Problem-solving strategies for the methods of heat transfer
Examine the situation to determine what type of heat transfer is involved.
Identify the type(s) of heat transfer—conduction, convection, or radiation.
Identify exactly what needs to be determined in the problem (identify the unknowns). A written list is very useful.
Make a list of what is given or can be inferred from the problem as stated (identify the knowns).
Solve the appropriate equation for the quantity to be determined (the unknown).
For conduction, equation
$\frac{Q}{t}=\frac{\text{kA}({T}_{2}-{T}_{1})}{d}$ is appropriate.
[link] lists thermal conductivities. For convection, determine the amount of matter moved and use equation
$Q=\text{mc}\text{\Delta}T$ , 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.
Insert the knowns along with their units into the appropriate equation and obtain numerical solutions complete with units.
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
$\sigma =5\text{.67}\times {\text{10}}^{-8}\phantom{\rule{0.25em}{0ex}}\text{J/s}\cdot {\text{m}}^{2}\cdot {\text{K}}^{4}$ 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
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 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?
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
$\text{30.}\mathrm{0\xba}\text{C}$ and the surrounding temperature is
$\text{15.}\mathrm{0\xba}\text{C}$ ? The emissivity of the roof is 0.900.
$-\text{21}\text{.}7\text{kW}$ Note that the negative answer implies heat loss to the surroundings.
(a) Cherry-red embers in a fireplace are at
$\text{850\xba}\text{C}$ and have an exposed area of
$0\text{.}\text{200}\phantom{\rule{0.25em}{0ex}}{\text{m}}^{2}$ and an emissivity of 0.980. The surrounding room has a temperature of
$\text{18}\text{.}\mathrm{0\xba}\text{C}$ . 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?
using a micro-screw gauge,the thickness of a piece of a A4 white paper is measured to be 0.5+or-0.05 mm. If the length of the A4 paper is 26+or-0.2 cm, determine the volume of the A4 paper in:
a). Cubic centimeters
b). Cubic meters
no life without physics ....that should tell you something
Exactly
Emmanuella
😎👍
E=MC^2
study of matter and energy and an inter-relation between them.
Minahil
that's how the mass and energy are related in stationery frame
Arzail
Ketucky tepung 10m
firdaus
Treeskin, 6m
Cloud gam water 2m
Cloud gam white 2m
And buur
firdaus
Like dont have but have
firdaus
Two in one
firdaus
Okay
firdaus
DNA card
firdaus
hey am new over hear
Shiwani
War right? My impesilyty again. Don't have INSURAN for me
firdaus
PUSH
firdaus
I give
firdaus
0kay
firdaus
Hear from long
firdaus
Hehehe
firdaus
All physics... Hahahaha
firdaus
Tree skin and two cloud have tokside maybe
firdaus
Sold thing
firdaus
PUSH FIRST. HAHAHAAHA
firdaus
thanks
firdaus
Kinetic energy is the energy due to montion of waves,electrons,atoms, molecule,substances an object s.
Emmanuella
Opjective 0
firdaus
Atom nber 0
firdaus
SOME N
firdaus
10.000m permonth. U use momentom with me
firdaus
hi
Hilal
plz anyone can tell what is meteor and why meteor fall in night? can meteor fall in the day
Hilal
meteor are the glowy (i.e. heated when the enter into our atmosphere) parts of meteoroids. now, meteoroids are the debris resulting from the collision of asteroids or comets.
yes, it occurs in daytime too, but due to the daylight, we cant observe it as clearly as in night
Arzail
thank's
Hilal
hello guys
Waka
wich method we use to find the potential on a grounded sphere
x-rays are electromagnetic Ray's produced when electrons with very high acceleration is brought to a stop by a target metal..
Felix
DNA CARD. DNA BLOOD(DARAH)
firdaus
@firdaus
What is this DNA card? can I get to know?
Arzail
determine how much less the mass of lithium with mass number of 7 and proton of 3 nucleus is compared to that of its constituents.the mass of neutral Li 6.015123 u, calculate the total binding energy and the binding energy per nucleon