Transfer of thermal energy

1. Textbook Chp 10, Pg 167-182 Transfer of thermal energy

2. Conduction Convection Radiation Topics

3. How to explainthe following two demos? 1) Hot, Cold and Tap water 2) Melting ice on metal and rubber. 1 – sensation of hotness/coldness is not due to temperature, but the flow of heat. 2 – ice melts faster on metal because it conducts heat away faster (which is also why metal feels cold) Transfer of thermal energy

4. Thermal Energy will only flow if there is a difference in temperature Thermal Energy will flow from hotter region to colder region Note: an object which is hotter does NOT necessarily mean it has more thermal energy Heat will stop flowing when the temperatures are the same – this is called Thermal Equilibrium thermal equilibrium

5. Conduction is the transfer of thermal energy (or heat) through a medium from a hotter to a colder region Usually occurs in solids Heat is transferred, but the actual substance does not Materials which can conduct heat faster are said to be good (thermal) conductors Materials which conduct heat slower are said to be good (thermal) insulators conduction

6. On a molecular level • molecules in hotter regions vibrate more vigorously than molecules in colder regions • these molecules collide with their neighbouring molecules, transferring kinetic energy to them • in addition, for metals (only) • Free Electron Diffusion occurs, with high energy electrons moving from hotter to colder regions conduction (microscopic)

7. Can only take place in a fluid (liquid of gas) • Relies on the difference in density • Hot fluid is less dense (rises) • Cool fluid is more dense (sinks) • Convection currents are formed • Substance is actually moving (along with the convection currents) convection

8. Radiation does not involve the movement of molecules (at your level) All objects emit electro-magnetic radiation. What kind of emission is radiated depends on the temperature of the object. Around room temperature, infra-red radiation is emitted When hotter (480°C or higher), visible light starts to be emitted. radiation

9. When an object is hotter than its surroundings, it is said to be emitting radiation. When an object is cooler than its surroundings, it is said to be absorbing radiation. A cooler object may also reflect radiation (i.e. not absorbed) A black object is a good emitter and absorber, but a poor reflector or radiation. A white/shiny object is a poor emitter and absorber, but a good reflector of radiation. Radiation

10. 3 Factors which affect rate of heat transfer by Radiation • 1) Color • Black faster rate, White/Shiny slower rate • 2) Difference in Temperature between object and surroundings • Greater difference btn Temperature, faster rate • 3) Surface Area • Greater surface area, faster rate Radiation

11. 10 min Quiz!