# First law of thermodynamics

Change in internal energy equals heat added minus work done by the system: change in U = Q - W. It is energy conservation.

Physics (university) · Part 06: Thermodynamics · Card 8 of 10 · Last updated 2026-10-19

## Analogy

A bank account: deposits are heat, withdrawals are work.

## Example

**Gas in a piston**

change in U = Q - W
= 500 - 200
= 300 J

## In practice

A gas absorbs 500 J and does 200 J of work. Its internal energy rises by 300 J.

## Practice

### Flashcards

- **What does the first law of thermodynamics say?** Change in internal energy = heat in - work done by the system.

### Interview questions

**A heat engine absorbs 1000 J from a hot reservoir at 500 K and rejects 700 J to a cold reservoir at 300 K. Find its efficiency and compare it with the Carnot limit.**

- Work = 1000 - 700 = 300 J
- Efficiency 30 percent
- Carnot limit 40 percent
- Real engine is below the limit

## Questions people ask

### What is internal energy?

Internal energy is the total energy of the particles inside a system, from their motion and from forces between them. For an ideal gas it depends only on temperature. Warm gas has more internal energy than cold gas of the same amount.

### What sign should work have in the first law?

In the form change in U = Q - W, work W is positive when the system does work on its surroundings, such as a gas expanding. It is negative when the surroundings compress the gas. Check your textbook, because some books write change in U = Q + W instead.

## Related concepts

- [Heat engines](https://studyvisually.com/learn/science/physics-university/physics-university-part-6/physics-university-part-6-concept-9)
- [Temperature](https://studyvisually.com/learn/science/physics-university/physics-university-part-6/physics-university-part-6-concept-1)
- [Thermal expansion](https://studyvisually.com/learn/science/physics-university/physics-university-part-6/physics-university-part-6-concept-2)
- [Specific heat](https://studyvisually.com/learn/science/physics-university/physics-university-part-6/physics-university-part-6-concept-3)

Web page: https://studyvisually.com/learn/science/physics-university/physics-university-part-6/physics-university-part-6-concept-8
Study this card in the app: https://studyvisually.com/app/learn?category=physics-university&card=physics-university-part-6-concept-8