opening the parachute leads to a new, much lower terminal velocity. Exam questions often ask students to sketch the velocity-time graph.
Examples
Once the raindrop reaches terminal velocity, it falls at a steady speed.
Drop the ball bearing into the oil and start timing once it has reached terminal velocity.
Common phrases
reach terminal velocity · terminal speed · air resistance · drag force
The ability of a system to do work or cause change. It comes in many forms, such as kinetic, potential, thermal and chemical, and it is measured in joules (J).
Where you use it
Used in every branch of physics. Energy is stored, transferred, converted (or transformed) and conserved; it is never 'used up', only changed into less useful forms. In atomic physics, small amounts are given in electronvolts (eV).
Examples
As the ball falls, its gravitational potential energy is converted into kinetic energy.
How much energy is transferred to the water when the heater runs for five minutes?
Common phrases
store energy · transfer energy · convert energy · energy loss · energy transfer
The energy transferred when a force moves an object through a distance in the direction of the force. It equals force times displacement and is measured in joules (J).
Where you use it
In physics, work has a precise meaning: work is done on or by an object. If you hold a heavy bag without moving it, you do no work in the physics sense, even though you get tired. When there is an angle, W = Fd cos θ.
Examples
The crane does 50 kJ of work lifting the beam to the top of the building.
Friction does negative work on the sliding block, so its kinetic energy decreases.
Usage note
Say 'do work', not 'make work'. In this meaning 'work' is uncountable, so never say 'a work' or 'works'.
Common phrases
do work · work done by · work done on · negative work · work-energy theorem
The rate at which energy is transferred or work is done, in other words energy per second. It is measured in watts (W), where one watt is one joule per second.
Where you use it
Used in mechanics (the power of a motor), electricity (P = VI) and waves. Common phrases: power output, power input, rated power and power loss. Do not mix up power and energy: a kettle has a power of 2 kW, but the energy it uses depends on time.
Examples
The motor lifts a 100 kg load by 3 m in 6 s, so its useful power output is about 500 W.
Power is energy transferred per unit time, so a 60 W bulb transfers 60 joules every second.
Usage note
In mathematics 'power' also means an exponent, as in 'ten to the power of six'. The context tells you which meaning is intended.
Common phrases
power output · power input · rated power · power loss · generate power
The energy an object has because it is moving. It depends on mass and speed and equals one half times mass times speed squared (½mv²).
Where you use it
Often shortened to KE or Eₖ. Because speed is squared, doubling the speed makes the kinetic energy four times larger, a point often used in road-safety discussions. Kinetic energy is gained, lost or converted.
Examples
A car travelling at 20 m/s has four times the kinetic energy it has at 10 m/s.
Some kinetic energy is lost as heat and sound when the ball hits the floor.
Common phrases
gain kinetic energy · lose kinetic energy · rotational kinetic energy · work-energy theorem
Energy stored in an object because of its position or condition, for example a raised book or a stretched spring. Near the Earth's surface, gravitational potential energy equals mgh.
Where you use it
Usually named by type: gravitational potential energy (GPE), elastic potential energy in springs and electric potential energy. Only changes in potential energy matter, so you choose a zero level: 'take the ground as zero'.
Examples
The roller coaster has its maximum potential energy at the top of the first hill.
Stretching a 200 N/m spring by 5 cm stores 0.25 J of elastic potential energy.
Common phrases
gravitational potential energy · elastic potential energy · store potential energy · change in potential energy
The principle that energy cannot be created or destroyed, only transferred between objects or converted from one form to another. The total energy of an isolated system stays constant.
Where you use it
One of the most used principles in problem solving. Students write 'by conservation of energy...' and set the energy at the start equal to the energy at the end. Also called the law or principle of conservation of energy.
Examples
By conservation of energy, the speed of the pendulum bob at the bottom is √(2gh).
Conservation of energy tells us that the 'lost' energy has gone into heating the brakes.
Usage note
In everyday English 'energy conservation' means saving energy (enerji tasarrufu). In physics, 'conservation of energy' is a law of nature, not a habit.
Common phrases
by conservation of energy · law of conservation of energy · apply conservation of energy · energy is conserved
The product of an object's mass and its velocity (p = mv). It is a vector and is measured in kilogram metres per second (kg·m/s).
Where you use it
Central to collisions, explosions and rocket motion. Momentum is gained, transferred and conserved. The plural in formal texts is 'momenta'. For rotating objects, physicists use angular momentum.
Examples
A 0.15 kg ball moving at 40 m/s has a momentum of 6 kg·m/s.
At the same speed, the truck has much more momentum than the car.
Usage note
In everyday English, 'gain momentum' means to grow stronger or faster, as in 'the campaign gained momentum'. In physics it is a precisely defined quantity.
Common phrases
linear momentum · angular momentum · change in momentum · momentum is conserved · transfer momentum
The effect of a force acting over a time interval, equal to force multiplied by time. It equals the change in an object's momentum and is measured in newton seconds (N·s).
Where you use it
Used to explain crumple zones, airbags and why you bend your knees when you land: spreading the same impulse over a longer time means a smaller force. On a force-time graph, the impulse is the area under the curve.
Examples
The airbag increases the collision time, so the same impulse is delivered with a smaller force.
Find the impulse from the area under the force-time graph.
Usage note
In everyday English, an impulse is a sudden wish to do something ('I bought it on impulse'). Physics uses only the force-times-time meaning.
Common phrases
impulse-momentum theorem · force-time graph · deliver an impulse · change in momentum
A collision in which the total kinetic energy of the objects is the same before and after the impact. Momentum is conserved in every collision, but kinetic energy only in elastic ones.
Where you use it
Contrasted with inelastic collisions, where some kinetic energy turns into heat, sound or deformation, and perfectly inelastic collisions, where the objects stick together. Collisions between gas molecules, or between billiard balls, are close to elastic.
Examples
Compare the total kinetic energy before and after to see whether it was an elastic collision.
In a head-on elastic collision between equal masses, the moving ball stops and the stationary one moves off with the same velocity.
The principle that the total momentum of a system stays constant if no external force acts on it. The momentum before a collision or explosion equals the momentum after it.
