PHY PHYSICS 231 Lecture 28: Conduction,Convection & Radiation Remco Zegers walk-in: Monday 9:15-10:15 Helproom

PHY Heat transfer to an object Q=cmTQ=cmT Energy transfer (J or cal) Specific heat (J/(kg o C) or cal/(g o C) Mass of object Change in temperature The amount of energy transfer Q to an object with mass m when its temperature is raised by  T:

PHY Phase Change GAS(high T) liquid (medium T) Solid (low T) Q=c gas m  T Q=c liquid m  T Q=c solid m  T Gas  liquid liquid  solid Q=mL f Q=mL v

PHY How can heat be transferred?

PHY Conduction Touching different materials: Some feel cold, others feel warm, but all are at the same temperature…

PHY Thermal conductivity metal wood T=37 0 C T=20 0 C The heat transfer in the metal is much faster than in the wood: (thermal conductivity)

PHY Heat transfer via conduction Conduction occurs if there is a temperature difference between two parts of a conducting medium Rate of energy transfer P P=Q/  t (unit Watt) P=kA(T h -T c )/  x=kA  T/  x k: thermal conductivity Unit:J/(ms o C) metal k~300 J/(ms o C) gases k~0.1 J/(ms o C) nonmetals~1 J/(ms o C)

PHY Example A glass window (A=4m 2,  x=0.5cm) separates a living room (T=20 0 C) from the outside (T=0 o C). A) What is the rate of heat transfer through the window (k glass =0.84 J/(ms o C))? B) By what fraction does it change if the surface becomes 2x smaller and the temperature drops to C?

PHY Another one. Heat reservoir Heat sink An insulated gold wire (I.e. no heat lost to the air) is at one end connected to a heat reservoir (T=100 0 C) and at the other end connected to a heat sink (T=20 0 C). If its length is 1m and P=200W what is its cross section (A)?

PHY And another Water 0.5L C C A=0.03m 2 thickness: 0.5cm. A student working for his exam feels hungry and starts boiling water (0.5L) for some noodles. He leaves the kitchen when the water just boils.The stove’s temperature is C. The pan’s bottom has dimensions given above. Working hard on the exam, he only comes back after half an hour. Is there still water in the pan? (L v =540 cal/g, k pan =1 cal/(ms 0 C)

PHY Isolation L1L1 L2L2 L3L3 inside ThTh TcTc A house is built with 10cm thick wooden walls and roofs. The owner decides to install insulation. After installation the walls and roof are 4cm wood+2cm isolation+4cm wood. If k wood =0.10 J/(ms 0 C) and k isolation =0.02 J/(ms 0 C), by what factor does he reduce his heating bill? P before =A  T/[0.10/0.10]=A  T P after =A  T/[0.04/ / /0.10]=0.55A  T Almost a factor of 2 (1.81)!

PHY Convection T high  low

PHY Radiation Nearly all objects emit energy through radiation: P=  AeT 4 : Stefan’s law (J/s)  =5.6696x10 -8 W/m 2 K 4 A: surface area e: object dependent constant emissivity (0-1) T: temperature (K) P: energy radiated per second.

PHY emissivity Ideal reflector e=0 no energy is absorbed Ideal absorber (black body) e=1 all energy is absorbed also ideal radiator!

PHY A BBQ The coals in a BBQ cover an area of 0.25m 2. If the emissivity of the burning coal is 0.95 and their temperature C, how much energy is radiated every minute?

PHY radiation balance If an object would only emit radiation it would eventually have 0 K temperature. In reality, an object emits AND emits radiation. P=  Ae(T 4 -T 0 4 ) where T: temperature of object T 0 : temperature of surroundings.

PHY example The temperature of the human body is 37 0 C. If the room temperature is 20 0 C, how much heat is given off by the human body to the room in one minute? Assume that the emissivity of the human body is 0.9 and the surface area is 2 m 2.