1. Heat Transfer Heat transfer Conduction Insulation Convection Radiation

2. Three Methods of Heat Transfer CONDUCTION CONVECTION RADIATION

3. Convection Convection is the method of heat transfer in liquids and gases. The warm air above the saucepan rises as it is less dense than the cooler air above it. Less dense warm air rises through more dense cooler air

4. Convection Currents heat 1. As the water is heated it becomes less dense and rises 2. On reaching the surface the water cools in contact with the air 3. As the water cools it becomes more dense and sinks 4. A cycle develops whereby warm water rises and cool water sinks, we call this a convection current

5. Hot-air Balloons How are hot-air balloons able to rise into the sky? A burner is used to heat the air inside a hot-air balloon. As the air is heated it expands and some of the air particles move out of the balloon. This means the air inside the balloon is less dense than the air outside the balloon. The balloon will then begin to rise into the sky.

6. Refrigerators How is convection used to keep food cool in a refrigerator? The cooler cools the air next to it by conduction. This cool air is more dense than the warmer air below it so the cool air sinks down. As the cool air sinks down it cools the food in the refrigerator. Cooler By the time the air reaches the bottom of the refrigerator it is warmer than the cool air above it and so rises back up. A convection current is set up as the cool air sinks and the slightly warmer air rises. The trays in the refrigerator must have gaps in so that the cool air can circulate

7. What Causes The Wind? Different parts of the Earth’s surface are heated by different amounts by the Sun’s radiation. The air close to the Earth’s surface gains heat energy from the ground by conduction. This warmer air is less dense than the cooler air above and so rises up through the cooler air. Cooler air then moves in to take the place of the warmer air that has risen up. These movements of air are what we can feel as the wind. A wind vane

8. Convection Currents Flying dinosaurs, such as Pteranadon, are thought to have used convection currents to glide and climb.

9. Exercise 10: Refrigerators Why will a refrigerator not cool food properly if it is too packed with food? Why will it work properly if you remove some of the food?

10. Exercise 11: Hot-air Balloons Use your knowledge of physics to describe how hot-air balloons are able to rise into the sky.

11. Exercise 12: Convection 1.Which is the most dense, cool air or warm air? 2.Which is the least dense, warm air or cool air? 3.Describe a simple experiment to demonstrate what a convection current is. Cool air Warm air Place a crystal of dye at the bottom of a beaker of water. Then heat the water. As you heat the water the dye will rise up with the warm water and then sink down with the cooler water. The cycle of rising and falling water is known as a convection current.

12. Exercise 13: Rising and Sinking 1.Why is the storage tank in hot-water heating systems usually located above the boiler? 2.What causes the wind? The warm water heated by the boiler will then rise to fill the storage tank. Also, the cooler water will sink to be heated by the boiler. The air close to the Earth’s surface gains heat energy from the ground by conduction. This warmer air is less dense than the cooler air above and so rises up through the cooler air. Cooler air then moves in to take the place of the warmer air that has risen up. These movements of air are what we can feel as the wind.

13. Exercise 14: Insert the Missing Words heat 1. As the water is ______ it becomes less dense and _____ 2. On reaching the surface the water _____ in contact with the air 3. As the water cools it becomes more ______ and sinks 4. A cycle develops whereby warm water rises and cool water sinks, we call this a __________ current heated rises cools dense convection