1. 1
   An electric room heater is placed near the floor of a room. After some time, the air near the ceiling becomes noticeably warmer than it was before, even though the heater itself is far below.

   (a)

   Which process mainly explains how thermal energy reaches the air near the ceiling?

   [1 mark]

   • AThe air near the ceiling was warmed only by direct sunlight through the windows
   • BConduction through the air molecules directly from the heater to the ceiling
   • CConvection currents carry warm, less dense air upward from the heater
   • DSound waves from the heater transfer thermal energy to the ceiling

   (b)

   Why does the air warmed directly above the heater rise upwards through the room?

   [1 mark]

   • ABecause warm air has a lower specific heat capacity than cool air
   • BBecause the warmed air becomes more dense than the surrounding cooler air
   • CBecause warm air always moves towards a source of light
   • DBecause the warmed air becomes less dense than the surrounding cooler air

   (c)

   Describe the complete cycle of a convection current that is set up in the room, from the air being heated near the heater to it eventually returning to be heated again.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A pan of water is heated on a stove from below. A student adds a few grains of coloured dye to the bottom of the pan and observes the dye rising and circulating through the water as it heats up.

   (a)

   What does this circulation of dye demonstrate?

   [1 mark]

   • ARadiation being absorbed by the dye particles
   • BConduction through the metal pan only
   • CConvection currents within the liquid water
   • DThe dye particles evaporating from the water

   (b)

   Why is it important that the water in this experiment is heated from below rather than from the side or the top?

   [1 mark]

   • ASo that the pan itself does not become hot
   • BSo that the water can be heated by conduction only, without any convection occurring
   • CSo that the dye dissolves more quickly in the water
   • DSo that the heated, less dense water at the bottom can rise freely, setting up a clear convection current

   (c)

   Explain, in terms of density and particle motion, why the water at the bottom of the pan rises when it is heated, carrying the coloured dye with it.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A coastal town experiences a noticeable sea breeze blowing from the sea towards the land during a sunny afternoon. During the day, the land heats up faster than the sea.

   (a)

   (a) Explain, in terms of convection, why a breeze blows from the sea towards the land during the day.

   [3 marks]

   (b)

   In the evening, after sunset, the land cools down faster than the sea. Explain fully what direction of breeze would now be expected, and describe the full convection cycle involving both the air above the land and the air above the sea.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An engineer is designing a domestic central heating system in which a radiator is deliberately mounted low down on a wall, near the floor of a room, rather than high up near the ceiling.

   (a)

   Explain fully, using the concept of convection currents and density changes, why mounting the radiator near the floor is a more effective design for heating the whole room than mounting an identical radiator near the ceiling, describing the complete convection cycle that would be set up in each case.

   [6 marks]

   (b)

   The same engineer is now designing a refrigerator and must decide whether to mount the cooling element near the top of the fridge's interior or near the bottom. Explain fully, using the concept of convection currents and density changes, why mounting the cooling element near the top of the fridge is the more effective design for cooling the whole interior, contrasting this with the reasoning used for the radiator in the room in part (a).

   [6 marks]

   Total for question 4: 12 marks

End of questions