Where you use it
Used for collisions, explosions, the recoil of guns and rocket propulsion. Students write 'total momentum before = total momentum after' and must keep track of directions with plus and minus signs. It holds even when kinetic energy is not conserved.
Examples
Using conservation of momentum, find the speed of the two trolleys after they stick together.
The recoil of the cannon is explained by conservation of momentum.
Common phrases
law of conservation of momentum · total momentum · momentum before and after · isolated system
The fraction of the energy put into a device that comes out as useful energy. It is given as a decimal or a percentage and can never be more than 100%.
Where you use it
Efficiency = useful output ÷ total input, using either energy or power. The rest is 'wasted', usually as heat. Common phrases: energy efficiency, highly efficient, improve the efficiency. LED lamps are far more efficient than filament lamps.
Examples
The motor takes in 500 J and gives 300 J to the load, so its efficiency is 60%.
Lubricating the gears improves the efficiency of the machine.
Usage note
Efficiency has no unit. Write 0.6 or 60%, never '60 J'.
Common phrases
energy efficiency · improve the efficiency · percentage efficiency · highly efficient · efficiency of a machine
A disturbance that travels through a medium or through space and carries energy from one place to another without carrying matter with it.
Where you use it
Waves are described as transverse (like light) or longitudinal (like sound), and as mechanical or electromagnetic. A wave travels, propagates, is reflected, refracted and absorbed. Its key quantities are speed, frequency, wavelength and amplitude, linked by v = fλ.
Examples
Sound is a longitudinal wave, but light is a transverse wave.
The waves in the ripple tank slow down when they move into shallower water.
The number of complete waves or oscillations that pass a point, or happen, each second. It is measured in hertz (Hz), where 1 Hz means one cycle per second.
Where you use it
Frequency is the inverse of the period (f = 1/T). In sound it decides pitch; in light it decides colour. Common phrases: high frequency, low frequency, natural frequency, frequency range. Values are often given in kHz, MHz or GHz.
Examples
The tuning fork vibrates at a frequency of 440 Hz.
If the frequency doubles while the wave speed stays the same, the wavelength is halved.
Common phrases
high frequency · natural frequency · frequency range · resonant frequency · angular frequency
The distance between two neighbouring points on a wave that are at the same stage of their cycle, such as two crests. It is shown by the Greek letter lambda (λ).
Where you use it
Measured in metres, but light is usually given in nanometres (visible light is roughly 400 to 700 nm) and radio in metres or kilometres. The wave equation v = fλ links wavelength, frequency and speed. People say 'short wavelength' and 'long wavelength'.
Examples
Red light has a longer wavelength than blue light.
Use the ruler to measure the wavelength of the water waves from one crest to the next.
Usage note
Wavelength is written as one word in English. Do not split it into 'wave length'.
Common phrases
short wavelength · long wavelength · de Broglie wavelength · measure the wavelength
The maximum distance that a point on a wave or an oscillating object moves away from its rest position. A larger amplitude means the wave carries more energy.
Where you use it
For sound, amplitude is linked to loudness; for light, to brightness. Measure it from the middle (equilibrium) line to a crest, not from crest to trough, which gives twice the amplitude. The energy of a wave is proportional to the amplitude squared.
Examples
Turning up the volume increases the amplitude of the sound wave, not its frequency.
The amplitude of the pendulum slowly decreases because of air resistance.
Common phrases
large amplitude · small amplitude · maximum amplitude · amplitude of oscillation
The time taken for one complete oscillation, or for one full wave to pass a point. It is measured in seconds and is the inverse of frequency (T = 1/f).
Where you use it
Symbol T. Used for pendulums, springs, orbits and waves. In experiments, students time ten oscillations and divide by ten to reduce timing errors. For a simple pendulum swinging through small angles, T = 2π√(L/g).
Examples
Time ten swings and divide by ten to find the period of the pendulum.
The Moon's orbital period is about 27 days.
Usage note
In everyday English 'period' means a length of time or, in American English, a full stop (.). In physics it always has the precise meaning above.
Common phrases
time period · orbital period · period of oscillation · period of a pendulum
A repeating back-and-forth motion in which the acceleration is proportional to the displacement from the centre and always points back towards it. A mass on a spring moves this way.
Where you use it
Abbreviated SHM. Its defining equation is a = −ω²x, and its displacement-time graph is a sine or cosine curve. The period of SHM does not depend on the amplitude, which is why a pendulum swinging through small angles keeps good time.
Examples
A mass bouncing on a spring is a good example of simple harmonic motion.
Show that the pendulum performs simple harmonic motion for small angles.
The effect produced when two or more waves meet and their displacements add together. They can reinforce each other (constructive) or cancel each other out (destructive).
Where you use it
Seen in Young's double-slit experiment, soap bubbles and noise-cancelling headphones. Constructive interference gives bright fringes or loud spots; destructive interference gives dark fringes or quiet spots. A stable pattern needs coherent sources.
Examples
The bright and dark fringes on the screen are caused by interference of light from the two slits.
Noise-cancelling headphones use destructive interference to reduce background sound.
Usage note
In everyday English, 'interference' means getting involved where you are not wanted, or noise on a radio. Turkish 'girişim' can also mean an initiative or a start-up, which are completely different English words.
The spreading out of a wave when it passes through a gap or around the edge of an obstacle. It is strongest when the gap is about the same size as the wavelength.
Where you use it
Explains why you can hear someone round a corner without seeing them: sound waves are long enough to spread through a doorway, but light waves are far too short. A diffraction grating with many slits is used to measure the wavelength of light.
Examples
Diffraction is easy to see when water waves pass through a narrow gap in a harbour wall.
We used a diffraction grating to find the wavelength of the laser.
Usage note
Do not confuse diffraction (kırınım) with refraction (kırılma). The Turkish words are as close as the English ones, so learn them as a pair.
The change in the observed frequency of a wave when the source and the observer move towards or away from each other. A siren sounds higher as it approaches and lower as it moves away.
