This document discusses the internal energy unit of an A-Level Physics course on thermal physics. It includes learning objectives about defining internal energy, how temperature increases relate to internal energy increases, and using the first law of thermodynamics. Specific topics covered include the internal energy of an ideal gas, changes in internal energy during phase changes, and demonstrations of work being done to change internal energies.

1. A-level Physics
Unit G484:
The Newtonian
World
Internal energy
Thermal physics

2. The energy of an ideal gas LOs
To do
Without referring to your notes,
1. Given that p = ⅓ ρ‹c2› and pV = nRT derive an expression for the total
k.e. N( ½ m‹c2› ) of the molecules of an ideal gas in terms of n, R and T.
2. Since N n, this could also be written 3/2 NkT, where k is the
Boltzmann constant. What is the average k.e. of a single molecule in
terms of k and T?
3. R is the molar gas constant. Explain why k could be thought of as the
molecular gas constant.
4. What is k in terms of R and the Avogadro constant, NA ?
Thermal physics

3. Lesson focus
• Internal energy
Learning objectives
At the end of the lesson you will be able to:
• define internal energy as the sum of the random distribution
of kinetic and potential energies of the molecules in a body;
• explain that a rise in the temperature of a body leads to an
increase in its internal energy.
Thermal physics

4. Learning outcomes
All of you should be able to
• give an exam quality definition of internal energy;
• describe the internal energy of an ideal gas;
• explain how we can measure the internal energy of an ideal gas;
• explain how the internal energy of a real gas can be changed;
• solve simple problems using the 1st law of thermodynamics.
Most of you should be able to
• solve more complex problems using the 1st law of thermodynamics.
Thermal physics

5. Internal energy LOs
Key definition
The internal energy of a body is the sum of the random distribution of
kinetic and potential energies of all the molecules in the body.
To do
Make your own notes on
• the internal energy of an ideal gas
• changes in internal energy during phase changes.
LO 1: define internal energy as the sum of the random distribution of
Thermal physics kinetic and potential energies of the molecules in a body

6. The 1st law of thermodynamics LOs
This states that:
the change in the internal energy of a system (ΔU) is equal to the sum of
the energy added by heating the system (ΔQ) and the energy entering the
system through work being done on it (ΔW).
i.e. ΔU = ΔQ + ΔW
Notes:
• This law is an expression of the principle of conservation of energy
(the amount of energy in a closed system does not change).
• It is not possible to measure the total internal energy of a body, only
changes in it.
LO 2: explain that a rise in the temperature of a body
Thermal physics leads to an increase in its internal energy

7. Doing work LOs
Demonstrations
• bike pump
• hammer and lead sealed
• CO2 cylinder bike pump
• fire tube thermal film, glued
to outside of pump
To do – record your observations and explain
1. how the gas particles gain energy (bike pump).
2. Why dry ice forms – what is doing work on what? (CO2 cylinder)
Thermal physics

8. Internal energy LOs
Questions
1. The tyres of a racing bike are generally inflated to pressures of p 6
atmospheres. When he valve is sharply depressed, ice forms around it.
Why?
Thermal physics