# Magnetic Field in Capacitor

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1. Jan 2, 2015

### Ben Whelan

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

A cylindrical parallel plate capacitor of radius R is discharged by an external current I. The
total electric field flux inside the capacitor changes at a rate dΦe/dt = I/ ε0. What is the
strength of the resulting magnetic field B(r, I) inside the capacitor at a radial distance r
from the centre axis? Start the answer with Maxwell’s extension to Ampere’s law.

2. Relevant equations

So using the line integral ∫B⋅dl= μ0 (I + Id )

3. The attempt at a solution

you get B⋅2πr= μ0 (I + Id )

However i don't understand what I and Id are?

I have the equation Id= ε0e/dt

however i don't understand why this is true or where this takes me?

2. Jan 3, 2015

### rude man

You have an equation and you don't know what the symbols represent? May I suggest your first step is to find that out.
(Hint: the problem tells you what I is; it's given.)

Last edited: Jan 3, 2015
3. Jan 3, 2015

### Ben Whelan

Sorry i should have qualified, I know what the symbols represent, and i realize that I_d is given, i also assume I is 0. However what i don't understand is why?

4. Jan 3, 2015

### rude man

Actually, I is given, not Id. I is the current in the wiring, not inside the capacitor. Inside the capacitor, I = 0 as you say.