# 14.7 Radiation  (Page 5/13)

 Page 5 / 13

## Problem-solving strategies for the methods of heat transfer

1. Examine the situation to determine what type of heat transfer is involved.
2. Identify the type(s) of heat transfer—conduction, convection, or radiation.
3. Identify exactly what needs to be determined in the problem (identify the unknowns). A written list is very useful.
4. Make a list of what is given or can be inferred from the problem as stated (identify the knowns).
5. Solve the appropriate equation for the quantity to be determined (the unknown).
6. For conduction, equation $\frac{Q}{t}=\frac{\text{kA}\left({T}_{2}-{T}_{1}\right)}{d}$ is appropriate. [link] lists thermal conductivities. For convection, determine the amount of matter moved and use equation $Q=\text{mc}\text{Δ}T$ , to calculate the heat transfer involved in the temperature change of the fluid. If a phase change accompanies convection, equation $Q={\text{mL}}_{\text{f}}$ or $Q={\text{mL}}_{\text{v}}$ is appropriate to find the heat transfer involved in the phase change. [link] lists information relevant to phase change. For radiation, equation $\frac{{Q}_{\text{net}}}{t}=\sigma eA\left({T}_{2}^{4}-{T}_{1}^{4}\right)$ gives the net heat transfer rate.
7. Insert the knowns along with their units into the appropriate equation and obtain numerical solutions complete with units.
8. Check the answer to see if it is reasonable. Does it make sense?

## Summary

• Radiation is the rate of heat transfer through the emission or absorption of electromagnetic waves.
• The rate of heat transfer depends on the surface area and the fourth power of the absolute temperature:
$\frac{Q}{t}=\sigma eA{T}^{4}\text{,}$

where $\sigma =5\text{.67}×{\text{10}}^{-8}\phantom{\rule{0.25em}{0ex}}\text{J/s}\cdot {\text{m}}^{2}\cdot {\text{K}}^{4}$ is the Stefan-Boltzmann constant and $e$ is the emissivity of the body. For a black body, $e=1$ whereas a shiny white or perfect reflector has $e=0$ , with real objects having values of $e$ between 1 and 0. The net rate of heat transfer by radiation is

$\frac{{Q}_{\text{net}}}{t}=\sigma eA\left({T}_{2}^{4}-{T}_{1}^{4}\right)$

where ${T}_{1}$ is the temperature of an object surrounded by an environment with uniform temperature ${T}_{2}$ and $e$ is the emissivity of the object .

## Conceptual questions

When watching a daytime circus in a large, dark-colored tent, you sense significant heat transfer from the tent. Explain why this occurs.

Satellites designed to observe the radiation from cold (3 K) dark space have sensors that are shaded from the Sun, Earth, and Moon and that are cooled to very low temperatures. Why must the sensors be at low temperature?

Why are cloudy nights generally warmer than clear ones?

Why are thermometers that are used in weather stations shielded from the sunshine? What does a thermometer measure if it is shielded from the sunshine and also if it is not?

On average, would Earth be warmer or cooler without the atmosphere? Explain your answer.

## Problems&Exercises

At what net rate does heat radiate from a $\text{275}{\text{-m}}^{2}$ black roof on a night when the roof’s temperature is $\text{30.}0º\text{C}$ and the surrounding temperature is $\text{15.}0º\text{C}$ ? The emissivity of the roof is 0.900.

Note that the negative answer implies heat loss to the surroundings.

(a) Cherry-red embers in a fireplace are at $\text{850º}\text{C}$ and have an exposed area of and an emissivity of 0.980. The surrounding room has a temperature of $\text{18}\text{.}0º\text{C}$ . If 50% of the radiant energy enters the room, what is the net rate of radiant heat transfer in kilowatts? (b) Does your answer support the contention that most of the heat transfer into a room by a fireplace comes from infrared radiation?

#### Questions & Answers

how many subject is in physics
the write question should be " How many Topics are in O- Level Physics, or other branches of physics.
effiom
how many topic are in physics
Praise
yh I need someone to explain something im tryna solve . I'll send the question if u down for it
a ripple tank experiment a vibrating plane is used to generate wrinkles in the water .if the distance between two successive point is 3.5cm and the wave travel a distance of 31.5cm find the frequency of the vibration
Tamdy
the range of objects and phenomena studied in physics is
what is Linear motion
straight line motion is called linear motion
then what
Amera
linear motion is a motion in a line, be it in a straight line or in a non straight line. It is the rate of change of distance.
Saeedul
Hi
aliyu
your are wrong Saeedul
Richard
Linear motion is a one-dimensional motion along a straight line, and can therefore be described mathematically using only one spatial dimension
Jason
is a one-dimensional motion along a straight line, and can therefore be described mathematically using only one spatial dimensions.
Praise
what is a classical electrodynamics?
Marga
what is dynamics
Marga
dynamic is the force that stimulates change or progress within the system or process
Oze
what is the formula to calculate wavelength of the incident light
if a spring is is stiffness of 950nm-1 what work will be done in extending the spring by 60mmp
State the forms of energy
machanical
Ridwan
Word : Mechanical wave Definition : The waves, which need a material medium for their propagation, e.g., Sound waves. \n\nOther Definition: The waves, which need a material medium for their propagation, are called mechanical waves. Mechanical waves are also called elastic waves. Sound waves, water waves are examples of mechanical waves.t Definition: wave consisting of periodic motion of matter; e.g. sound wave or water wave as opposed to electromagnetic wave.h
correct
Akinpelu
what is mechanical wave
a wave which require material medium for its propagation
syed
The S.I unit for power is what?
watt
Okoli
Am I correct
Okoli
it can be in kilowatt, megawatt and so
Femi
yes
Femi
correct
Jaheim
kW
Akinpelu
OK that's right
Samuel
SI.unit of power is.watt=j/c.but kw.and Mw are bigger.umots
syed
What is physics
study of matter and its nature
Akinpelu
The word physics comes from a Greek word Physicos which means Nature.The Knowledge of Nature. It is branch of science which deals with the matter and energy and interaction between them.
Uniform
why in circular motion, a tangential acceleration can change the magnitude of the velocity but not its direction
reasonable
Femi
because it is balanced by the inward acceleration otherwise known as centripetal acceleration
MUSTAPHA
What is a wave
Tramsmission of energy through a media
Mateo
is the disturbance that carry materials as propagation from one medium to another
Akinpelu
mistakes thanks
Akinpelu
find the triple product of (A*B).C given that A =i + 4j, B=2i - 3j and C = i + k
Difference between north seeking pole and south seeking pole