Heat

1. Heat

2. How hot is the sun? • Surface 6000oC • Core 15000000oC

3. Temperature • Temperature, is a measure of the hotness or coldness of a body. • The units we use is the Celsius Scale but the SI units are the Kelvin

4. What is absolute zero? • The temperature of a body depends on how fast the molecules can vibrate. As they cool down the molecules slow down and absolute zero is the temperature at which there is the least amount of movement • Temperature Game

5. Converting Temperatures • The Fahrenheit Scale • Chill Coke

6. How does a thermometer work? • To measure the temperature of a substance we need to use a thermometer that has an appropriate scale and uses a physical quantity that changes measurably with temperature i.e. Thermometric Property

7. Examples of Thermometric Properties • Length of liquid column • Emf of thermocouple • Resistance • Colour

8. Why do different thermometers give different readings? • Because different thermometers use different thermometric properties the change in temperature is measured differently and different thermometric properties do not change proportionally. • We use a mercury thermometer as standard.

9. Practical Thermometers • Clinical • Oven • Boiler • Car

10. Heat Capacity • Why is the apple in an apple pie hotter than the pastry when they are at the same temperature when they come out of the oven? • All substances have the capacity to store heat but some substances are better at storing the heat than others.

11. How do storage heaters work? • In a storage heater there are bricks with a very high heat capacity. They are heated at night and then the heat is released slowly by the bricks during the day. • Fire Walking

12. Heat Capacity Formula

13. Latent Heat

14. Latent Heat • Why doesn’t ice melt immediately after a big freeze when the sun starts shining? • The ice is solid and for it to change state heat must be added. This heat is used to break the bonds between the molecules in the solid before the ice turns to water. This is called Latent Heat.

15. Latent Heat Formula • Latent heat of fusion is the heat required to change the substance from a solid to a liquid • Latent heat of vaporisation is the heat required to change the substance from a liquid to a gas.

16. How does sweating cool you down? • When sweat evaporates it takes the latent heat of vaporisation from your body, thus cooling you down.

17. How does a fridge work? • A fridge uses a heat pump to transfer heat from one place to another. The heat is taken from the fridge by a refrigerant liquid. It is passed through an expansion valve where the liquid is turned into a vapour taking the heat from the fridge. The liquid is circulated through the compressor where it is turned back into a liquid and the heat is given out through the fins at the back of the fridge.

18. Heat Pump

19. How do double glazed windows work? • Heat is transferred in 3 ways; Conduction (heat transfer through molecules) Convection (heat transfer through a fluid) Radiation (heat transfer by electromagnetic waves) • Double glazed windows use the fact that air is a bad conductor of heat and therefore if you trap as much air as possible between the glass the window will be well insulated

20. 3 forms of Heat Transfer • Hot Balloons

21. U-Value • The U value is the measurement of heat transmission through a material. The U value of a material is a gauge on how well heat passes through the material and the lower the U value, the greater the resistance to heat and therefore has a better insulating value. The U value is generally used in the building and construction industry. U-Value is expressed in units of W/m2

22. How does my domestic heating system work? • The hot water in the tank rises by convection to the top of the tank. The hot water outlet is at the top of the tank. The water circulates through the house by convection currents. For better efficiency a pump is also used to circulate the hot water

23. How do solar panels work? • Most solar panels in Ireland are composed of the panel, consisting of black pipes and a water tank. The water in the pipes are heated by radiation from the sun and the water circulated by convection.