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Energy Transfer Questions - GCSE Level

Q4. The diagram shows potatoes being baked in a gas oven. Each potato has a metal skewer pushed through it.

(a) Explain how heat is transferred by the process of convection from the gas flame at the bottom of the oven to the potatoes at the top of the oven. (3 marks)

any three from:

The (air) particles / molecules / atoms near to the flame gain kinetic energy from the flame, that makes them move faster.They therefore move further apart OR the air near the flame expands - as it becomes warmer, becoming less densethan the cooler air surrounding it. The warm/hot air rises taking the heat energy to the potaoes at the top of the oven.

Remember warm air rise not 'heat' - never say 'heat rises'.

Do not accept 'move more' or 'move with a bigger amplitude' or 'vibrate more'. With a gas translational energy is so much more significant than vibrational energy.
If you say the air particles expand you get no marks here - they don't expand - they move further apart - see notes on convection

If credit is to be given for answers in terms of 'particles' it must be clear they are 'air particles' not the (fuel) 'gas particles'.

(b) The metal skewers help the potatoes to cook by transferring heat to the inside of the potatoes. By what method is heat transferred through a metal skewer? (1 mark)

Conduction

(c) When the potatoes are taken from the oven, they start to cool down. Suggest one factor that will affect how fast a potato cools down. (1 mark)

Any one from:

• temperature (not 'heat') of the potato

• temperature (not 'heat') of (or how hot is) the surroundings / room / surface / atmosphere

• size / mass / weight / volume of the potato

• shape of the potato

• surface area of the potato

• nature of the surface of the potato

• type of surface it is placed on

• if is is placed in a draught or not

• type of potato

• whether the skewers are left in or taken out

(d) If the potatoes need to be kept hot, they may be wrapped in shiny aluminium foil. Why does this help to keep the potatoes hot? (1 mark)

The foil reflects heat back towards potato OR It reduces heat loss by radiation - the foil is a poor emitter (of heat radiation).

'It reduces heat loss' is insufficient for a mark.

'It reflects hot air' is incorrect (it is the infra red energy that is reflected).

Heat is 'trapped by the foil' is incorrect - heat is never 'trapped' - warm air can be trapped but not heat energy.

(6 marks total)