Where you use it
Applies to both sound and light. In astronomy, light from galaxies moving away from us is redshifted, which is evidence that the universe is expanding. Police speed guns, medical ultrasound and weather radar all use the Doppler effect. Also called the Doppler shift.
Examples
The Doppler effect explains why the ambulance siren drops in pitch as it passes us.
Astronomers use the Doppler effect to measure how fast stars are moving towards or away from us.
The large increase in amplitude that happens when a system is pushed at a frequency equal to its own natural frequency. Energy is then transferred to the system very efficiently.
Where you use it
Useful in musical instruments, radio tuning and MRI scanners, but harmful in bridges, buildings and machines, where engineers add damping to limit it. A system 'resonates' or 'is in resonance'; the matching frequency is the resonant frequency.
Examples
A singer can break a wine glass through resonance if she hits its natural frequency loudly enough.
Engineers added dampers to the footbridge to reduce resonance when crowds walked across it.
Common phrases
resonant frequency · natural frequency · forced oscillation · damping · in resonance
A wave in which the particles of the medium vibrate back and forth along the direction the wave travels, forming compressions and rarefactions. Sound is the main example.
Where you use it
Contrasted with transverse waves, where the vibrations are at right angles to the direction of travel, as in light and waves on a string. Longitudinal waves need a medium to travel through. Seismic P-waves are longitudinal, while S-waves are transverse.
Examples
Push and pull the end of the slinky to make a longitudinal wave.
Sound is a longitudinal wave made of compressions and rarefactions in the air.
The bouncing back of light, sound or another wave when it hits a surface. For a flat mirror, the angle of reflection equals the angle of incidence.
Where you use it
Angles are measured from the normal, an imaginary line at right angles to the surface. Smooth surfaces give specular (mirror-like) reflection; rough surfaces give diffuse reflection. As a verb: light is reflected off or from a surface. An echo is a reflection of sound.
Examples
The law of reflection says that the angle of incidence equals the angle of reflection.
About a second later we heard the echo, a reflection of our voices from the cliff.
Common phrases
angle of reflection · law of reflection · diffuse reflection · reflection off a surface · plane mirror
The change in direction of a wave as it passes from one material into another, caused by a change in its speed. It is why a straw looks bent in a glass of water.
Where you use it
Described by Snell's law, n₁ sin θ₁ = n₂ sin θ₂, where n is the refractive index. Light bends towards the normal when it slows down (air to glass) and away from the normal when it speeds up. Lenses and rainbows depend on refraction.
Examples
Refraction makes the pool look shallower than it really is.
Measure the angle of refraction for five different angles of incidence.
Usage note
Turkish 'kırılma' also means breaking. In optics, always translate it as refraction; for a broken object, English uses 'broken' or 'fractured'.
Common phrases
angle of refraction · refractive index · Snell's law · refraction of light
A curved piece of glass or plastic that bends light by refraction to form an image. A converging lens brings light rays together; a diverging lens spreads them out.
Where you use it
Converging lenses are also called convex lenses, and diverging lenses concave lenses. Lenses are used in cameras, glasses, microscopes and the human eye. Ray diagrams show where the image forms. The plural is 'lenses'.
Examples
A converging lens with a focal length of 10 cm forms a real image of the candle on the screen.
People who are short-sighted wear glasses with diverging lenses.
Usage note
'Lens' is already singular, so do not write 'a lense'. Turkish uses 'lens' mainly for contact lenses and camera lenses; in optics lessons the word is 'mercek'.
The full range of electromagnetic waves, ordered by wavelength or frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
Where you use it
All parts of the spectrum travel at the same speed in a vacuum, about 3.00 × 10⁸ m/s. Going from radio waves to gamma rays, wavelength decreases while frequency and photon energy increase. Visible light is only a narrow band of the whole spectrum.
Examples
X-rays and radio waves are both part of the electromagnetic spectrum.
In the electromagnetic spectrum, ultraviolet lies just beyond violet light.
Usage note
The plural of 'spectrum' is 'spectra'. In Turkish texts you will see both 'spektrum' and 'tayf'.
Common phrases
visible spectrum · electromagnetic radiation · electromagnetic waves · regions of the spectrum
The complete reflection of light back into an optically denser material, such as glass or water, when it meets the boundary at an angle greater than the critical angle.
Where you use it
Abbreviated TIR. It only happens when light travels from a higher to a lower refractive index. For a boundary with air, the critical angle C satisfies sin C = 1/n. Used in optical fibres, endoscopes, prism binoculars and the sparkle of diamonds.
Examples
Optical fibres carry light over long distances by total internal reflection.
Increase the angle until the ray no longer leaves the block: that is total internal reflection.
Common phrases
critical angle · optical fibre · refractive index · undergo total internal reflection
The distance from the centre of a lens or mirror to its focal point, where parallel rays of light meet or appear to come from. It is usually given in centimetres or metres.
Where you use it
Symbol f. The shorter the focal length, the more powerful the lens. Lens power is measured in dioptres (D): power = 1/f, with f in metres. It appears in the lens equation, which in one common sign convention is 1/f = 1/u + 1/v.
Examples
A lens with a focal length of 0.5 m has a power of 2 dioptres.
Focus the image of a distant window on the screen to estimate the focal length of the lens.
Common phrases
short focal length · long focal length · focal point · principal focus · lens equation
A basic property of matter that makes particles feel electric forces. Charge can be positive or negative; like charges repel and unlike charges attract. It is measured in coulombs (C).
Where you use it
Often just 'charge'. Objects become charged, gain or lose charge, and charge flows round a circuit. The charge of an electron is about −1.6 × 10⁻¹⁹ C; its size is called the elementary charge (e). Charge is always conserved.
Examples
Rubbing the balloon on your hair transfers negative electric charge to the balloon.
Each of the two small spheres carries an electric charge of 3 μC.
Common phrases
positive charge · negative charge · point charge · charge carrier · conservation of charge
The rate at which electric charge flows past a point in a circuit. It is measured in amperes (A), and one ampere is one coulomb of charge per second.
Where you use it
Usually just 'current', symbol I. Current flows through a component and is measured with an ammeter connected in series. Conventional current goes from positive to negative, the opposite way to electron flow. Values are in amps (A) or milliamps (mA).
