# Food Engineering Questions and Answers – Unit Operations – Heat Transfer by Radiation

This set of Food Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Unit Operations – Heat Transfer by Radiation”.

1. Statement 1: Radiation is the transfer of heat by electromagnetic waves.
Statement 2: When heat is radiated from a hot source, the amount of heat received by the body depends on the nature of the body.
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: Radiation is the transfer of heat by electromagnetic waves. When heat is radiated from a hot source, the amount of heat received by the body depends on the nature of the body. E.g. A black body will absorb more energy than any other body from the same source which is exposed to both the bodies in the same manner.

2. Statement 1: Radiation needs a medium to travel.
Statement 2: Thermal radiation takes place at a certain temperature for every body.
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: Radiation needs NO medium to travel. Thermal radiation is emitted by every body at a temperature above 0 K.

3. Statement 1: Radiation is NOT heat.
Statement 2: Maximum value of absorptivity is 1.
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: Radiation is NOT heat, and if it is absorbed and converted to heat, it is no longer heat. Maximum value of absorptivity is 1.

4. The emission of radiation by any body is independent of the radiation by any body next to it.
a) True
b) False

Explanation: The emission of radiation by any body is independent of the radiation by any body next to it.

5. A food industry keeps food cool in a very crude manner, by radiation. It paints its walls white and not black, because of _____
a) Kirchhoff’s Law
b) Stephan-Boltzmann Law
c) None of the mentioned
d) Kirchhoff’s Law & Stephan-Boltzmann Law

Explanation: Kirchhoff’s law states that at temperature equilibrium, the absorptivity of a grey body is equal to its emissivity. Since white paint has less emissivity than that of black paint, the wall is painted white.
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6. Which of the following reason does NOT affect radiation between two parallel bodies?
a) Angle of vision
b) Orientation of surfaces
c) Emissivity of both
d) Smoothness and roughness

Explanation: The angle of vision does not affect radiation between two parallel bodies. But, it is an important factor when it comes to curved surfaces.

7. Freezer is at -18 deg C. Will will be Radiative heat transfer if meat is at 298k, its emissivity is 0.82 and average area is 0.045m2.
Either use Stefan Boltzmann law or take hr (radiative heat transfer co-efficient) as 0.227.
ε = ( Tav/100 )3
a) 7.61
b) 6.94
c) 8.43
d) None of the mentioned

Explanation: Stefan Boltzmann law:
T1 = 298, T2 = 255. σ= 5.67*10-8 W /m2K4
T14 = 7.88 * 109
T24 = 4.22* 109
Using, q = A εσ (T14 – T24)
q = 5.7 * 10-8 * 0.82* 0.45 * (7.88-4.22) * 109 = 7.61.
Tav= (298 + 255) /2
hr = 3.93 W/m2 K
q = A εσ (T1 -T2) = 3.93* 0.045 * (298-255) = 7.61.

8. Statement 1: Radiation dryers are used to dry grains and reduce their moisture content before storage.
Statement 2: Baking ovens are used both for making the dough and after that’s made, making biscuits by cutting them and baking them further. This entire process uses radiation.
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: Both the statements pertaining to radiation are true.

9. In deep freezers, the walls absorb heat from food materials by _____ and keep them cool.
a) Conduction
b) Convection
d) None of the mentioned

Explanation: In deep freezers, the walls absorb heat from food materials by radiation and keep them cool.

a) Ultraviolet rays
c) Visible range
d) None of the mentioned

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