potential energy question

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

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.

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

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?
Also, watch your sig. figs.

wml

well, actually I'm still confused but I entered the answer and it was right >,< I didn't skip any words... what I put on the first post is exactly I have on my paper