ALL OF RADIATION TO MEMORISE

Question 1

What is radiation?

Answer 1

A conversion of a bodies thermal energy into electromagnetic energy.

Question 2

What type of object/matter emits radiation?

Answer 2

All matter above absolute zero.

Question 3

What type of object/matter absorbs radiation?

Answer 3

All matter absorbs radiation to some degree.

Question 4

What is a black body?

Answer 4

A black body is one which absorbs all radiation incident on it. Black bodies do NOT reflect.

Question 5

What is planks law of radiation?

Answer 5

B(T) = (2*h*c^2/lamda^5)(1/exp(h*c/lamda*k_b*T)-1) 
where: h = plancks constant
c = speed of light
lambda = wavelength
B = spectral radiance
k_b = boltzmann constant

Question 6

What is Weins displacement law?

Answer 6

lambda = b/T
where lambda = peak wavelength
b = weins displacement constant
T = Absolute temp

Question 7

What is the emissive power per unit area of a black body ?

Answer 7

Eb = sigma * T^4

Question 8

What happens when radiation is incident on a surface? ANY TYPE OF SURFACE.

Answer 8

Draw diagram. A fraction is absorbed(Alpha), a fraction is reflected (rho) and a fraction is transmitted (tao). However, usually solid bodies don’t transmit thermal radiation so we can write: alpha + rho = 1

Question 9

Can you derive kirchoffs law (emissivity)?

Answer 9

Yes or no?

Question 10

A larger emissivity means?

Answer 10

Closer to black body behaviour.

Question 11

In reality, what does emissivity depend on?

Answer 11

Wavelength and temperature

Question 12

What is G?

Answer 12

Irradiation - the total amount of radiation incident on a surface per unit area and per unit time

Question 13

What is J?

Answer 13

Radiosity - the total amount of radiation leaving a surface per unit time and per unit area

Question 14

How can radiosity be expressed?

Answer 14

J = total radiation emitted + total radiation reflected = emissivityEb + reflectivityG and since reflectivity = 1 - absorption and absorption = emissivity then we get J = emissivity * Eb + (1-emissivity)* G

Question 15

Thermal resistance circuit for radiative energy leaving a surface?

Answer 15

draw resistance.. q = (Eb - J)/(1-e)/eA

Question 16

Thermal resistance circuit for radiative heat flux between two surfaces?

Answer 16

draw resistance.. q=(J1-J2)/(1/A1F12)

Question 17

Can you draw a radiation network for 2 surfaces that see each other and nothing else?

Answer 17

Yes or no?

Question 18

What are the two limiting cases in radiation?

Answer 18

1. If the surface is perfectly insulating, it must re-radiate all of the energy incident on its surface as there is nowhere else for it to go. Therefore, J=Eb and no current will flow through the resistance.
2. If the surface has a very large area then 1-e/eA is very small. Therefore the thermal resistance will also be small. So V=IR will mean that Eb=J.

Question 19

Why may a thermometer placed in a flow of gas not indicate the temp of the gas?

Answer 19

Energy will be transferred via convection to the thermometer. At the same time though, the thermometer will be exchanging energy with the enclosing walls through radiation. As a result, the thermometer will not have the same temp as the gas, it will have a temp dictated by some equilibrium of convection and radiation.

Question 20

What are ways of minimising heat transfer via radiation?

Answer 20

1. Employ highly reflective surfaces (low emissivity).. this will make the thermal resistance high
2. Use larger areas if possible
3. Minimise the amount each surface sees of the other to decrease the shape factor
4. If these can’t be done, use a radiation shield. This can add many resistances to the resistive path, minimising q.

Question 21

Can you draw a resistive path for two objects (i) without a radiation shield and (ii) with a radiation shield

Answer 21

Yes or no?