O-LevelPhysicsThermal Physics5054/22 · Oct/Nov 2022 · Q715 Marks

Copper is sometimes chosen as a material to make pans for heating liquids. A copper pan contains water at room temperature.

(a)The top surface of the water in the pan cools as water evaporates. A convection current causes the rest of the water to cool. Explain how this convection current is produced.[3]
(b)(i)The copper pan is now placed on a hotplate that is switched on. Name the process by which thermal energy passes through the base of the pan.[1]
(b)(ii)Copper remains a solid when heated up to 1000°C. State one other reason why it is sensible to make a pan out of copper.[1]
(c)(i)The water in the pan is heated from 17°C until it starts to boil at 100°C. The specific heat capacity of water is 4200 J/(kg°C) and the pan contains 750 g of water. Calculate the energy needed to increase the temperature of the water from 17°C to 100°C.[3]
(c)(ii)State two ways in which boiling differs from evaporation.[2]
(c)(iii)At 100°C the temperature of the water stops increasing, even though energy is still being transferred to the water. Explain, in terms of molecules, why thermal energy is needed to change the state of the water from liquid to gas.[2]
(c)(iv)The specific latent heat of vaporisation of water is 2.3 × 10⁶ J/kg. Thermal energy is transferred to the boiling water at a rate of 1300 J/s. Calculate the mass of water that boils away in the 10 minutes immediately after the water reaches 100°C.[3]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[B1] (surface) water contracts (as it cools) or molecules (at surface) become closer
[B1] (surface / cold) water / its density increases or density greater than rest of water
[B1] surface / cold / dense water sinks or deeper / warm / less dense water rises / moves upwards
[B1] conduction and no other mechanism of thermal transfer
[B1] it is a good conductor (of thermal energy)
[C1] 0.75 or 83 seen or 100 – 17 seen
[C1] (E =) mcΔT or 0.75 × 4200 × 83 or 750 × 4200 × 83 or 2.6 × 10⁸
[A1] 2.6 x 10⁵ J
[B1] occurs (only) at a specific temperature / boiling point
[B1] occurs throughout the liquid or produces bubbles or does not produce cooling or does not depend on draught / surface area
[B1] does work against the forces / breaks the bonds (between the molecules) or increases the potential energy (of the molecules)
[B1] so that / as the molecules move apart / separate (and become gas molecules)
[C1] (m =) E/ lv or Pt/ lv or 10 × 60 or 600 or 7.8 × 10⁵
[C1] 1300 × 60 × 10/2.3 × 10⁶ or 1300 × 10/2.3 × 10⁶
[A1] 0.34 kg

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