# Homework Help: Electric potential of a conductor.

1. Aug 21, 2010

### Terocamo

1. The problem statement, all variables and given/known data
I know the definition of electric potential is the work done to bring a test charge from infinity to a particular point.
I wonder what that means, whether it is the potential of the surface of the conductor or otherwise.

Also, I dont understand the concept of bringing a earthed object close to a charged conductor will
decrease the magnitude of potential of that charged object.
eg. my notes mention the induced charge inside the eathed object contrubutes a positive potential
to make the potential of the negative charged conductor LESS negative. Itsn't an earthed object
have zero potential? So how can it contrubute any potential.

Thz

2. Aug 21, 2010

### kuruman

What do you mean by "otherwise"? A conductor is an equipotential which means that all points that make up this conductor whether on the surface or underneath the surface are at the same potential.

The earthed conductor is always at zero potential. If you bring it close to a negatively charged conductor, free electrons inside the earthed conductor will be pushed into the earth as they are repelled by the negative charges on the other conductor. The loss of negative charges in the earthed conductor raises the potential of the negatively charged conductor because it is essentially a gain of positive charge in that region of space.

3. Aug 21, 2010

### Terocamo

Thz first, you clear some of my wrong concepts, but i am still puzzled by how the potential is raised.
I now understand the earthed object gains positive charge and produce a field to compensate
the potential due to the negative charge conductor in order to maintain zero potential. (I hope
thats a right concept )
Is it that potential due to the earthed object raised the potential of the charged object?

4. Aug 21, 2010

### Terocamo

Ive got another question about electric field.

There is a spherical metallic shell of uniform thickness which is neutral. A positive charge is put inside it
but not anywhere close to the centre of the spherical shell. Why is it the charge deposit on the outer and inner surface of the
shell distrubute unitformly?
Itsnt the electrostatic force stronger if closer? I thought the negative charge will gathered
at a pt close the the positive charge and it will disturb the distrubution of charge.

5. Aug 21, 2010

### kuruman

Yes. The potential of the earthed object is always zero.

6. Aug 21, 2010

### kuruman

The charge on the outer surface only is distributed uniformly if the charge inside the cavity is off center. To get uniform distribution on the inner surface, you need to have the charge in the center. You can change the charge distribution on the inner surface by moving the charge in the cavity around, but that does not affect the charge distribution on the outer surface because the electric field inside the conductor is always zero so the inside distribution cannot communicate to the outside distribution what is happening to it.

7. Aug 21, 2010

### Terocamo

Thz again, it is very helpful.