Question

Explain why:
a) The freezer compartment in a fridge is placed at the top.
b) A fridge doesn't work properly if the food is too tightly packed inside.
c) A radiator quickly warms all air in a room, even though air is a very poor conductor of heat.
d) Warm water rises when surrounded by cooler water.

Answer 1

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Answer 2

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1)In a refrigerator, the freezer at the top of the unit contains the evaporator, which cools the air. Cold air is more dense than warmer air, so it sinks to the bottom of the freezer. There the cold air flows through vents (usually inside the meat keeper) into the refrigerator cabinet where it sinks to the bottom of the fridge (the crisper). Air warmed by the contents of the fridge rises as the cold air displaces it, and the 'warm' (only a few degrees difference) air enters into the freezer compartment where the cycle begins again.

Hence, the freezer in a refrigerator is fitted near the top so that it can cool the whole interior by setting up convection currents.

2)Because warm always rises and cooler air sinks. ... In time, all the air will be cooled. Explain why a refrigerator does not work properly if the food is too tightly packed inside. Because a tightly packed food will prevent the circulation of warm and cool air.

3)The metal radiator heats the air touching it by conduction. The warm air is less dense than the cold air around it so the warm air rises by convection. One cubic metre of air has a mass of only 1.1kg so the heat capacity of the air in the room is very low. It takes very little heat emergy to heat up a roomful of air

4)When fluids (liquids and gases) are heated, they expand and therefore become less dense. Any object or substance that is less dense than a fluid will float in that fluid, so hot water rises (floats) in colder water. When fluids are cooled, they contract and therefore become more dense.