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Homework Help: Temperature(thermal radiation transfer)

  1. Mar 1, 2010 #1
    1. A solid cylinder of radius r1 = 2.3 cm, length h1 = 4.2 cm, emissivity 0.88, and temperature 24°C is suspended in an environment of temperature 55°C. (a) What is the cylinder's net thermal radiation transfer rate P1? (b) If the cylinder is stretched until its radius is r2 = 0.52 cm, its net thermal radiation transfer rate becomes P2. What is the ratio P2/P1?

    2. P1 = ([tex]\sigma[/tex]) ([tex]\epsilon[/tex]) A1(Tenv4 - T4)

    A1 = 2([tex]\pi[/tex]) r12 + 2 ([tex]\pi[/tex]) r h

    3. A1 = 9.393362*10-3m2
    P1 = (5.67*10-8)(.88)(9.393362*10-3)((273+55)4-(273+24)4)) = 1.77W

    I have (b) .. but for some reason (a) is wrong.. here is answer for (b) [tex]\frac{P2}{P1}[/tex] = 2.87
    Last edited: Mar 2, 2010
  2. jcsd
  3. Mar 1, 2010 #2
    Ok I just did it in mathmatica .. and it gave the same answer .. If
    my intuition is incorrect then the only possible solution would be
    that P1 is somehow =([tex]\sigma[/tex]) ([tex]\epsilon[/tex]) A1((24+273)4) ... but thats not how the question reads
  4. Mar 2, 2010 #3
    I asked a friend he agrees with me.. but are we both wrong? not sure why the answer is considered wrong
  5. Mar 2, 2010 #4
    had another person agree.. anyway I could get someone to review this problem and confirm?
  6. Mar 3, 2010 #5
    any attempts?
  7. Mar 25, 2010 #6
    ok I've resolved the problem. Both the book, and their online website have two distinct wrong equations to use.. The correct one would involve flipping the Tenv^4 with the T^4.. so it would read (T^4 - Tenv^4). So, I dub this problem as resolved in case anyone was interested.
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