Examples
The ammeter shows an electric current of 0.4 A through the lamp.
In a series circuit, the electric current is the same at every point.
Usage note
As an adjective, 'current' means 'güncel', as in 'current research'. In a circuit question, the noun almost always means akım.
Common phrases
conventional current · alternating current (AC) · direct current (DC) · current flows · measure the current
The energy transferred to or by each coulomb of charge between two points in a circuit, also called potential difference. It is measured in volts (V).
Where you use it
Physics textbooks, especially British ones, prefer 'potential difference' (p.d.); 'voltage' is more common in speech and in the US. A voltmeter is connected in parallel across a component. You say 'the voltage across', not 'the voltage through'.
Examples
The voltage across the resistor is 6 V when the current is 0.2 A.
A 9 V battery provides a higher voltage than a 1.5 V cell.
Usage note
Do not translate 'gerilim' as 'tension'. In physics, tension is the pulling force in a rope or string.
Common phrases
voltage across · potential difference · supply voltage · high voltage · measure the voltage
How strongly a component or material opposes the flow of electric current. It equals voltage divided by current and is measured in ohms (Ω).
Where you use it
Symbol R. The resistance of a wire increases with its length and decreases with its cross-sectional area; for most metals it also rises with temperature. Resistances in series simply add. In mechanics, 'air resistance' is another name for drag.
Examples
The resistance of the filament lamp increases as it gets hotter.
Calculate the total resistance of two 10 Ω resistors connected in parallel.
Usage note
Turkish uses 'direnç' for both the property (resistance) and the component (resistor). In English, a resistor is the part you put in a circuit; resistance is its value.
Common phrases
electrical resistance · total resistance · internal resistance · air resistance · resistance of a wire
The rule that the current through a metal conductor is directly proportional to the voltage across it, as long as its temperature stays the same. It is usually written V = IR.
Where you use it
One of the first equations students use in circuits. Components that follow it are called ohmic, and their current-voltage graph is a straight line through the origin. Filament lamps and diodes are non-ohmic. Say 'using Ohm's law' or 'by Ohm's law'.
Examples
Using Ohm's law, a 12 V supply across a 4 Ω resistor gives a current of 3 A.
The diode does not obey Ohm's law, so its graph is curved.
Common phrases
obey Ohm's law · using Ohm's law · ohmic conductor · non-ohmic component
A closed path through which electric current can flow, made of a power source, wires and components such as lamps, resistors and switches.
Where you use it
Components are connected in series (one after another) or in parallel (side by side). You build, set up or draw a circuit; a circuit diagram uses standard symbols. An open circuit has a gap; a short circuit lets current bypass the load.
Examples
Set up the circuit as shown in the diagram, then close the switch.
In a parallel circuit, each lamp gets the full supply voltage.
Usage note
Pronounced like 'SUR-kit'. The 'ui' is not pronounced as two vowels, so do not say 'sir-ku-it'.
Common phrases
series circuit · parallel circuit · circuit diagram · short circuit · complete the circuit
The region around a magnet or a current-carrying wire where magnetic forces can be felt. Its strength is measured in teslas (T), and it is shown with field lines.
Where you use it
Symbol B. Outside a magnet, field lines go from the north pole to the south pole. A charge moving through a magnetic field feels a force, and a changing field induces a current. The Earth's magnetic field at the surface is roughly 50 microteslas.
Examples
The wire is placed in a uniform magnetic field of 0.2 T.
Iron filings show the shape of the magnetic field around the bar magnet.
Common phrases
uniform magnetic field · magnetic field lines · field strength · Earth's magnetic field · magnetic flux density
The region around a charged object where another charge feels an electric force. Its strength is the force per unit charge, measured in newtons per coulomb (N/C) or volts per metre (V/m).
Where you use it
Symbol E. Field lines start on positive charges and end on negative ones. Between two parallel charged plates the field is uniform. Common phrases: field strength, field lines, uniform field, place a charge in a field.
Examples
The electric field between the plates is 5000 V/m, so an electron there feels a constant force.
Draw the electric field lines around a single positive point charge.
Common phrases
electric field strength · uniform electric field · electric field lines · field around a charge
The production of a voltage, and often a current, in a conductor when the magnetic field through it changes. It is the principle behind generators and transformers.
Where you use it
Described by Faraday's law (the induced emf depends on the rate of change of magnetic flux) and Lenz's law (the induced current opposes the change that causes it). You talk about an induced current and an induced emf. Pushing a magnet into a coil is the classic demonstration.
Examples
Pushing the magnet into the coil makes the needle move: that's electromagnetic induction.
Power stations use electromagnetic induction to generate electricity.
Common phrases
Faraday's law · Lenz's law · induced current · induced emf · magnetic flux
A component that stores electric charge and energy in the electric field between two conducting plates separated by an insulator. Its capacitance is measured in farads (F).
Where you use it
Capacitors are charged and discharged; the charge stored is Q = CV. Physics courses study how current and voltage change exponentially during charging and discharging, with a time constant RC. Values are usually in microfarads (μF).
Examples
The capacitor discharges through the resistor, and after one time constant the voltage has fallen to about 37% of its initial value.
A camera flash uses a capacitor to release a lot of energy very quickly.
Common phrases
charge a capacitor · discharge a capacitor · capacitance · time constant · parallel-plate capacitor
The energy that a source such as a battery or generator gives to each coulomb of charge passing through it. Despite its name, it is not a force, and it is measured in volts.
Where you use it
Usually shortened to emf, said letter by letter, with the symbol ε or E. The voltage at the terminals is less than the emf because of the source's internal resistance: V = ε − Ir. An induced emf comes from electromagnetic induction.
Examples
The cell has an electromotive force of 1.5 V and an internal resistance of 0.5 Ω.
When the coil spins faster, a larger electromotive force is induced.
Usage note
Many students think emf is a force because of its name. It is an energy per unit charge, just like voltage.
Common phrases
emf of a cell · induced emf · internal resistance · terminal voltage
Energy that flows from a hotter object to a colder one because of the temperature difference between them. Like all energy, it is measured in joules (J).
