O-LevelPhysicsThermal Physics5054/21 · Oct/Nov 2014 · Q1015 Marks

The casing of an electric kettle is made of white plastic. Fig. 10.1 shows the heating element positioned in the base of the kettle. [Figure 10.1]

(a)(i)The heating element supplies thermal energy to the water at the bottom of the kettle. Describe and explain how the thermal energy is transferred throughout the water.[3]
(a)(ii)Explain why a kettle with its heating element in the water at the top of the kettle does not heat the water uniformly.[1]
(b)(i)The kettle is powered by a 230V supply. It is switched on for 3.5 minutes and there is a current of 9.6 A in the heating element. Calculate the thermal energy produced in the heating element in this time.[2]
(b)(ii)The kettle contains 1.6kg of water that was at an initial temperature of 22°C. The specific heat capacity of water is 4200J/(kg°C). Calculate the maximum possible temperature of the water.[3]
(b)(iii)Suggest one reason why the temperature of the water, after 3.5 minutes, is less than the value calculated in (b)(ii).[1]
(c)(i)Explain one advantage of using plastic for the casing of a kettle,[1]
(c)(ii)choosing white as the colour for the outside of the casing.[1]
(d)(i)The kettle is switched on again and the water reaches its boiling point. It starts to boil and the kettle remains switched on. State the meaning of boiling point.[1]
(d)(ii)Explain, in terms of molecules, what happens to the thermal energy that is supplied when the water is boiling.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[3] (a) (i) heated/hot water expands or density of heated/hot water decreases
(heated/hot water) rises
convection (current)/circulation set up or (heated/hot water) rises and cold water sinks
[1] (a) (ii) convection transfers heat upwards or less dense/heated/hot water (already) at top
[2] (b) (i) (Q = ) VIt or 230 × 9.6 × 3.5 or 230 × 9.6 × 3.5 × 60 or 7728
4.6(368) × 10⁵ J
[3] (b) (ii) (∆T = ) Q/ mc or 4.6(3680) × 10⁵/1.6 × 4200
69 (°C)
91 °C
[1] (b) (iii) evaporation or thermal energy/heat in plastic casing/element/surroundings (i.e. air or environment)
[1] (c) (i) poor conductor (heat or electricity) or less heat lost/cooler to touch or less risk of shock
[1] (c) (ii) poor emitter and less heat lost/of radiation/IR (not poor absorber)
[1] (d) (i) temperature where liquid and vapour/gas coexist or where liquid (not substance) boils
(at atmospheric pressure)(allow becomes vapour/gas)
[2] (d) (ii) (work done) against/overcoming forces between molecules or molecules gain P.E. (ignore K.E. increases)
changes to P.E./molecules separate

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