1. HEAT TRANSFER By the end of this presentation you should be able to: Describe conduction, convection and radiation Appreciate that if two areas were at the same temperature, there would not be any transfer of thermal energy.

2. CONDUCTION Conduction is the transfer of heat from a hotter area to a cooler area through a solid. The substance does not move as a whole. The particles vibrate more vigorously when they are heated and the heat energy is then transferred from one particle to the next.

3. The picture below shows the structure of table salt, which is a solid. Like many other solids the atoms are very close to each other. If one of the atoms begin to vibrate, then …..

5. In a metal, in addition to the atoms being very close to each other, there are also electrons which are free to move about. These electrons can carry heat energy along a metal much faster than vibrations can transfer it. So this makes metals better conductors than most other solids. + + + + + + + + + + + +

6. Solids are generally better conductors than liquids or gases. Suggest a reason for this. Conduction involves heat energy being transferred through vibrations. Since the particles of a solid are tightly packed, vibrations can be passed to neighbouring particles quickly and easily.

7. In a liquid, the particles are a little further away (although at least half are touching). It is difficult for vibrations to be passed along continuously.

8. In a gas, the particles are very far apart. Conduction is virtually impossible!

9. CONVECTION Convection is the movement of heat from a hotter region to a cooler region with movement of the substance as a whole. This is the way heat moves through liquids and gases. - a water molecule. The water at the bottom expands as it heats up before the water above. It then rises, then cools, and sinks again.

10. The paragraph below relates to convection. Copy it and fill in the blanks. Convection is the movement of heat through a ________ or a _____. The heat energy is transferred from a ________ area to a ________ area. When the fluid is heated it ________. It then becomes less _______ than its surroundings so it _______. Cooler liquid or gas then _______ to take its place. Convection __________ are set up in this way. liquid gas hotter cooler expands dense risesfalls currents Pick from these words : Currents liquid expands hotter cooler gas rises falls dense

11. Radiation is the movement of heat from a hotter region to a cooler region via waves.

12. Radiation does not need a substance to move through, it can occur in a vacuum! This is how energy gets from the Sun to the Earth. WHEN AN OBJECT IS WARM, IT EMITS RADIATION….. Black, matt surfaces emit radiation better than white, shiny surfaces. WHEN AN OBJECT IS COOL, IT ABSORBS RADIATION FROM OTHER WARM OBJECTS….. Black, matt surfaces absorb radiation better than white, shiny surfaces.

13. The human body is always radiating heat that is invisible You can use an infra-red camera to detect it. 25 o C 30 o C 35 o C

14. When an object is cooling, it often loses its heat energy through more than one of these processes at the same time. Conduction through solids. Convection currents within the liquid, and in the air above. Radiation from the warm cup in all directions.