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.
Last updated
Analogy
A bank account: deposits are heat, withdrawals are work.
Example
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.