The First Law of Thermodynamics
The First Law of Thermodynamics
- Internal energy changes when energy crosses the boundary of the system.
- For this chapter, energy crosses the boundary in two ways: by heating or by doing work.
Start with the direction of each transfer
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Heating
- Energy enters the system: q is positive.
- Energy leaves the system: q is negative.
Work
- Work is done on the system: W is positive.
- The system does work: W is negative.
Common mistake
The first law uses for work done on the system. Work done by the system has the opposite sign.
Conservation of energy for the system
Definition
2 marksWhat is meant by the first law of thermodynamics?
Model answer: The increase in internal energy equals the work done on the system plus the energy transferred to the system by heating.
Symbols
- = increase in internal energy (J)
- = energy transferred to the system by heating (J)
- = work done on the system (J)
The wording and both energy directions are worth separate marks (9702/42/O/N/25 Q4(a)).
Worked example
Both transfers enter the gas
A gas receives 250 kJ by heating. At the same time, 500 kJ of work is done on it. Find the increase in internal energy.
- Both transfers enter the system, so both values are positive.
- Add the signed values.
Answer
Your turnquick check
A gas receives 250 kJ by heating and does 200 kJ of work on its surroundings. Find the change in internal energy.
Show worked answer
- Heating enters the gas, but work leaves it.
- Use the signed values.
Answer