HEAT TRANSFER - 4

Q: A metallic plate has a black spoy in the center.the plate is heated to 1000c and taken to dark room. the black spot glows why ?
A: It radiates in visible region.

Q: The surfaces of thermoflask is silvered why ?
A: It doesnot a absorb heat from the surroundings also the silver surface is a bad emitter.

Q: If air is a bad conductor of heat why do we not fell warm without clothes.
A: This is because when we are without clothes, air carrier away heat from our body due to convection and hence we feel cold.

Q: A body with large reflectivity is a poor emitter why ?
A: This is because a body with large reflectivity is a poor absorber of heat and poor absorbers are poor reflectors.


ASSIGNMENTS:

EASY:

E1. Two plates each of Area A₁ thickness t₁ and t₂ respectively are joined from a single plate of thickness (t₁ + t₁). If the temperature of right and left side are T₁ and T₂.

Find out
(a) Rate of flow of heat
(b) Temperature of interface

Solution: Thermal Resistance are in series
R =


    [T: Interface temp.]



E2. Two bodies A and B have thermal emissitivities of 0.01 and 0.81 respectively. The outer surface areas of the two bodies are same. thetwo bodies emit total radient power at the same rate. The wavelength _B corresponding to maximum spectral radiancy in the radiation from B is shifted from the wavelength corresponding to maximum spectrial radiancy in the radiation from A by 1.00 pm. If the temperature ofa is 5802 k. Calculate
(a) Temperature of B.
(b) wavelength of B.

Sol: By Stefan's Law
P = AT⁴

P_A = P_B and A_A = A<sub>B

A</sub>T_A4 = _BT_B⁴

T_B = 1934 k

_AT_A = _BT_B     [Wien's Law]

_B = 3_A,   also   _B - _A = 1µm

_B - 3_B = 1µm

_B = 1.5 µm


E-3: The emmisitivity of tungstun is approx 0.35. A tungstun sphere 1cm in radius is suspended within a large evacuated enclosure shose wall are at 100 k. What power input is required to maintain the sphere at 3000k if heat conduction along the support is neglected ?

Solution: H_net = A(t⁴ - T₀⁴)

A = 4r²

Pinput = A(T⁴ - T₀⁴)

          = 2019.8 watts


E4. The rods of Cu, brass and steel are welded together to form a y shaped structure. The cross sectional area of each rod is 4cm². the end of copper rod is maintained at 100c and the end of brass and steel rods at 0⁰c. Assume that there is no loss of heat from the surface of
of the rods. the length of rod au, cu, 46cm brass 13cm and steel 12 cm.
(a) What is the temperature of the function point ?
(b) What is the heat current in the copper rod ?