Where you use it
In physics, heat is energy in transfer, not something an object 'contains'; what an object contains is internal (thermal) energy. Heat is transferred, absorbed, released or lost. As a verb, you heat water or a gas. Symbol: Q.
Examples
The metal block absorbs heat from the hot water until both reach the same temperature.
On its own, heat always flows from the hotter body to the cooler one.
Usage note
Heat and temperature are different: a bathtub of warm water holds far more thermal energy than a cup of boiling water, even though its temperature is lower.
Common phrases
heat transfer · absorb heat · release heat · heat loss · latent heat
A measure of how hot or cold something is, linked to the average kinetic energy of its particles. In science it is measured in kelvin (K) or degrees Celsius (°C).
Where you use it
Temperature rises, falls, is measured with a thermometer and is kept constant in experiments. In gas law calculations you must use kelvin: add 273 to the Celsius value. Write 300 K, not '300 °K', and read it as 'three hundred kelvin'.
Examples
Keep the temperature of the water bath at 40 °C throughout the experiment.
When the temperature of the gas rises at constant volume, its pressure increases.
Usage note
Americans use Fahrenheit in daily life, so you may meet °F in news stories or recipes, but physics always uses kelvin or Celsius.
Common phrases
room temperature · temperature difference · temperature rise · absolute temperature · constant temperature
The force acting on each unit of area of a surface, found by dividing force by area. It is measured in pascals (Pa), where one pascal is one newton per square metre.
Where you use it
Used in mechanics (p = F/A), fluids (pressure increases with depth) and gases, where pressure comes from particles hitting the container walls. Atmospheric pressure at sea level is about 101 kPa. Pressure is applied or exerted, and it rises or drops.
Examples
Sharp knives cut well because they apply a large pressure over a tiny area.
The energy needed to raise the temperature of 1 kg of a substance by 1 K (or 1 °C). Water has a high value, about 4200 J/(kg·K).
Where you use it
Symbol c, used in Q = mcΔT, and often shortened to 'specific heat'. A material with a high specific heat capacity heats up and cools down slowly, which helps keep coastal climates mild. It is measured in the lab with an electric heater, a joulemeter and a thermometer.
Examples
Because water has a high specific heat capacity, it is used in central heating systems.
Use Q = mcΔT to find the specific heat capacity of the aluminium block.
Usage note
'Heat capacity' alone means the energy needed to warm a whole object by 1 K, not 1 kg of it. In physics, 'specific' usually means 'per unit mass'.
Common phrases
specific heat capacity of water · high specific heat capacity · measure the specific heat capacity · heat capacity
The principle that the change in a system's internal energy equals the heat supplied to it minus the work it does. It is the conservation of energy applied to heat and work.
Where you use it
Written ΔU = Q − W when W is the work done by the gas, but some books write ΔU = Q + W with W as the work done on the gas, so always check the sign convention. Used for engines, refrigerators and gas processes such as isothermal and adiabatic changes.
Examples
By the first law of thermodynamics, if the gas absorbs 500 J of heat and does 200 J of work, its internal energy rises by 300 J.
The first law of thermodynamics tells us that no engine can give out more energy than it takes in.
Common phrases
laws of thermodynamics · internal energy · work done by the gas · sign convention · second law of thermodynamics
A measure of how spread out, or disordered, the energy of a system is. In an isolated system the total entropy never decreases, which is one way of stating the second law of thermodynamics.
Where you use it
Symbol S, measured in joules per kelvin (J/K). Used to explain why heat flows from hot to cold, why no heat engine can be 100% efficient and why some processes are irreversible. Popular science links it to 'the arrow of time'.
Examples
When the ice melts in the warm room, the total entropy of the ice and the room increases.
A refrigerator can lower the entropy inside it only by increasing the entropy of the kitchen.
Usage note
'Disorder' is a helpful picture for beginners, but physicists define entropy precisely through heat and temperature (ΔS = Q/T for a reversible process) or by counting microstates.
Common phrases
increase in entropy · change in entropy · second law of thermodynamics · entropy of a system
The equation pV = nRT, which links the pressure, volume, temperature and amount of an ideal gas. Real gases follow it closely at low pressure and high temperature.
Where you use it
Here n is the number of moles and R ≈ 8.31 J/(mol·K); physicists also write pV = NkT, with N molecules and Boltzmann's constant k. Temperature must be in kelvin. Also called the ideal gas equation or the equation of state of an ideal gas.
Examples
Use the ideal gas law to find the pressure of 2 mol of gas in a 0.05 m³ container at 300 K.
The ideal gas law assumes that the molecules take up no volume and do not attract each other.
Common phrases
ideal gas equation · molar gas constant · Boltzmann constant · Boyle's law · Charles's law
The movement of thermal energy from a hotter place to a colder one. It happens in three ways: conduction, convection and radiation.
Where you use it
Conduction happens through solids by particle collisions, convection through liquids and gases as warmer regions rise, and radiation through infrared waves that can even cross a vacuum. Insulation, double glazing and vacuum flasks are designed to reduce heat transfer.
Examples
A vacuum flask reduces heat transfer by conduction, convection and radiation.
The rate of heat transfer through the wall depends on its thickness and on the temperature difference.
Usage note
'Radiation' here means infrared radiation from warm objects, not radioactivity. English uses the same word for both, so check the context.
Common phrases
rate of heat transfer · conduction · convection · thermal radiation · reduce heat transfer
The energy absorbed or released when a substance changes state, for example from solid to liquid, without any change in its temperature.
Where you use it
Specific latent heat (L) is the energy per kilogram: Q = mL. There is a latent heat of fusion for melting and a latent heat of vaporisation for boiling; for water these are about 334 kJ/kg and 2260 kJ/kg. It explains why steam causes such bad burns.
Examples
While the ice is melting, the thermometer stays at 0 °C because the energy goes into latent heat.
Sweating cools you down because the evaporating water takes latent heat from your skin.
Common phrases
specific latent heat · latent heat of fusion · latent heat of vaporisation · change of state
The lowest possible temperature, 0 K or −273.15 °C, at which particles have the least energy they can possibly have. It can be approached but never actually reached.
