Textbook Chp 10 Pg 167 182 TRANSFER OF

Textbook Chp 10, Pg 167 -182 TRANSFER OF THERMAL ENERGY

TOPICS � Conduction � Convection � Radiation

TRANSFER OF THERMAL ENERGY � How to explain the 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)

THERMAL EQUILIBRIUM � 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

CONDUCTION � 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 (MICROSCOPIC) � 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

CONVECTION � 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)

RADIATION � 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 � 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 � 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 � 3) difference btn Temperature, faster rate Surface Area � Greater surface area, faster rate

10 MIN QUIZ!