The first law of thermodynamics

Summary and Exam Tips for The First Law of Thermodynamics

The first law of thermodynamics is a subtopic of Thermodynamics, which falls under the subject Physics in the Cambridge International A Levels curriculum. This law is fundamentally based on the conservation of energy principle, stating that energy cannot be created or destroyed, only transformed. It is mathematically expressed as $\Delta U = q + W$, where $\Delta U$ is the change in internal energy, $q$ is the heat added to the system, and $W$ is the work done on the system.

When a gas expands, it does work on its surroundings, which is significant in applications like steam engines. The work done by a gas at constant pressure is given by $W = p \Delta V$, where $p$ is the external pressure and $\Delta V$ is the change in volume. In a cylinder with a movable piston, the force exerted by the gas is $F = p \times A$, leading to the derivation of the work equation.

During expansion, the gas does work ($-W$), decreasing its internal energy, while during compression, work is done on the gas ($+W$), increasing its internal energy. Graphically, constant pressure processes are represented by horizontal lines, and constant volume processes by vertical lines on a $p$ vs. $V$ graph.

Exam Tips

• Understand Key Equations: Be familiar with the equation $\Delta U = q + W$ and how it applies to different scenarios of expansion and compression.
• Graph Interpretation: Practice interpreting $p$ vs. $V$ graphs, focusing on the area under the curve for work done.
• Conceptual Clarity: Ensure you understand the concepts of energy conservation and how work and heat affect internal energy.
• Real-World Applications: Relate the first law to practical examples like steam engines to better grasp its significance.
• Problem-Solving Practice: Solve problems involving the calculation of work done by or on a gas, especially under constant pressure or volume conditions.