Where you use it
It is the zero point of the kelvin scale, so ordinary temperatures in kelvin are never negative. It can be estimated by extending a pressure-temperature graph for a gas back to zero pressure. Laboratories have cooled atoms to within a billionth of a kelvin of absolute zero.
Examples
Extend the line on the pressure-temperature graph to estimate absolute zero.
Unless high pressure is applied, helium stays liquid even very close to absolute zero.
Common phrases
kelvin scale · approach absolute zero · absolute temperature · extrapolate to absolute zero
A single packet, or quantum, of electromagnetic energy. Its energy depends only on the frequency of the radiation: E = hf, where h is Planck's constant.
Where you use it
Used when light behaves like a stream of particles, as in the photoelectric effect and atomic spectra. Photons are emitted or absorbed by atoms, have no mass and always travel at the speed of light in a vacuum. Blue photons carry more energy than red ones.
Examples
When the electron drops to a lower energy level, the atom emits a photon.
Calculate the energy of a photon of green light with a frequency of 5.6 × 10¹⁴ Hz.
Common phrases
photon energy · emit a photon · absorb a photon · stream of photons
The smallest separate amount of a physical quantity, such as energy, that can be absorbed or emitted. Light energy, for example, comes in quanta called photons.
Where you use it
The plural is 'quanta'. The word is used even more often as an adjective: quantum physics, quantum mechanics, quantum number, quantum state. 'Quantised' means a quantity can only take certain separate values, like the energy of an electron in an atom.
Examples
Planck proposed that energy is emitted in small packets, each one a quantum.
In quantum mechanics, the energy of an electron in an atom is quantised.
Usage note
In everyday English, a 'quantum leap' means a huge change, but in physics a quantum jump is a tiny change between energy levels.
Common phrases
quantum mechanics · quantum physics · quantum number · quantised energy
The release of electrons from a metal surface when light of high enough frequency shines on it. It showed that light energy comes in packets called photons.
Where you use it
Below a threshold frequency no electrons are emitted, however bright the light. Einstein explained it in 1905 with hf = φ + KEmax, where φ is the work function, and his Nobel Prize was awarded mainly for this work. It is used in light detectors such as photomultiplier tubes.
Examples
In the photoelectric effect, brighter light releases more electrons, but not faster ones.
Ultraviolet light produces the photoelectric effect in zinc, but red light does not.
Common phrases
threshold frequency · work function · photoelectrons · stopping potential
The idea that light and matter can behave both as waves and as particles, depending on the experiment. Electrons, for example, can produce interference patterns.
Where you use it
A key idea of quantum physics. Light shows its wave nature in diffraction and interference, and its particle nature in the photoelectric effect. For matter, the de Broglie wavelength λ = h/p gives the wavelength of a moving particle.
Examples
Electron diffraction is strong evidence for wave-particle duality.
The double-slit experiment is the classic way to introduce wave-particle duality.
One of the fixed amounts of energy that an electron in an atom is allowed to have. Electrons move between levels by absorbing or emitting photons.
Where you use it
The lowest level is the ground state and higher ones are excited states. Energies are often given in electronvolts as negative values, for example −13.6 eV for the ground state of hydrogen. Transitions between levels produce line spectra.
Examples
An electron falling from the n = 3 to the n = 2 energy level in hydrogen gives off red light.
Each element has its own set of energy levels, so each one produces its own line spectrum.
Common phrases
ground state · excited state · energy level diagram · line spectrum · electron transition
A rule of quantum physics stating that certain pairs of quantities, such as a particle's position and momentum, cannot both be known exactly at the same time.
Where you use it
Also called Heisenberg's uncertainty principle and written Δx Δp ≥ ħ/2. It is a property of nature, not a limit of our instruments, and a similar relation links energy and time. Popular articles often explain it wrongly as the observer disturbing the particle.
Examples
According to the uncertainty principle, the more precisely we know the electron's position, the less precisely we know its momentum.
Heisenberg published the uncertainty principle in 1927.
Common phrases
Heisenberg's uncertainty principle · position and momentum · energy-time uncertainty · quantum uncertainty
Einstein's 1905 theory stating that the laws of physics are the same for all observers moving at constant velocity, and that the speed of light in a vacuum is the same for all of them.
Where you use it
It leads to time dilation (moving clocks run slow), length contraction and E = mc². Its effects are only large at speeds close to the speed of light, but GPS satellites must still correct for them. General relativity (1915) extends the theory to include gravity.
Examples
Special relativity explains why muons created high in the atmosphere live long enough to reach the ground.
This term we'll study special relativity, starting with time dilation.
Common phrases
theory of special relativity · time dilation · length contraction · general relativity · relativistic speeds
The smallest unit of a chemical element, made of a tiny positive nucleus surrounded by negatively charged electrons. Most of an atom is empty space.
Where you use it
Atoms are about 10⁻¹⁰ m across, while the nucleus is roughly 100,000 times smaller. Physicists talk about atomic structure, models of the atom (Thomson, Rutherford, Bohr) and atoms that are excited, ionised or neutral. The adjective is 'atomic'.
Examples
Rutherford's experiment showed that the atom has a small, dense nucleus.
A neutral atom has the same number of electrons as protons.
Common phrases
atomic structure · atomic number · neutral atom · model of the atom · excited atom
A very light particle with a negative charge of about −1.6 × 10⁻¹⁹ C, found around the nucleus of every atom. Moving electrons carry the current in metal wires.
Where you use it
Its mass is about 9.11 × 10⁻³¹ kg, roughly 1/1840 of a proton's. Electrons are emitted, absorbed and accelerated, and they move between energy levels. The electronvolt (eV) is the energy an electron gains when accelerated through 1 V.
Examples
When ultraviolet light hits the zinc plate, electrons are emitted from its surface.
In a copper wire the electrons drift slowly, even though the current starts almost instantly.
Common phrases
free electrons · electron beam · charge on an electron · emit electrons · electronvolt (eV)
The very small, dense centre of an atom, made of protons and neutrons. It contains almost all of the atom's mass and all of its positive charge.
