# Energy of an uniformly charged bar of length L

1. Sep 21, 2009

### DaTario

Hi All

I would like to know why is it so difficult to calculate by integration the electric potential energy of an uniformily charged bar of length L and total charge Q. I have tried hard, thinking it would be as easy as the case of a uniformly charged sphere but my efforts failed.

Why these two systems are so different ?

Best Regards

DaTario

2. Sep 21, 2009

### Staff: Mentor

Because there is less symmetry in the bar case than in the sphere case.

3. Sep 21, 2009

### DaTario

Ok with respect to the simmetry, but how this argument relates to divergence in the computation of the energy's integral?

Thanks,

DaTario

4. Sep 21, 2009

### clem

The radius of your line charge is zero. This means an infinite E field at the wire.
The energy calculation is infinite because of this. The energy of a point charge is also infinite. You need finite fields to get finite energies.

5. Sep 22, 2009

ok. good.

thanks

DaTario

6. Oct 1, 2009

### DaTario

Let me just provoke a little further. If my bar (or rod) of length L were a cylinder surface uniformly charged, then no infinite would appear in the energy calculation. Is it?

best regards,

DaTario

7. Oct 2, 2009

### clem

That's right.