# Homework Help: Laplace Equation in a Annulus

1. Nov 21, 2012

### joelcponte

1. The problem statement, all variables and given/known data

Heat equation in a annulus, steady state solution.

u(a,θ,t) = Ta
u(b,θ,t) = Tbcos(θ)

2. Relevant equations

Using separation of Variables

$\frac{}{}\frac{1}{r}\frac{d}{d r}(r\frac{d R}{d r}) + \frac{1}{r^2}\frac{d^2\Theta}{d \theta} = 0$

3. The attempt at a solution

I found

u(r,θ,t) = $\alpha_0 + \beta_0 ln(r) + \sum (\alpha_n r^n + \beta_n r^{-n})(\gamma_n cos(n\theta) + \sigma_n sin(n\theta))$

u(r,θ,t) = $\alpha_0 + \beta_0 ln(r) + \sum (\alpha_n r^n + \beta_n r^{-n})cos(n\theta)$

(this is before applying the boundary conditions)

2. Nov 21, 2012

### vela

Staff Emeritus
Your answer is correct. The second solution is what you get after you apply the boundary condition at r=b.

3. Nov 21, 2012

### joelcponte

Hi, I can't see how does the sin term disappear when I use the boundary condition at b =\

4. Nov 21, 2012

### vela

Staff Emeritus
Actually, I was a little careless. What they did was to say that both boundary conditions are even functions of θ, so the solution must also be an even function of θ, which implies you can throw out the sine terms. So you're not exactly applying the boundary conditions yet, but you are using some information gleaned from them.

5. Nov 21, 2012

### joelcponte

Oh, really? I didn't know that! Do you know anywhere I can read about it?