State the first law of thermodynamics.

\Delta U = Q + W: the internal energy increases by the energy transferred to it by heating plus the work done on it.

Define specific heat capacity.

The energy required to raise the temperature of 1 kg of a substance by 1 K.

Define specific latent heat.

The energy needed to change the state of 1 kg of a substance without a change in temperature.

What happens to the molecules' energy during melting?

Their potential energy increases while the mean kinetic energy stays the same.

Why does temperature stay constant during a change of state?

Temperature depends on the mean kinetic energy, which is unchanged.

Why is the latent heat of vaporisation greater than of fusion?

The molecules must be completely separated against the attractive forces, not just loosened.

State the equation for continuous flow.

P = \dot{m}c\Delta\theta (plus L if there are losses).

How does the continuous-flow method eliminate energy loss?

Use two flow rates with the same temperature rise, so L is the same and cancels in P1 - P2 = (\dot{m}1 - \dot{m}2)c\Delta\theta.

Thermal energy transferAQA A-Level Physics: Flashcards

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Define internal energy.

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Define internal energy.: The sum of the randomly distributed kinetic and potential energies of the particles in a body.
State the first law of thermodynamics.: $\Delta U = Q + W$ : the internal energy increases by the energy transferred to it by heating plus the work done on it.
State the equation for a change of temperature.: $Q = mc\Delta\theta$
Define specific heat capacity.: The energy required to raise the temperature of 1 kg of a substance by 1 K.
State the equation for a change of state.: $Q = ml$
Define specific latent heat.: The energy needed to change the state of 1 kg of a substance without a change in temperature.
What happens to the molecules' energy during melting?: Their potential energy increases while the mean kinetic energy stays the same.
Why does temperature stay constant during a change of state?: Temperature depends on the mean kinetic energy, which is unchanged.
What are the units of specific heat capacity?: J kg⁻¹ K⁻¹
Why is the latent heat of vaporisation greater than of fusion?: The molecules must be completely separated against the attractive forces, not just loosened.
State the equation for continuous flow.: $P = \dot{m}c\Delta\theta$ (plus $L$ if there are losses).
How does the continuous-flow method eliminate energy loss?: Use two flow rates with the same temperature rise, so $L$ is the same and cancels in $P_1 - P_2 = (\dot{m}_1 - \dot{m}_2)c\Delta\theta$ .

Exam questions on Thermal energy transfer

An electric kettle has a heating element of power 2.2 kW. It contains 2.0 kg of water at 20 °C, which is heated to 100 °C. The specific heat capacity of water is 4180 J kg⁻¹ K⁻¹.
In practice the kettle takes longer than the time calculated in part (b). Suggest two reasons why.2 marks
A gas is trapped in a cylinder by a piston. The piston is pushed in quickly, so that 450 J of work is done on the gas, and during the process 120 J of energy is transferred from the gas to its surroundings by heating.
Use the idea of internal energy to explain why the temperature of the gas rises when it is compressed quickly.2 marks
A beaker contains 0.250 kg of ice at 0 °C. It is heated by a 150 W immersion heater, and all of the energy from the heater is transferred to the ice. The specific latent heat of fusion of ice is 3.34 × 10⁵ J kg⁻¹.
The temperature of the ice remains at 0 °C while it melts, even though energy is being supplied. Explain this in terms of the energies of the molecules.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).