Is Helmholtz equation a Poisson Equation?

[yungman]

Helmholtz equation:$\nabla^2 u=-ku$ is the same form of $\nabla^2 u=f$.

So is helmholtz equation a form of Poisson Equation?

 

[SteamKing]

They're both second order PDEs, but the Poisson f is a more general function, not necessarily related to the unknown function u. If the function f is 0, then the Poisson equation reduces to the Laplace equation.

In the solution of certain types of the Helmholtz equation, the separation of variables can be utilized.

http://en.wikipedia.org/wiki/Helmholtz_equation

http://en.wikipedia.org/wiki/Poisson's_equation

The generality of 'f' in Poisson's equation makes it trickier to solve than Laplace.

 

[yungman]

Thanks for the reply, I understand the difference between the two. But Helmholtz is also in form of Poisson, only when $f=-ku$. So, can I say Helmholtz is a subset or one form of Poission Equation?

Thanks

 

[SteamKing]

Thanks for the reply, I understand the difference between the two. But Helmholtz is also in form of Poisson, only when $f=-k\nabla^2 u$. So, can I say Helmholtz is a subset or one form of Poission Equation?

Thanks

I think you mean when f = -ku

FWIW, sure, go ahead.

 

[yungman]

I think you mean when f = -ku

FWIW, sure, go ahead.

Yes, my bad. What is FWIW?

Thanks

 

[SteamKing]

FWIW = For What It's Worth

 

[dextercioby]

Leaving the chat speak aside, generally speaking the only connection between Poisson's equation and Helmholtz equation is that they are both elliptic 2nd order linear PDEs. One is not a particular case of the other, as posts 2 and especially 3,4 above insinuate.

 

[yungman]

Thanks everyone.