Paper 3 Thermal Physics Route Answers

Q1 Integrated thermal explanation

1. Hot water and gas are initially at different temperatures, so energy is transferred from the hotter water to the cooler gas until thermal equilibrium is approached. [2]
2. At constant volume, $p\propto T$, so $p_2/p_1=330/290=1.14$. [2]
3. Higher temperature means greater mean molecular kinetic energy. Molecules hit the walls more often and/or with larger momentum changes, increasing force per unit area. [3]
4. No boundary work is done because the container volume is fixed. [1]
5. Energy is supplied by heating and no work is done by expansion, so internal energy increases. [2]

Q2 Thermal route comparison

1. Latent heat is not temperature rise, and work is an energy transfer by macroscopic force/displacement, not ‘heat’. [2]
2. $E=mL$ is used for change of state at constant temperature for mass $m$. [2]
3. Heat and work describe energy crossing a system boundary by different mechanisms. Their values depend on the process path, unlike internal energy which depends on state. [3]