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Heat Questions - GCSE Level

Q17. The diagram shows a fridge with a freezer compartment.

The temperature of the air inside the freezer compartment is –5 °C

(a) The air inside the fridge forms a convection current when the fridge door is closed. Explain why.

Air near freezer compartment at the top of the fridge is cooled (or loses energy). The cool air is denser than the warm air because the particles are closer together than in warmer air. So the cooler air falls and less dense air at bottom of the fridge is displaced and moves upwards/rises to be cooled by the freezer compartment. The process is then repeated.

Be careful not to say on cooling 'the particles get smaller' or 'condense' - they don't - the air becomes denser because the air molecules are more tightly packed.

Also, never say 'heat rises' - it doesn't! Warm fluid rises because of density difference no heat energy's wanting to 'go up'!

[4 marks]

(b) The table below shows information about four fridges.

Fridge
Volume in litres
Energy used in one year in kWh
A
250
300
B
375
480
C
500
630
D
750
750

 

A householder concludes that the energy used in one year is directly proportional to the volume of the fridge.

Explain why her conclusion is not correct. Use data from the table in your answer.

If they were proportional then if volume was doubled, energy use would also double.

or volume ÷ energy would be a constant ratio.

This is not the case as 500 is 2×250 but 630 is not 2×300 or similar use of data.

[2 marks]

(c) New fridges are more efficient than fridges made twenty years ago.

Give one advantage and one disadvantage of replacing an old fridge with a new fridge.

Ignore the cost of buying a new fridge.

Advantage on from:

reduces emissions into atmosphere

lower input power or uses less energy or wastes less energy

costs less to run

Disadvantage one from:

land fill

energy waste in production

cost or difficulty of disposal

transport costs

[2 marks]

(8 marks total)