(adsbygoogle = window.adsbygoogle || []).push({}); Please help!!! Electric Field Problem due tomorrow

1. The problem statement, all variables and given/known data

We want to analyze how a charged object picks up a neutral piece of metal. A small circular disk of foil is lying flat on a table. The foil has radius R and thickness t. A glass ball w/ positive charge Q is at height h above the foil.

The ball's electric field polarizes the foil. The foil surfaces with charges +q and -q then act as the the plates of a parallel plate capacitor with separation t. But the foil is a conductor in electrostatic equilibrium so the electric field inside the foil must be zero. Write this condition as a mathematical statement and use it to find the charge on the upper surface of the foil.

2. Relevant equations

3. The attempt at a solution

I know that the electric field of this sphere is Q/4[tex]\pi[/tex][tex]\epsilon[/tex]_{o}r^2

Should the electric field from the ball equal the electric field from the top surface of the plate?

Ecapacitor = [tex]\eta[/tex]/[tex]\epsilon[/tex]_{o}

in the case of the foil disk [tex]\eta[/tex]=q/4[tex]\pi[/tex]r^{2}

so the electric field is then q/4[tex]\pi[/tex]R^{2}[tex]\epsilon[/tex]_{o}

If I set this equal to the electric field of the ball I get

q/4[tex]\pi[/tex]R^{2}[tex]\epsilon[/tex]_{o}=Q/4[tex]\pi[/tex][tex]\epsilon[/tex]_{o}r^2

q=QR^{2}/h^{2}

The answer is actually q=QR^{2}/4h^{2}

Could someone help me understand this better?

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Please help Electric Field Problem due tomorrow

**Physics Forums | Science Articles, Homework Help, Discussion**