1st Law of Thermodynamics

Objectives

Lesson outcomes

Describe internal energy as the sum of random molecular kinetic and potential energies.
Relate a temperature rise to an increase in internal energy.
Use W = p delta V for gas work at constant pressure and distinguish work done by and on the gas.
Use delta U = q + W to account for heating and work done on a system.

Syllabus

CIE 9702 syllabus points

4 linked

16.1.1 understand that internal energy is determined by the state of the system and that it can be expressed as the sum of a random distribution of kinetic and potential energies associated with the molecules of a system
16.1.2 relate a rise in temperature of an object to an increase in its internal energy
16.2.1 recall and use W = p∆V for the work done when the volume of a gas changes at constant pressure and understand the difference between the work done by the gas and the work done on the gas
16.2.2 recall and use the first law of thermodynamics ∆U = q + W expressed in terms of the increase in internal energy, the heating of the system (energy transferred to the system by heating) and the work done on the system

Lesson Notes

Student guidance and lesson notes

Overview

This lesson brings the thermal physics and gas models together. You describe internal energy, calculate work done during volume changes at constant pressure, and use the first law of thermodynamics to track energy transfers into or out of a gas system.

What You Need to Know

Internal energy is the sum of the random molecular kinetic and potential energies in a system.
A temperature rise means the internal energy of an object increases.
At constant pressure, gas work is W = p delta V.
Work done by the gas and work done on the gas have opposite signs, so define the system clearly.
The first law is delta U = q + W, where q is heating of the system and W is work done on the system.

How to Work Through It

Identify the system and decide whether energy is entering or leaving by heating.
Decide whether the gas is expanding or being compressed.
Calculate any constant-pressure work using W = p delta V.
Substitute signs carefully into delta U = q + W and interpret the result.

Check Your Understanding

What happens to internal energy when temperature increases?
If a gas expands at constant pressure, is work done by the gas or on the gas?
What does a negative value of delta U tell you about the system?

Common Mistakes

Treating q, W, and delta U as always positive quantities.
Mixing up work done by the gas with work done on the gas.
Forgetting that W = p delta V only applies for constant pressure in this course context.
Describing internal energy as only kinetic energy and leaving out molecular potential energy.

Next Steps

Practise sign conventions until each term has a clear physical meaning.
Use the revision lesson to connect temperature, ideal gases, kinetic theory, and thermodynamics.