Convection How can heat energy circulate through liquids and gases? What uses are there of convection?
Density Density is a measure of how heavy something is for its volume. e.g. lead is dense and has a relatively large mass in a small volume. Polystyrene is not dense – it has a small mass for quite a large volume.
What happens to the spacing of particles as we heat a gas or liquid? They move further apart we have a less dense gas because the volume is bigger.
Convection – example in action
What do you think happens to gas if we heat it? As the gas is heated, the particles move more _______. This causes the particles to move _____ apart. This means that the volume of the gas ________. This causes the density of the gas to decrease. This causes the gas to rise.
What happens to gas if we heat it? As the gas is heated, the particles move more quickly. This causes the particles to move further apart. This means that the volume of the gas increases. This causes the density of the gas to decrease. This causes the gas to rise.
Convection currents We get a circulation of gas or liquid (fluid.) Warm fluids expand and rise. At the top they contract and sink. The whole process continues to cycle in this way.
Example – here we have a sea breeze.
Sea breeze During they day, the land is warmer than the sea. The warm air over land expands and rises. It cools over the sea and returns to the land. This provides a refreshing sea breeze which is very welcome on a hot day.
Convection heater Please explain in 3 sentences how this heater is able to warm the whole room.
Convection heater The air near the heater is warmed, expands and rises. At the top of the room the air cools, contracts and becomes more dense. The air then sinks to the ground and the whole process continually repeats.
Convection in a liquid. Please explain in 3 or 4 sentences how the liquid in this saucepan becomes hot.
Convection in a liquid. Heat is conducted to the water at the bottom. This warms the liquid near the bottom which expands, becomes less dense and rises. As it reaches the top, the liquid cools, contracts and becomes more dense and so sinks. The process repeats in a cycle.
Convection Cool fluids contract, become more dense and sink. When a fluid is heated it expands. The process repeats in a cycle continuously. Warm fluids expand and become less dense. Convection is to do with expansion and contraction.
Convection cannot happen in solid. Use your knowledge of particles to explain why heat energy cannot transfer through a solid by convection but it can in a liquid or gas.
Why convection does not occur in solids. In a solid the particles (atoms) are very close together and are in fixed positions. For convection to occur, the particles must move further apart so the density can decrease. This cannot happen in solids and so convection cannot occur.