# How is this solution derived?

1. Mar 10, 2015

### ShayanJ

In a physics textbook I'm reading, the PDE $\frac{\partial p}{\partial t}=-\mu E \frac{\partial p}{\partial x}+D \frac{\partial^2 p}{\partial x^2}-\frac{p-p_o}{\tau}$ is given where $\mu, \ E, \ D, \ p_o$ and $\tau$ are constants. It is then stated(yeah, just stated!) that the solution for E=0 is $p(x,t)=\frac{N}{\sqrt{4 \pi D t}} \exp{\left( -\frac{x^2}{4 D t}-\frac{t}{\tau} \right)}+p_o$ and that for nonzero E, the only change to the solution is $x \rightarrow x-\mu E t$. But I'm really wondering how did the author get this solution. Its obvious that he didn't use separation of variables. But I know no other method for solving it. What method is used?
Thanks

2. Mar 10, 2015

### the_wolfman

The equation is similar to the heat equation, and the solution is similar to the fundamental solution to the heat equation. This solution to the heat equation is a well known results, and you can find multiple derivations on-line. The general idea is to note that the heat equation is invariant under the transform $x'=ax$ and $t' =a^2 t$. This implies a solution of the form $u(x,t) = u\left(x^2/t \right)$.

It's not clear how to go from the heat equation to your equation. I suspect that there is a clever substitution.

3. Mar 10, 2015

### ShayanJ

But the solution I stated in the OP contains $\frac{1}{\sqrt t}$ and $t$, not only $\frac{x^2}{t}$.

4. Mar 10, 2015

### Staff: Mentor

Do you understand the problem that was being solved for the case where E = 0, say with τ infinite?

Chet

5. Mar 10, 2015

### HomogenousCow

Shift p to p-p0 (p nought) to get rid of the inhomogeneous term. (Doesn't affect the derivatives)
Then fourier transform the equation, the derivation is similar to that of the heat kernel. Throw p0 to the other side and you get that expression.
Sorry for the informal language.

EDIT:I'm bad with LaTex so here's a photo of the first few steps. Just do the same trick with the heat kernel here and you're good.

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Last edited: Mar 10, 2015
6. Mar 10, 2015

### ShayanJ

Its Haynes-Shockley experiment in semiconductor physics and the equation is basically the continuity equation for holes. The first two terms in the R.H.S. are the divergence of drift and diffusion current and the third term is the sink term corresponding to the recombination process. With $\tau \to \infty$, no recombination happens and with E=0, the peak of the hole distribution doesn't move.

Thanks man, that worked out nicely.

7. Mar 11, 2015

### HomogenousCow

No prob. Did you have difficulty with the inverse fourier transform

8. Mar 11, 2015

### ShayanJ

I'd have if I've done it myself, but I used wolframalpha.com!