Summary
The First Law of Thermodynamics states that the change in internal energy of a system is equal to the heat added to the system plus the work done on the system. It is based on the principle of conservation of energy.
- Work Done by a Gas — Work done when a gas changes volume at constant pressure is W = p ΔV.
Example: In a steam engine, expanding steam pushes a piston to turn the engine. - Gas in a Cylinder — The force exerted by gas on a piston is F = p × A, where A is the cross-sectional area.
Example: A gas in a cylinder with a movable piston. - Expansion and Compression — Expansion does work by the gas, decreasing internal energy; compression does work on the gas, increasing internal energy.
Example: Adiabatic expansion and compression in a piston-cylinder system. - Constant Pressure — In a p vs. V graph, a horizontal line indicates constant pressure, with the area under the line representing work done.
Example: Isothermal process with constant pressure. - Constant Volume — In a p vs. V graph, a vertical line indicates constant volume, with no work done.
Example: Isochoric process with constant volume.
Exam Tips
Key Definitions to Remember
- Work done by a gas: W = p ΔV
- First Law of Thermodynamics: ΔU = q + W
Common Confusions
- Confusing work done by the gas with work done on the gas
- Misinterpreting the signs of ΔU, q, and W in the First Law
Typical Exam Questions
- What happens to internal energy when a gas expands?
Internal energy decreases as work is done by the gas. - How is work calculated when a gas is compressed at constant pressure?
Work is calculated as W = p ΔV, where ΔV is the change in volume. - What does a vertical line in a p vs. V graph indicate?
It indicates a constant volume process with no work done.
What Examiners Usually Test
- Understanding of the First Law of Thermodynamics
- Ability to calculate work done during expansion and compression
- Interpretation of p vs. V graphs for constant pressure and volume processes