First Law of Thermodynamics
Energy is conserved: ΔU = Q − W.
About this topic
The first law is energy conservation applied to thermal systems: the change in internal energy equals heat added minus work done by the system, ΔU = Q − W. Heat flowing in or work done on the gas raises its internal energy.
In an isothermal process temperature is constant and so ΔU = 0, meaning heat in equals work out. In an adiabatic process there is no heat exchange (Q = 0), so the gas cools as it expands and warms as it is compressed. At constant volume no work is done, and at constant pressure the work is W = PΔV.
Worked example: a gas absorbs 500 J of heat and expands, doing 200 J of work on a piston. Its internal energy rises by 500 − 200 = 300 J. A bicycle pump gets warm because compressing the air quickly does work on it that raises its temperature, and aerosol cans cool when spraying because the gas expands.
The law governs engines, refrigerators, weather systems and cooking. It tells us you cannot get more energy out of a system than you put in, which forbids perpetual motion machines of the first kind. Take care with signs: work done by the gas is positive, and work done on the gas is negative in this convention. Some textbooks use the opposite convention.
Key ideas
- Heat added raises internal energy
- Work done by gas lowers it
- Applies to engines
Quick check
Use cases: where First Law of Thermodynamics helps
- Homework: review the key idea behind First Law of Thermodynamics before you start a problem set.
- Exam revision: use the quick check to test whether you can recall First Law of Thermodynamics without notes.
- Visual learning: play with the interactive lab to see how First Law of Thermodynamics changes when you adjust its inputs.
- Teaching a friend: explain First Law of Thermodynamics in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is First Law of Thermodynamics?
Energy is conserved: ΔU = Q − W.
What will I learn in First Law of Thermodynamics?
Heat added raises internal energy. Work done by gas lowers it. Applies to engines.
Is First Law of Thermodynamics part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the First Law of Thermodynamics lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study First Law of Thermodynamics?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.