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
is appropriate.
[link] lists thermal conductivities. For convection, determine the amount of matter moved and use equation
, to calculate the heat transfer involved in the temperature change of the fluid. If a phase change accompanies convection, equation
or
is appropriate to find the heat transfer involved in the phase change.
[link] lists information relevant to phase change. For radiation, equation
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:
where
is the Stefan-Boltzmann constant and
is the emissivity of the body. For a black body,
whereas a shiny white or perfect reflector has
, with real objects having values of
between 1 and 0. The net rate of heat transfer by radiation is
where
is the temperature of an object surrounded by an environment with uniform temperature
and
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
black roof on a night when the roof’s temperature is
and the surrounding temperature is
? 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
and have an exposed area of
and an emissivity of 0.980. The surrounding room has a temperature of
. 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?
Astronomy (from Ancient Greek ἀστρονομία (astronomía) 'science that studies the laws of the stars') is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution.
Rafael
vjuvu
Elgoog
what is big bang theory?
Rosemary
what type of activity astronomer do?
Rosemary
No
Richard
the big bang theory is a theory which states that all matter was compressed together in one place the matter got so unstable it exploded releasing All its contents in the form of hydrogen
according to the theory of astronomers why the moon is always appear in an elliptical orbit?
Gatjuol
hi !!! I am new in astronomy....
I have so many questions in mind ....
all of scientists of the word they just give opinion only.
but they never think true or false ...
i respect all of them...
I believes whole universe depending
on true ...থিউরি
Govinda
hello
Jackson
hi
Elyana
we're all stars and galaxies a part of sun. how can science prove thx with respect old ancient times picture or books..or anything with respect to present time .but we r a part of that universe
there many theory to born universe but what is the reality of big bang theory to born universe
Asmit
what is the exact value of π?
Nagalakshmi
by big bang
universal
there are many theories regarding this it's on you believe any theory that you think is true ex. eternal inflation theory, oscillation model theory, multiple universe theory the big bang theory etc.
Aarya
I think after Big Bang!
Michele
from where on earth could u observe all the stars during the during the course of an year
is that so. the question was in the end of this chapter
Karuna
in theory, you could see them all from the equator (though over the course of a year, not at pne time). stars are measured in "declination", which is how far N or S of the equator (90* to -90*). Polaris is the North star, and is ALMOST 90* (+89*).
So it would just barely creep over the horizon.
Christopher
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