Retrieval Practice
Thermal Energy Transfer — AQA A-Level Physics
Q1. Define internal energy.
The sum of the randomly distributed kinetic and potential energies of all the particles in a body.
Q2. State two ways to increase the internal energy of a system.
- Do work on the system (e.g. compress a gas).
- Transfer thermal energy to the system (heat it).
Q3. State the first law of thermodynamics.
The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it.
Q4. Define specific heat capacity.
The amount of thermal energy required to raise the temperature of 1 kg of a substance by 1 K (or 1 degree C) without a change of state.
Q5. Write the equation for thermal energy change.
delta Q = mc delta theta, where m = mass, c = specific heat capacity, delta theta = temperature change.
Q6. State the specific heat capacity of water.
4200 J kg^{-1} \(K^{-1}\).
Q7. Explain why the continuous flow method eliminates heat losses.
- Two experiments are run with the same temperature rise but different flow rates and power.
- Heat loss to surroundings is assumed constant in both, so subtracting the two energy equations cancels E_lost.
Q8. Write the continuous flow equation for specific heat capacity.
c = (Q_2 - Q_1) / ((m_2 - m_1) delta theta), where Q = IVt for each run.
Q9. Define specific latent heat of fusion.
The thermal energy required to convert 1 kg of solid to liquid with no change in temperature.
Q10. Define specific latent heat of vaporisation.
The thermal energy required to convert 1 kg of liquid to gas with no change in temperature.
Q11. Write the equation for energy during a change of state.
Q = mL, where m = mass and L = specific latent heat.
Q12. Why is the latent heat of vaporisation much larger than the latent heat of fusion?
- Boiling requires particles to be completely separated against all intermolecular forces, plus work must be done against atmospheric pressure.
- Melting only requires particles to move just far enough apart to flow past each other.
Q13. What happens to kinetic and potential energy during a change of state?
- Kinetic energy stays constant (temperature does not change).
- Potential energy changes because particle separation changes.
Q14. How do you approach a thermal equilibrium calculation where ice is added to a hot drink?
- Energy balance: heat ice to 0 degrees C (mc delta theta) + melt ice (mL) + heat meltwater to T_f (mc delta theta) = energy lost by hot drink (mc delta theta).
- Solve for T_f.