# Magnetic Field in Capacitor

• Ben Whelan

## Homework Statement

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.
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## Homework Equations

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

## 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?
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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:
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.)

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?

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?
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.