# The Magnetic Field in a Charging Capacitor

1. Mar 9, 2009

### cupcake

A parallel-plate capacitor of capacitance C with circular plates is charged by a constant current I. The radius a of the plates is much larger than the distance d between them, so fringing effects are negligible. Calculate B(r), the magnitude of the magnetic field inside the capacitor as a function of distance from the axis joining the center points of the circular plates.

according to Ampère's law as extended by Maxwell:
$$\oint \vec{B} \cdot d\vec{l}= \mu_0\left(I+ \epsilon_0 \frac{d\Phi}{dt}\right).$$

what should i do then?

2. Mar 10, 2009

### cupcake

anyone can help me???

3. Mar 10, 2009

### nealh149

Pick an amperian surface, and calculate either current or electric flux through that surface. That will give you the value of the right side of the equation. Then solve for B.

4. Mar 11, 2009

### cupcake

how to calculate flux through the that surface??