O-LevelPhysicsThermal Physics5054/21 · May/June 2018 · Q1015 Marks

Describe two differences between boiling and evaporation. 1. 2.

(a)(i)Describe two differences between boiling and evaporation. 1. 2.[4]
(a)(ii)Explain, using ideas about molecules, why thermal energy is needed to boil a liquid.[2]
(b)(i)In one type of bathroom shower, cold water passes through a metal pipe which contains an electric heater. The cold water is heated and emerges from the shower head. The temperature of the cold water before heating is measured and the hot water emerging from the shower in 1.0 minute is collected in a container. Measurements and other data are: temperature of water before heating = 16°C temperature of water after heating = 37 °C volume of water collected in 1.0 minute = 4.6×10⁻³ m³ specific heat capacity of water = 4200J/(kg°C) density of water = 1000kg/m³ Calculate the mass of water leaving the shower in 1.0s. mass = ................................................................[2][2]
(b)(ii)Calculate the thermal energy (heat) gained by the water in 1.0s. energy = ................................................................[3][3]
(b)(iii)To calculate the efficiency of the shower, the value in (b)(ii) and the value of one other quantity are needed. 1. State what other quantity is needed. 2. Describe how these two values are used to calculate the efficiency of the shower.[2]
(b)(iv)The metal pipe that contains the electric heater is earthed. Explain why this is necessary.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[B1] Pair 1 boiling occurs at one temperature / constant temperature evaporation occurs at all / any temperature
[B1] Pair 2 boiling occurs within / throughout liquid or causes bubbles evaporation occurs at surface
[B1] Pair 3 boiling requires a heat source / affected by rate of heating evaporation causes cooling / affected by wind etc.
[B1] Pair 4 temperature remains constant during boiling evaporation temperature drops / liquid cools
[B1] (thermal energy) used to break bonds or work done against intermolecular forces
[B1] molecules move further apart or P.E. increases or (molecules) push back the atmosphere
[C1] (M=) DV in any form algebraic or numerical
[A1] 0.077 kg/s or 0.077 kg
[C1] (E=) mcT in any form algebraic or numerical
[C1] 37 – 16 or 21 (°C) seen
[A1] 6800 J
[B1] input / electrical energy or power mentioned
[B1] (efficiency =) output (energy / power) ÷ input (energy / power)
[B1] avoid electrocution / shock / current through body or avoid pipe being live or to allow a current / charges to earth / ground or fuse blows to stop electrocution
[B1] when live / heater touches metal / water

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