Where you use it
The plural is 'nuclei'. Protons and neutrons together are called nucleons. A nucleus is described by its proton number (atomic number, Z) and nucleon number (mass number, A). Nuclear physics studies how nuclei decay, split and fuse.
Examples
A carbon-12 nucleus contains six protons and six neutrons.
Most alpha particles went straight through the gold foil, which showed that the nucleus is tiny.
Usage note
Pronounced NOO-klee-us in American English and NYOO-klee-us in British English. Biology also uses 'nucleus' for the centre of a cell (hücre çekirdeği).
One of two or more forms of the same element whose atoms have the same number of protons but different numbers of neutrons. Isotopes behave the same chemically but have different masses.
Where you use it
Isotopes are named with their mass number: carbon-12, carbon-14, uranium-235. Some are stable and others are radioactive (radioisotopes). Isotopes are used in carbon dating, as medical tracers and as fuel in nuclear power.
Examples
Carbon-14 is a radioactive isotope used to date ancient wood and bone.
Deuterium is an isotope of hydrogen with one proton and one neutron.
Usage note
The stress is on the first syllable: EYE-so-tope. Turkish speakers often stress the last syllable instead.
Common phrases
radioactive isotope · stable isotope · isotope of · radioisotope
The random process in which an unstable nucleus breaks down by giving off radiation, such as alpha particles, beta particles or gamma rays, and often turns into a different element.
Where you use it
Decay is random: you cannot predict when one nucleus will decay, only the probability. Nuclei decay into daughter nuclei, and the activity of a sample is measured in becquerels (Bq). Common phrases: alpha decay, beta decay, decay chain, decay constant.
Examples
Radioactive decay is random, but a large sample still decays in a predictable way.
In this radioactive decay, uranium-238 emits an alpha particle and becomes thorium-234.
The time it takes for half of the radioactive nuclei in a sample to decay, or for its activity to fall to half. It is constant for a given isotope.
Where you use it
Half-lives range from fractions of a second to billions of years. After two half-lives a quarter remains, after three an eighth. Used in carbon dating (carbon-14 has a half-life of about 5730 years) and in choosing safe medical tracers. Plural: half-lives.
Examples
Technetium-99m has a half-life of about six hours, which makes it useful in medical imaging.
If the half-life is 8 days, only one eighth of the original sample is left after 24 days.
Usage note
A half-life is not half of a 'total life'. A sample does not disappear after two half-lives; a quarter of it is still there.
Common phrases
short half-life · long half-life · measure the half-life · radioactive half-life
The splitting of a heavy nucleus, such as uranium-235, into two smaller nuclei, usually after it absorbs a neutron. It releases a large amount of energy and more neutrons.
Where you use it
The released neutrons can cause further fissions, producing a chain reaction. In nuclear power stations, control rods absorb neutrons to keep the reaction steady. The energy comes from a small loss of mass, given by E = mc².
Examples
Each nuclear fission of uranium-235 releases about 200 MeV of energy.
The reactor uses nuclear fission to heat water and produce steam.
The joining of two light nuclei, such as isotopes of hydrogen, to form a heavier nucleus. It releases huge amounts of energy and is what powers the Sun and other stars.
Where you use it
Fusion needs extremely high temperatures and pressures so that the positive nuclei can overcome their electric repulsion. Researchers are developing fusion reactors, such as tokamaks, as a possible future energy source. It is the opposite of fission, which splits heavy nuclei.
Examples
In the core of the Sun, nuclear fusion turns hydrogen into helium.
Nuclear fusion produces much less long-lived radioactive waste than fission.
Usage note
Turkish speakers sometimes mix up fusion and fission because both are loanwords in Turkish (füzyon, fisyon). Remember: fusion means joining (kaynaşma), fission means splitting (bölünme).
A simple sketch of one object with labelled arrows for all the forces acting on it, each drawn in its own direction. It is the usual first step in solving a forces problem.
Where you use it
Teachers say 'start with a free-body diagram' or 'draw the FBD'. Only forces acting on the chosen object appear, never the forces it exerts on other things. Typical labels: W or mg (weight), N (normal force), T (tension), f (friction).
Examples
Draw a free-body diagram for the box on the ramp before you write any equations.
Your free-body diagram is missing the friction force acting up the slope.
The range within which the true value of a measurement probably lies, written with a ± sign. It shows how reliable a measurement or result is.
Where you use it
Every lab report needs uncertainties: absolute uncertainty (±0.1 cm), percentage uncertainty (±2%) and the uncertainty in the final result. The uncertainty of a ruler reading is often taken as half the smallest division. You estimate, calculate and reduce uncertainty.
Examples
The length is 24.5 ± 0.1 cm, so the percentage uncertainty is about 0.4%.
Repeating the measurement and taking an average reduces the random uncertainty.
Usage note
In the lab, uncertainty is about the quality of a measurement. The quantum 'uncertainty principle' is a different idea about nature itself.
Common phrases
absolute uncertainty · percentage uncertainty · measurement uncertainty · estimate the uncertainty · within the uncertainty
Describes two quantities that change at the same rate, so that if one doubles the other doubles too. Their graph is a straight line through the origin.
Where you use it
Used constantly in explanations and exam answers: 'the extension is directly proportional to the load'. The symbol is ∝. If the straight line does not pass through the origin, examiners call it a linear relationship, not direct proportionality.
Examples
Within the limit of proportionality, the extension of a spring is directly proportional to the force applied.
The graph is a straight line through the origin, so the current is directly proportional to the voltage.
Common phrases
directly proportional to · direct proportion · is proportional to · constant of proportionality
Describes two quantities linked so that when one doubles, the other halves. Their product stays constant, and a graph of one against the other is a curve, not a straight line.
Where you use it
Common in gas laws (at constant temperature, pressure is inversely proportional to volume) and waves (wavelength is inversely proportional to frequency). To get a straight line, plot one quantity against 1/x of the other. It is not the same as an inverse square law.
Examples
At constant temperature, the pressure of a gas is inversely proportional to its volume.
Plot t against 1/v to check whether the time is inversely proportional to the speed.
