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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 Q t = kA ( T 2 T 1 ) d size 12{ { {Q} over {t} } = { { ital "kA" \( T rSub { size 8{2} } - T rSub { size 8{1} } \) } over {d} } } {} is appropriate. [link] lists thermal conductivities. For convection, determine the amount of matter moved and use equation Q = mc Δ T size 12{Q= ital "mc"ΔT} {} , to calculate the heat transfer involved in the temperature change of the fluid. If a phase change accompanies convection, equation Q = mL f size 12{Q= ital "mL" rSub { size 8{f} } } {} or Q = mL v is appropriate to find the heat transfer involved in the phase change. [link] lists information relevant to phase change. For radiation, equation Q net t = σ e A T 2 4 T 1 4 size 12{ { {Q rSub { size 8{"net"} } } over {t} } =σ`e`A` left (T rSub { size 8{2} } rSup { size 8{4} } - T rSub { size 8{1} } rSup { size 8{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:
    Q t = σ e A T 4 , size 12{ { {Q} over {t} } =σ`e`A`T rSup { size 8{4} } } {}

    where σ = 5 .67 × 10 8 J/s m 2 K 4 is the Stefan-Boltzmann constant and e size 12{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

    Q net t = σ e A T 2 4 T 1 4 size 12{ { {Q rSub { size 8{"net"} } } over {t} } =σ`e`A` left (T rSub { size 8{2} } rSup { size 8{4} } - T rSub { size 8{1} } rSup { size 8{4} } right )} {}

    where T 1 size 12{T rSub { size 8{1} } } {} is the temperature of an object surrounded by an environment with uniform temperature T 2 size 12{T rSub { size 8{2} } } {} and e size 12{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.

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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?

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Why are cloudy nights generally warmer than clear ones?

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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?

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

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Problems&Exercises

At what net rate does heat radiate from a 275 -m 2 size 12{"275""-m" rSup { size 8{2} } } {} black roof on a night when the roof’s temperature is 30. C and the surrounding temperature is 15. C size 12{"15" "." 0°C} {} ? The emissivity of the roof is 0.900.

21 . 7  kW size 12{ - "21" "." 7`W} {}
Note that the negative answer implies heat loss to the surroundings.

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(a) Cherry-red embers in a fireplace are at 850º C and have an exposed area of 0 . 200  m 2 and an emissivity of 0.980. The surrounding room has a temperature of 18 . 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?

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Questions & Answers

what there factors affect the surface tension of a liquid
Promise Reply
formula for impedance
muyiwa Reply
ehat is central forces
Nita Reply
what is distance?
Jonathan Reply
What does mean ohms law imply
Victoria Reply
ohms law state that the electricity passing through a metallic conductor is directly proportional to the potential difference across its end
muyiwa
what is matter
folajin Reply
Anything that occupies space
Kevin
Any thing that has weight and occupies space
Victoria
Anything which we can feel by any of our 5 sense organs
Suraj
Right
Roben
thanks
Suraj
what is a sulphate
Alo
any answers
Alo
the time rate of increase in velocity is called
Blessing Reply
acceleration
Emma
What is uniform velocity
Victoria
Greetings,users of that wonderful app.
Frank Reply
how to solve pressure?
Cruz Reply
how do we calculate weight and eara eg an elefant that weight 2000kg has four fits or legs search of surface eara is 0.1m2(1metre square) incontact with the ground=10m2(g =10m2)
Cruz
P=F/A
Mira
can someone derive the formula a little bit deeper?
Bern
what is coplanar force?
OLADITI Reply
forces acting and lying on d same plane
Promise
what is accuracy and precision
Peace Reply
How does a current follow?
Vineeta Reply
follow?
akif
which one dc or ac current.
akif
how does a current following?
Vineeta
?
akif
AC current
Vineeta
AC current follows due to changing electric field and magnetic field.
akif
you guys are just saying follow is flow not follow please
Abubakar
ok bro thanks
akif
flows
Abubakar
but i wanted to understand him/her in his own language
akif
but I think the statement is written in English not any other language
Abubakar
my mean that in which form he/she written this,will understand better in this form, i write.
akif
ok
Abubakar
ok thanks bro. my mistake
Vineeta
u are welcome
Abubakar
what is a semiconductor
Vineeta Reply
substances having lower forbidden gap between valence band and conduction band
akif
what is a conductor?
Vineeta
replace lower by higher only
akif
convert 56°c to kelvin
Abubakar
How does a current follow?
Vineeta
A semiconductor is any material whose conduction lies between that of a conductor and an insulator.
AKOWUAH
what is Atom? what is molecules? what is ions?
Abubakar Reply
atoms are the smallest unit of an element which is capable of behaving as a single unit
Promise
a molecule is d smallest unit of a substances capable of independent existence and can also retain the chemical proper ties of that substance
Promise
an ion is referred to as freely moving charged particles
Promise
What is a molecule
Samuel Reply
Is a unit of a compound that has two or more atoms either of the same or different atoms
Justice
A molecule is the smallest indivisible unit of a compound, Just like the atom is the smallest indivisible unit of an element.
Rachel
what is a molecule?
Vineeta
Practice Key Terms 5

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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