# Black hole thermodynamics

1. Jul 19, 2016

### MrPhysicsGuy

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
S=kc^3A/4ħG

R(s)=2GM/c^2

Eq. (3) A=πR^2

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

2. Jul 20, 2016

### collinsmark

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

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

So are you asking where the $S = \frac{4 \pi k G M^2}{\hbar c}$ comes from? Here's a wiki link on Black Hole thermodynamics that should help:
https://en.wikipedia.org/wiki/Black_hole_thermodynamics

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

When expressing that in terms of area, make sure you use the area equation for a sphere ($A = 4 \pi r^2$). 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.