Usage note
In an inverse square law, such as gravity or light intensity, a quantity falls with the square of the distance: double the distance and it becomes four times smaller.
Common phrases
inversely proportional to · inverse proportion · inverse square law
To obtain an equation or result step by step from basic laws or other equations, using logic and algebra instead of simply quoting it.
Where you use it
A common verb in exams and lectures: 'derive an expression for...', 'show that...', 'derive the equation from first principles'. The noun is 'derivation'. Do not confuse 'derive' with 'differentiate' (türev almak), even though the Turkish words look alike.
Examples
Derive an expression for the maximum height reached by the ball.
The lecturer derived the lens equation from the geometry of the ray diagram.
Usage note
A 'derivative' is the result of differentiation in calculus (türev), while a 'derivation' is the process of deriving a formula (türetme). They are different words.
Common phrases
derive an expression · derive an equation · derive from first principles · derivation of
An error that shifts every reading in the same direction by a similar amount, for example because of a badly calibrated instrument or a zero error. Repeating the measurement does not remove it.
Where you use it
Contrasted with random error, which scatters readings on both sides of the true value. Systematic errors affect accuracy; random errors affect precision. On a graph, a systematic error often shows up as a line that misses the origin when it should pass through it.
Examples
The balance read 0.2 g with nothing on it, so there is a systematic error in all our mass readings.
Reading the scale from the same wrong angle every time causes a systematic error.
Usage note
In English lab reports, 'accuracy' means closeness to the true value, while 'precision' means how close repeated readings are to each other. Turkish often uses 'hassasiyet' loosely for both.
Common phrases
random error · zero error · parallax error · source of error · reduce systematic error
A straight line or smooth curve drawn on a graph so that it passes as close as possible to all the plotted points, showing the overall trend of the data.
Where you use it
Students draw it by eye or with software (least-squares fitting), then use its gradient and intercept to find physical quantities. Never just join the dots. Points far from the line may be anomalous results that need checking.
Examples
Draw a line of best fit and use its gradient to find the spring constant.
The line of best fit does not pass through the origin, which suggests a zero error.
Usage note
British English says 'gradient' for the steepness of a line, while American English usually says 'slope'. Both are understood.
Common phrases
draw a line of best fit · gradient of the line · y-intercept · anomalous result · curve of best fit
To split a force or another vector into two parts at right angles to each other, usually horizontal and vertical, so that each direction can be dealt with separately.
Where you use it
Typical phrases: resolve a force into components, resolve horizontally, resolve vertically, resolve parallel to the slope. A force F at an angle θ to the horizontal has components F cos θ (horizontal) and F sin θ (vertical).
Examples
Resolve the weight into components parallel and perpendicular to the slope.
Resolving vertically, the vertical components of the two tensions must add up to the weight of the lamp.
Usage note
In everyday English, 'resolve' means to solve a problem or settle a disagreement. In mechanics it has this special meaning of splitting into components.
To leave a small effect, such as air resistance or friction, out of a calculation on purpose because it makes little difference to the result.
Where you use it
A standard textbook verb: 'neglect air resistance', 'friction can be neglected', 'the mass of the string is negligible'. In conclusions, students discuss whether it was reasonable to neglect an effect. Similar expressions: ignore, assume to be zero.
Examples
Neglecting air resistance, how long does the stone take to reach the ground?
We neglected the heat lost to the surroundings, so our value for c is too high.
Usage note
In everyday English, 'neglect' means failing to look after someone or something, as in 'child neglect'. In physics it is a neutral, deliberate simplification.
Common phrases
neglect air resistance · neglect friction · negligible mass · can be neglected
Ten questions from this glossary. Pick the right meaning for each term and see your score.
How to learn physics vocabulary
Read real texts from the field: open physics textbooks, lab manuals, past exam papers and popular science articles use this vocabulary in its natural context.
Learn as pairs the words that Turkish handles differently: speed and velocity (sürat and hız), mass and weight, heat and temperature, distance and displacement, refraction and diffraction.
Practise reading equations and units aloud the way physicists do: 'F equals m a', 'nine point eight metres per second squared', 'three times ten to the eight metres per second'. English uses a decimal point, not a comma.
Learn the verbs of problem solving and lab work, because exam questions are built on them: assume, neglect, resolve, derive, substitute, plot, measure, estimate.
Write short lab reports in English: what you measured and how, your results with units and uncertainties, what the graph shows and how the experiment could be improved.
Frequently asked questions
How can I learn physics in English?
Read real texts from the field and notice how the words are used: English-language textbooks, lab manuals, exam papers and popular science articles. Learn each term with its usual partners (apply a force, the current flows, neglect air resistance), read equations aloud and write your own solutions and lab reports in English from the start.
Which physics terms should I learn first?
Start with the words you meet in the first weeks of any course: force, mass, velocity, acceleration, energy, work, power, momentum, weight, friction, vector, wave, frequency, wavelength, charge, current, voltage, heat and temperature. Then move on to optics, circuits, thermodynamics, atomic and quantum physics. The terms in this glossary follow the same order, from most to least essential.
What is the difference between speed and velocity, and how do I translate 'hız'?
Speed only tells you how fast something moves, so it is a scalar. Velocity also gives the direction, so it is a vector. Turkish school physics calls them 'sürat' and 'hız', but everyday Turkish uses 'hız' for both. If direction matters, as in kinematics problems, use velocity; for everyday statements and fixed phrases such as 'speed of light', use speed.
How do I read equations and units aloud in English?
Read symbols as letters and operations as words: F = ma is 'F equals m a', v² is 'v squared', ½mv² is 'a half m v squared'. For units, say 'per' for a slash and 'squared' for a power of two: m/s² is 'metres per second squared'. Powers of ten are read 'times ten to the', so 3 × 10⁸ is 'three times ten to the eight' and 10⁻³ is 'ten to the minus three'.
Why do British and American physics books use different words?
Both traditions describe the same physics but have their own favourite terms. British books often say resultant force, moment, potential difference (p.d.), gradient and metre; American books prefer net force, torque, voltage, slope and meter. Both sets are correct and widely understood. Pick one style and use it consistently in your own writing.
