1. RADIATION
1.1. MOVES ENERGY THROUGH A VACCUM
1.2. Q/t = σAeT^4
1.2.1. Q = HEAT ENERGY t = TIME e = EMISSIVITY (0 <= e <= 1) σ = 5.67x10^-8 Wm^-2 K^-4 (STEFAN'S CONSTANT) A = AREA IN m^2 T = TEMPERATURE IN K
1.2.2. EXAMPLE : A dirty solid aluminium sphere (m =11.3g, area 12.5cm^2) has a temperature of 427*c. what is the rate of loss of heat through radiation ? assume e= 0.60 for the sphere
1.2.2.1. SOLUTION : (5.67 x10^-8)(12.5x10^-4)(0.6)(700^4) = 10.2W
2. CONDUCTION
2.1. PARTICLES TO PARTICLES BY DIRECT COLLSION
2.2. Q/ t= kA/L(T2-T1)
2.2.1. Q = HEAT ENERGY T1 = TEMPERATURE ON COOL SURFACE T2 = TEMPERATURE ON HOT SURFACE k = THERMAL CONDUCTIVITY A = AREA OF SLAB L = THICKNESS OF SLAB t = TIME
2.2.2. EXAMPLE : A square slab of a thickness of 4cm and measuring 25cm on a side has a 40*c temperature difference between its faces. how much heat flows through it per hour ? the thermal conductivity is 1.05Wm^-1 Degree celsius ^-1
2.2.2.1. SOLUTION : (1.05 x (0.25^2)x (40)x (3600)) / 0.04 = 236.25kJ
3. CONVECTION
3.1. TRANSPORTED BY MOVEMENT OF MOLECULES THEMSELVES OVER A LARGE DISTANCE
3.1.1. Q/t = hA(Ts-Tf)
3.1.1.1. Q = HEAT ENERGY Ts= TEMPERATURE OF SOLID Tf = TEMPERATURE OF SURROUNDING A = AREA t = TIME h = CONVECTION HEAT TRANSFER COEFFICENT
3.1.1.2. EXAMPLE : Air at 20*C blows over a hot plate 50 by 75 cm maintained at 250*C. the convection heat transfer coefficent is 25Wm^-2 *C^-1. Calculate the rate of heat transfer.
3.1.1.2.1. SOLUTION : (250-20)/(1/25)(0.375) = 2.156kW