Energy Transfers & Particle MotionOxford AQA IGCSE Physics: Subtopic test

1. 1
   An electric radiator has metal cooling fins fitted to the rear panel to help warm a room more quickly.

   (a)

   Which statement best explains why the fins increase the rate at which energy is transferred to the surrounding air?

   [1 mark]

   • AThe fins increase the surface area in contact with the air, allowing more particle collisions to transfer energy per second
   • BThe fins reduce the surface area in contact with the air, concentrating the heat in one place
   • CThe fins are made of an electrical insulator, which speeds up energy transfer
   • DThe fins prevent any convection currents from forming in the room

   (b)

   Once the radiator has warmed the air near it, this warm air rises to the ceiling and cooler air sinks down to replace it, setting up a circulating current around the room. Which process is being described?

   [1 mark]

   • AConduction
   • BConvection
   • CRadiation
   • DSublimation

   (c)

   Explain, in terms of particles, why the heated air near the radiator rises and sets up a convection current in the room.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An elephant living in a hot climate has very large ears with a wide surface area compared to its body size, while a arctic fox living in a cold climate has small, compact ears.

   (a)

   Which statement explains why the elephant's large ears help it to lose excess body heat?

   [1 mark]

   • AThe large ears act as insulators, trapping heat inside the elephant's body
   • BThe large ears reduce the surface area available for heat loss, keeping the elephant warm
   • CThe large surface area of the ears increases the rate at which energy is transferred from the elephant to its surroundings
   • DThe size of an animal's ears has no effect on the rate of energy transfer

   (b)

   On a particularly hot day, the difference between the elephant's body temperature and the surrounding air temperature becomes smaller than usual. What effect does this have on the rate at which the elephant loses energy to its surroundings by heating?

   [1 mark]

   • AEnergy transfer stops completely
   • BThe rate of energy transfer increases
   • CThe rate of energy transfer stays exactly the same
   • DThe rate of energy transfer decreases

   (c)

   Explain why the arctic fox's small, compact ears are an advantage for an animal that lives in a very cold climate.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A bimetallic strip made of brass and steel, joined firmly together along their length, is used inside a thermostat to switch a heater on and off.

   (a)

   Explain, in terms of particle behaviour, why brass expands more than steel for the same rise in temperature, and describe what happens to the shape of the strip as a result when it is heated.

   [3 marks]

   (b)

   Explain how the bending of the bimetallic strip described above allows the thermostat to switch the heater off once a set temperature has been reached, and describe one everyday hazard caused by the thermal expansion of a material.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   On a warm, still afternoon, a shallow dish of water is left in a garden in direct sunlight, and its level gradually falls even though the water never reaches its boiling point; nearby, a jug of cold lemonade develops droplets of moisture on its outside surface as the afternoon goes on.

   (a)

   A shallow dish of water is left in direct sunlight on a warm, breezy day, and its water level gradually falls even though the water never reaches its boiling point. Using kinetic theory, explain how evaporation occurs at temperatures below the boiling point, and explain how increasing the air temperature, increasing the wind speed, and increasing the surface area of the dish would each affect the rate of evaporation.

   [6 marks]

   (b)

   Water vapour in the air near the cold jug of lemonade condenses to form droplets on the outside of the jug. Using kinetic theory, explain why condensation occurs on the cold jug, and explain why this would happen more readily on a humid afternoon than on a dry afternoon.

   [6 marks]

   Total for question 4: 12 marks

End of questions