2. HEAT ENERGY What is heat? A form of electromagnetic energy, measured in joules or calories. Molecules move about more or vibrate and take up more room if heated—which is why things expand if heated It is also why substances change from: solids liquidsgaseswhen heated.

3. Temperature and Heat The temperature of an object indicates the average kinetic energy of its molecules. Temperature is measured in degrees Celsius - °C Temperature is NOT the same as heat energy although the two quantities are related. e.g. a beaker of water at 60 °C is hotter than a bath of water at 40 °C BUT the bath contains more joules of heat energy

4. Heating and Cooling If an object has become hotter, it means that it has gained heat energy. If an object cools down, it means it has lost heat energy If an object cools down to absolute zero, the molecules have ceased to move at all.

5. 0 th Law of Thermodynamics Heat energy always moves from: WARMER object COOLER object A spoon at 20 °C in a cup of water at 30°C - gains heat energy and heats up – its temperature rises A spoon at 20 °C in a cup of water at 10°C loses heat energy and cools down – its temperature falls.

6. Heat energy movement Energy transfer: Conduction Convection Radiation

7. Conduction Heat is transferred through a material by being passed from one molecule to the next. No molecules move down the material. Heat energy is passed on from molecule to molecule. Molecules at the warm end move faster and this then causes the next molecules to move faster and so on in a domino effect. In this way heat in an object travels from: the HOT end the cold end

8. CONDUCTION heat transfer Occurs by the molecules hitting each other and so transferring energy. Surprisingly the electrons are the key agents. Can happen in solids, liquids and gases, Happens best in solids-molecules are very close together Conduction does not occur very quickly in liquids or gases. (Remember test-tube demo.)

9. Conductors Materials that conduct heat quickly are called conductors All metals are good conductors of heat Copper is a very good conductor of heat Pans for cooking are usually made with a copper or aluminium bottom and plastic handles

10. Insulators/poor conductors Materials that conduct heat slowly or poorly are called insulators. Glass, wood, plastic and rubber are poor conductors (good insulators). Nearly all liquids, including water, are poor conductors (good insulators). Gases, including air, are very poor conductors. So wool or a winter ski jacket feels warm not because of the type of material but because of its ability to trap a lot of air. A bird fluffs its feathers so as to trap air, which is a good insulator. Starbuck double-walled mugs keep coffee warm because of the trapped air. Think about it… steel is a good conductor of heat— hence a very poor insulator. A refrigerator has insulation material around it to keep it cold – reducing the amount of heat conducted to the inside from the warmer room.

11. Convection Convection can only take place in materials where molecules can move around inside the material, i.e. a liquid or gas. The heat is carried by the molecules themselves moving Convection currents. Convection occurs because an area with warm molecules expands and becomes less dense than the cooler areas nearby. Hence the warm area rises. Cooler molecules fall into the space left by the warm molecules and a convection current is set up.

12. Convection Currents Hot liquids and gases expand and rise while the cooler liquid or gas falls 1. Hot air rises – it has expanded 2. Goes across 3. Then down as it cools and contracts 4. And across

13. Convection currents illustrated As molecules are heated, they expand and rise since they are less dense. When they cool, they become more dense and fall. Since air expands and contracts much more than liquids, convection is much more effective in air.

14. Convection in air The sun can cause large convection currents—WINDS During the daytime the land warms up to a higher T o than the sea, because water has a much higher heat capacity (5x) than the land. The warm air rises over the land and cool air falls over the sea. So we feel a cool sea breeze blowing in towards us as we stand on the shore in the daytime. At night, the breeze direction is reversed, as the water takes longer to cool down than does land.

15. Convection currents on the beach

16. Radiation The transfer of heat directly from the source to the object by an electromagnetic wave, travelling as rays. Heat radiation is also known as All objects that are warmer than their surroundings give out heat as infrared radiation Heat transfer by radiation does not need molecules to transfer heat (as do conduction and convection), but can radiate energy across empty space. Infrared Radiation

17. Emitters Hot objects emit (give out) heat. Cool objects absorb (take in) heat. Different types of surfaces emit heat at different speeds. A dull black surface loses energy more quickly – it is a good radiator. A bright, shiny or white surface is a poor radiator. Marathon runners needing to keep warm at the end of races are covered in shiny blankets which reduce radiation and therefore heat loss by emission.

18. Emitters of heat A bright shiny can  Poor radiator A dull black can  Good Radiator

19. Absorbers Cooler objects absorb (take in) heat. Substances absorb heat at different speeds. Dull, black surfaces absorb heat quickly. Bright, shiny surfaces absorb heat slowly. In hot countries, people wear bright white clothes and paint their homes white to reduce absorption of infrared energy from the sun. Petroleum storage tanks are sprayed silver so as to reflect the sun’s rays. Good emitters are also good absorbers of heat.

20. Absorbers A shiny, bright can  Poor absorber A dull black can  Good absorber

21. Key Words Heat Radiation Convection Conduction Infrared Insulator Conductor Temperature Emitter Absorber Transfer joules calories