Question 1

What is displacement current?

Accepted Answer

Displacement current is the effective current associated with a changing electric field, even when no charged particle crosses the dielectric gap. Maxwell defined it as I_d = epsilon_0 d(Phi_E)/dt, linking changing electric flux to the magnetic field.

Question 2

Why is displacement current needed for a charging capacitor?

Accepted Answer

Current cannot remain continuous around the circuit if the wire current suddenly disappears inside the capacitor gap. The changing electric field provides the missing effective current in Maxwell's corrected Ampere law, while ordinary conduction current remains zero across the dielectric.

Question 3

What happens to the magnetic field when the capacitor is fully charged?

Accepted Answer

When the capacitor is fully charged, dE/dt becomes zero, so the displacement current and its magnetic-field contribution disappear. The electric field can remain stored between the plates, but a persistent magnetic field should not be shown without continuing current.

Question 4

How are E, B, and propagation related in an electromagnetic wave?

Accepted Answer

In a vacuum electromagnetic wave, the electric field, magnetic field, and direction of propagation are mutually perpendicular. Their magnitudes are related by B = E/c, where c = 1/sqrt(epsilon_0 mu_0) and is approximately 3.0 x 10^8 metres per second.

Question 5

What is the right-hand rule for the magnetic field around a charging capacitor?

Accepted Answer

Point your right thumb in the direction of the displacement current, which is along the changing electric field. Your curled fingers show the azimuthal direction of the magnetic field around the capacitor axis. The same rule is used for conduction current in a straight wire.

Displacement Current & Maxwell's Prediction — The Current That Isn't There

About this lab

Practice