Internal energy Cambridge International AS & A Level Physics revision
Not started
Learn it
In plain words
Even a cup of tea sitting still is full of motion: its molecules are jiggling about and tugging on one another. Add up all that hidden kinetic and potential energy and you have the tea's internal energy. Warm it up and the total goes up.
4 things to know
- The internal energy of a system is the sum of the randomly distributed kinetic and potential energies of its molecules. It depends only on the state the system is in.
- A rise in temperature means the molecules have more kinetic energy, so the internal energy rises.
- During a change of state the molecules' potential energy changes while their kinetic energy, and so the temperature, stays the same.
- In an ideal gas there are no forces between the molecules, so there is no potential energy. Its internal energy is all kinetic and depends only on its temperature: (3/2)NkT in total.
Tips and tricks
- Internal energy is not the kinetic energy of the object moving as a whole. Throwing a ball faster does not change its internal energy.
- There are two ways to change internal energy: heating, and doing work.
It lands in your notebook with its questions as flashcards.
Internal energy: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the internal energy of a system?
It is the energy hidden inside, at the scale of molecules.
What does the internal energy of an ideal gas depend on?
It is all kinetic energy, and that depends on temperature.
What is the potential energy of the molecules of an ideal gas?
There are no forces between the molecules.
A gas is heated at constant volume. What happens to its internal energy?
Its temperature rises.
A ball is thrown faster than before, at the same temperature. What happens to its internal energy?
The motion of the whole ball is not random molecular motion.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Internal energy
Cambridge International AS & A Level Physics 9702 · 6 marks · papermunch.org
Name ______________________________ Date ______________
State what is meant by the internal energy of a system.[2]
Show answerHide answer
The sum of the random kinetic and potential energies of all its molecules.
Ice melts at 0 °C. State what happens to the kinetic energy and the potential energy of its molecules, and to its internal energy.[2]
Show answerHide answer
The kinetic energy stays the same, because the temperature does. The potential energy increases. So the internal energy increases.
Find the internal energy of an ideal gas of 2.0 × 10²³ molecules at 300 K.[2]
Show answerHide answer
1.2 × 10³ J. 1.5 × 2.0 × 10²³ × 1.38 × 10⁻²³ × 300.
Answers: Internal energy
- 1. The sum of the random kinetic and potential energies of all its molecules.
- 2. The kinetic energy stays the same, because the temperature does. The potential energy increases. So the internal energy increases.
- 3. 1.2 × 10³ J. 1.5 × 2.0 × 10²³ × 1.38 × 10⁻²³ × 300.