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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

full meaning of GPS system
Anaele Reply
how to prove that Newton's law of universal gravitation F = GmM ______ R²
Kaka Reply
sir dose it apply to the human system
Olubukola Reply
prove that the centrimental force Fc= M1V² _________ r
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prove that centripetal force Fc = MV² ______ r
Kaka
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mitul Reply
griffts bridge derivative
Ganesh Reply
below me
please explain; when a glass rod is rubbed with silk, it becomes positive and the silk becomes negative- yet both attracts dust. does dust have third types of charge that is attracted to both positive and negative
Timothy Reply
what is a conductor
Timothy
hello
Timothy
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why below you
Timothy
no....I said below me ...... nothing below .....ok?
dust particles contains both positive and negative charge particles
Mbutene
corona charge can verify
Stephen
when pressure increases the temperature remain what?
Ibrahim Reply
what is frequency
Mbionyi Reply
define precision briefly
Sujitha Reply
CT scanners do not detect details smaller than about 0.5 mm. Is this limitation due to the wavelength of x rays? Explain.
MITHRA Reply
hope this helps
what's critical angle
Mahmud Reply
The Critical Angle Derivation So the critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees. Make particular note that the critical angle is an angle of incidence value. For the water-air boundary, the critical angle is 48.6-degrees.
dude.....next time Google it
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Chidalu
pls who can give the definition of relative density?
Temiloluwa
the ratio of the density of a substance to the density of a standard, usually water for a liquid or solid, and air for a gas.
Chidalu
What is momentum
aliyu Reply
mass ×velocity
Chidalu
it is the product of mass ×velocity of an object
Chidalu
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Sean Reply
then you can edit your work anyway you want
Wat is the relationship between Instataneous velocity
Oyinlusi Reply
Instantaneous velocity is defined as the rate of change of position for a time interval which is almost equal to zero
Astronomy
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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