2. H EAT C ONDUCTION  Solids can be divided into two types, regarding conduction of heat: Topic 3.1 Extended A – Heat transfer  Conductors and nonconductors.  Metals are usually GOOD conductors of heat, whereas nonmetals are usually BAD conductors of heat.  When atoms of one portion of a material are in contact with vibrating atoms of another portion, the vibration is transferred from atom to atom.  Since we associate motion with temperature, high T portions vibrate more than low T portions, so we can imagine the vibration “impulse” to travel through the material, from high T to low T. HOTHOT COLDCOLD HOTHOT FYI: Metals are good heat conductors because they have lots of free electrons (the same reason they are good electrical conductors). FYI: The rate at which heat energy is transferred from the hot to the cold side depends directly on the cross-sectional area of the conductor and inversely with the length of the conductor. A d

3. H EAT C ONDUCTION  The rate at which heat is conducted is given by Topic 3.1 Extended A – Heat transfer QtQt  That rate is proportional to the cross-sectional area A and the difference in temperature  T, and inversely proportional to the distance between the two temperature sources. Thus A d QtQt  ATdATd  Of course the great physics Wallahs have found the proportionality constant k (called the thermal conductivity) so that we have equality: QtQt = kA  T d Thermal Conductivity FYI: Every material has its own thermal conductivity k. By the way, don't confuse it with Boltzman's constant k B.

4.  A brick has a thermal conductivity of 0.71 J/m·s·C°, and dimensions shown. 20°C H EAT C ONDUCTION Topic 3.1 Extended A – Heat transfer 100°C 2.5 m0.5 m 0.75 m  Find the rate at which heat energy is conducted through the brick. QtQt = kA  T d = 0.71(0.5)(0.75)(100-20) 2.5 FYI: The cross-sectional area is (0.5 m)(0.75 m). Why? = 8.52 J/s

5. H EAT C ONVECTION Topic 3.1 Extended A – Heat transfer  Another form of heat transfer is called convection.  Convection requires a fluid as a medium of heat transfer.  For example, hot air is less dense than cold air, so it rises.  But as it rises it cools, and so becomes denser and sinks.  We thus obtain a cycle, which forms a circulation called a convection current.  Convection currents drive many interesting physical systems.

6. A TMOSPHERIC C ONVECTION - THUNDERHEADS

7. O CEANIC C ONVECTION - CURRENTS

8. S OLAR C ONVECTION - SUNSPOTS

9. M ANTLE C ONVECTION – CONTINENTAL DRIFT

12. A TMOSPHERIC C ONVECTION – NUCLEAR DETONATION

13. H EAT R ADIATION Topic 3.1 Extended A – Heat transfer  Another form of heat transfer is called radiation.  Convection requires a fluid as a medium of heat transfer, and conduction requires physical contact, but radiation can transfer energy (called radiant energy) through space. FYI: Radiant energy is transported by electromagnetic waves at the speed of light. In fact, visible light is a form of radiant energy. 10 4 10 6 10 8 10 10 10 12 10 14 10 16 10 18 Frequency f / Hz Radio, TV Infrared Light X-Rays The Electromagnetic Spectrum Microwaves Ultraviolet Light Gamma Rays

14. H EAT R ADIATION Topic 3.1 Extended A – Heat transfer  The rate at which an object radiates energy is given by Stefan's law: P =  AeT 4 Stefan's law  where  is the Stefan-Boltzmann constant and is given by  = 5.67  10 -8 W/m 2 ·K 4  A is the surface area of the radiating object, T is the temperature in kelvins, and e is the emissivity of the object.  The emissivity of the object is a unitless constant between 0 and 1, where 0 is a non-emitter, and 1 is a perfect emitter.  In general, a good emitter is also a good absorber.  A perfect emitter is called a blackbody (e = 1).

15. H EAT R ADIATION Topic 3.1 Extended A – Heat transfer  A small sphere covered with lamp black makes a pretty good blackbody and has an emissivity somewhat close to 1.  An even better blackbody can be made by drilling a small hole in a hollow block. The hole itself acts as a blackbody. e = 0.5 tallow candle e = 0.9 better blackbody i n c i d e n t t h e r m a l r a d i a t i o n FYI: A cavity blackbody, such as the one shown below, traps almost all the incident radiation, acting like a perfect absorber. Thus the hole acts like a nearly perfect absorber of radiation. FYI: We will return to blackbody radiation next year when we cover QUANTUM PHYSICS.

16. H EAT R ADIATION Topic 3.1 Extended A – Heat transfer  The equation P =  AeT 4 assumes the surrounding temperature is 0 K. For the situation where it is not (almost every situation you can imagine) we use P =  Ae(T 4 - T 4 )  where T s is the absolute temperature of the surroundings. s  Note that if T s > T, then the object has a positive rate of energy change, which we take to mean the object is absorbing energy. Its internal energy will increase and its temperature will climb.  Note that if T s < T, then the object has a negative rate of energy change, which we take to mean the object is losing energy. Its internal energy will decrease and its temperature will fall. Radiation Exchange