# Potential energy question

1. Mar 12, 2006

### wml

An infinite sheet of charge that has a surface charge density of 21 nC/m2 lies in the yz plane, passes through the origin and is at a potential of 1.3 kV.
A long wire having a linear charge density of 50 nC/m lies parallel to the y axis and intersects the x axis at 3.2m.
The permittivity of a vacuum is 8.85419*10^-12 C2/N.m2

What is the potential energy of a 2.4 nC charge placed at x = 1.5 m in units of J.

can anybody help me..
I dont know how to use the 1.3 kV potential
I used V=（入/ 2Eoл）× Ln(r1/r2) , U=V×q
but I entered the answer, it was wrong

Last edited: Mar 12, 2006
2. Mar 12, 2006

### Meir Achuz

The problem is not stated correctly.
If the infinite sheet has a uniform charge density, and there are other charges present, it can’t be an equipotential.

3. Mar 12, 2006

### wml

never mind~ I got the right answer~!!!! ^^: finally

Vb-Va=-21n×1.5/2Eo -> Vb=1300-1779.661= -479.661

VB-VA(0)=50n/ (2лEo) × Ln(3.2/1.7) = 568.7519 3.2-1.5=1.7

U=q(Vb+VB) = 2.4n × (568.7519-479.661)=2.13818×10^-7

4. Mar 13, 2006

### Meir Achuz

But that answer is wrong because of my first post.
If there is a charged wire and a point charge present, the plane cannot be at a constant potential of 1.3 kV.
Is it stated that that is the potential at the origin or on the whole plane?