First law of thermodynamics

How it is used

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.

Example sentences

1. 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.

2. The first law of thermodynamics tells us that no engine can give out more energy than it takes in.

Common collocations

• laws of thermodynamics
• internal energy
• work done by the gas
• sign convention
• second law of thermodynamics

Related terms

• conservation of energyThe 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.
• heatEnergy 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).
• entropyA 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.
• ideal gas lawThe 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.
• efficiencyThe 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%.

More terms: Heat and Thermodynamics

• temperature
• pressure
• specific heat capacity
• heat transfer
• latent heat
• absolute zero