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Black hole thermodynamics

  1. Jul 19, 2016 #1
    1. The problem statement, all variables and given/known data
    I would very much like getting some help with my problem regarding the equations in some black hole thermodynamics.

    "Using the expression for the Schwarzschild radius, the entropy of a black hole of event-horizon area A=πR^2 can be written in terms of its mass using Eq. (1) as S=4πkGM^2/ħc. As mass is lost, the change in entropy will be dS=8πkGmdm/ħc..."

    I don't understand how they got S=4πkGM^2/ħc and dS=8πkGmdm/ħc.

    2. Relevant equations
    Eq. (1) Entropy

    Eq. (2) Schwarzshild radius

    Eq. (3) A=πR^2

    3. The attempt at a solution
    Thanks for helping and have a wonderful day :)

  2. jcsd
  3. Jul 20, 2016 #2


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    First there's a mistake in the problem statement. The equation that was given was [itex] A = \pi R^2 [/itex]. That's the area of a circle, not a sphere. You should use the area equation for a sphere,
    [tex] A = 4 \pi R^2. [/tex]

    Assuming that [itex] S = \frac{4 \pi k G M^2}{\hbar c} [/itex] is correct, you should be able to derive [itex] dS = \frac{8 \pi k G M \ dM}{\hbar c} [/itex] easily enough; it is just a simple derivative.

    So are you asking where the [itex] S = \frac{4 \pi k G M^2}{\hbar c} [/itex] comes from? Here's a wiki link on Black Hole thermodynamics that should help:

    The Schwartzschild radius is typically given by [itex] R = \frac{2 M G}{c^2} [/itex], by the way.

    When expressing that in terms of area, make sure you use the area equation for a sphere ([itex] A = 4 \pi r^2 [/itex]). Don't use the area equation for a circle.

    That, the Bekenstein–Hawking formula given in the above link, and a bit of substitution should get you to your answer.
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