What is specific latent heat?

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

Why does temperature stay constant during a change of state?

The energy supplied changes the arrangement of the molecules rather than raising their temperature.

Which is larger for water: latent heat of fusion or of vaporisation?

Vaporisation, 2.26 × 10⁶ J kg⁻¹ against 3.34 × 10⁵ J kg⁻¹.

Why is a measured L too large when energy is lost to the surroundings?

Less mass changes state than the supplied energy should give, so E/m is too large.

How is a thermistor circuit calibrated as a thermostat?

Record p.d. against temperature in a water bath, then read off the temperature for the switching p.d.

Why stir the water bath when calibrating?

So the temperature is uniform and the thermistor and thermometer agree.

Specific heat capacity and specific latent heatEdexcel International A Level Physics: Flashcards

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What is specific heat capacity?

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What is specific heat capacity?: The energy needed to raise the temperature of 1 kg of a substance by 1 K without a change of state.
State the equation for energy transferred in heating.: E = mcΔθ
What is the unit of specific heat capacity?: J kg⁻¹ K⁻¹
What is specific latent heat?: The energy needed to change the state of 1 kg of a substance without a change in temperature.
State the equation for a change of state.: E = Lm
What is the unit of specific latent heat?: J kg⁻¹
Why does temperature stay constant during a change of state?: The energy supplied changes the arrangement of the molecules rather than raising their temperature.
Which is larger for water: latent heat of fusion or of vaporisation?: Vaporisation, 2.26 × 10⁶ J kg⁻¹ against 3.34 × 10⁵ J kg⁻¹.
How is electrical energy found for a heater?: E = Pt = VIt
Why is a measured L too large when energy is lost to the surroundings?: Less mass changes state than the supplied energy should give, so E/m is too large.
What happens to an NTC thermistor's resistance as temperature rises?: It falls.
How is a thermistor circuit calibrated as a thermostat?: Record p.d. against temperature in a water bath, then read off the temperature for the switching p.d.
Why stir the water bath when calibrating?: So the temperature is uniform and the thermistor and thermometer agree.

Exam questions on Specific heat capacity and specific latent heat

A student heats 0.50 kg of water in a well-insulated beaker using a 60 W immersion heater. The specific heat capacity of water is 4180 J kg⁻¹ K⁻¹. The heater is switched on for 520 s.
The student measures a temperature rise of 12 K, not 15 K. Calculate the percentage of the electrical energy supplied that was transferred to the water.2 marks
A catering company uses ice to keep food chilled and steam to cook it. The specific latent heat of fusion of ice is 3.34 × 10⁵ J kg⁻¹ and the specific latent heat of vaporisation of water is 2.26 × 10⁶ J kg⁻¹. A cool box contains 0.30 kg of ice at 0 °C, which absorbs energy from the food at an average rate of 20 W.
Calculate the energy released when 0.040 kg of steam at 100 °C condenses to water at 100 °C.2 marks
A student determines the specific latent heat of vaporisation of water. A beaker of water is placed in an insulated container on a top-pan balance, and a 12.0 V electric heater with an ammeter in series is lowered into the water. Once the water is boiling steadily, the student records the balance reading and switches the heater on for a measured time, then records the balance reading again.
The ammeter reads 4.20 A and the heater is on for 300 s. The balance reading falls by 6.0 g. Calculate the specific latent heat of vaporisation of water from these results.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).