Convection

Learning objectives

Demonstration We are going to do an experiment to see how heat (thermal energy) moves through a liquid. We will put potassium permanganate in the bottom of a beaker. We are going to heat the beaker from below and see what happens.

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How does the thermal energy move? At the start, the potassium permanganate is at the bottom.

As we heat the water, convection currents form. Convection currents happen because as the water heats up the particles spread out and it gets lighter.

Because the hot water is lighter, it rises up above the cooler water which is heavier. As the water moves, it is taking the potassium permanganate with it.

When the water gets to the top, it is no longer being heated. This means it cools down and becomes heavier, falling to the bottom again.

Convection Convection happens when particles with a lot of thermal energy in a liquid or gas move, and take the place of particles with less thermal energy. The heat source causes the fluid to expand and rise upwards. The hot fluid gives up its heat energy to cooler parts of the fluid. A convection current of warm fluid is set up. A roomful of air can be heated by convection.

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Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated than they do when they are cold. As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size. The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up.

Convection Thermal energy is transferred from hot places to cold places by convection.

Lava Lamps!

Task. Have a go at completing the descriptions of what happened when the food colouring and water was heated together.

Finally… Design a poster that compares convection and conduction. Show how the thermal energy is transferred to the particles for each.

Draw diagrams showing what happened as the beaker was heated and convection currents formed. Write underneath what is happening. Try and mention particles in your answer.