A 0.210–A current is charging a capacitor that has circular plates 11.6 cm in radius. The plate separation is 4.00 mm.(a) What is the time rate of increase of electric field between the plates? How is the charge on the plates related to the current? V/(m·s)(b) What is the magnetic field between the plates 5.00 cm from the center? What is the electric flux through a circle of radius 5.00 cm lying parallel to the plates, midway between them? T
Question
A 0.210–A current is charging a capacitor that has circular plates 11.6 cm in radius. The plate separation is 4.00 mm.(a) What is the time rate of increase of electric field between the plates? How is the charge on the plates related to the current? V/(m·s)(b) What is the magnetic field between the plates 5.00 cm from the center? What is the electric flux through a circle of radius 5.00 cm lying parallel to the plates, midway between them? T
Solution
(a) The time rate of increase of electric field between the plates can be calculated using the formula for the current I = dQ/dt, where Q is the charge and t is the time.
The charge on a capacitor is given by Q = CV, where C is the capacitance and V is the voltage. For a parallel plate capacitor, the capacitance is given by C = εA/d, where ε is the permittivity of free space, A is the area of one of the plates, and d is the separation between the plates.
The electric field E between the plates of a capacitor is given by E = V/d.
Substituting the expression for Q from the equation Q = CV into the equation for the current gives I = d(CV)/dt = C dV/dt.
Substituting the expression for E from the equation E = V/d into the equation for the current gives I = C d(Ed)/dt = CdE.
Solving for dE/dt gives dE/dt = I/C.
Substituting the expression for C from the equation C = εA/d gives dE/dt = I/(εA/d) = Id/(εA).
The area of the plates can be calculated using the formula for the area of a circle, A = πr^2, where r is the radius of the plates.
Substituting this into the equation for dE/dt gives dE/dt = Id/(επr^2).
Substituting the given values gives dE/dt = (0.210 A)(0.00400 m)/[(8.85 x 10^-12 C^2/N*m^2)(π)(0.116 m)^2] = 2.02 V/(m·s).
(b) The magnetic field B between the plates can be calculated using Ampere's law, which states that the integral of B over a closed loop is equal to μI, where μ is the permeability of free space and I is the current.
For a circular loop of radius r, the integral of B over the loop is B*2πr, so B = μI/(2πr).
Substituting the given values gives B = (4π x 10^-7 T*m/A)(0.210 A)/(2π(0.0500 m)) = 2.64 x 10^-6 T.
The electric flux Φ through a circle of radius r lying parallel to the plates, midway between them, can be calculated using the formula Φ = EA, where E is the electric field and A is the area of the circle.
Substituting the given values gives Φ = (2.02 V/m)(π(0.0500 m)^2) = 0.159 N*m^2/C.
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