# Use the Fourier transform directly to solve the heat equation

1. Apr 12, 2014

### richyw

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

Use the Fourier transform directly to solve the heat equation with a convection term
$$u_t =ku_{xx} +\mu u_x,\quad −infty<x<\infty,\: u(x,0)=\phi(x), assuming that u is bounded and k > 0. 2. Relevant equations fourier transform inverse fourier transform convolution thm 3. The attempt at a solution taking the FT of both sides i get [tex]U_t=-k w^2U-iw\mu U$$
$$U(0,t)=\Phi(w,0)$$
I solved the ode and got
$$U(w)=e^{(\mu i w- w^2k)t}$$
but now I am a bit confused on the next step, is this where I want to get my initial condition involved, or do I want to try and get it back as u(x,t) using inverse FT. I can see that my solution is a gaussian multiplied by another function of F, so I think I might be able to use convolution thm?

2. Apr 13, 2014

### vela

Staff Emeritus
Don't you mean $U(\omega,0) = \Phi(\omega,0)$?

You left out the arbitrary constant when you solved for $U(\omega,t)$. You should have $U(\omega,t) = A(\omega) e^{(i\mu\omega-k\omega^2)t}.$