Physics: Electricity & Magnetism Flashcards

An electron moves in a circular path in a uniform magnetic field. If the magnetic field strength is doubled, the radius of the circular path:

Answer: Halves

The radius r = mv/(qB); doubling B with fixed speed and mass halves the radius.

A conducting loop is placed in a region where the magnetic flux through it is increasing at a constant rate. The induced EMF in the loop:

Answer: Is constant and opposes the change

By Faraday's Law, EMF = −dΦ/dt; a constant rate of flux change produces a constant induced EMF.

Two point charges +2Q and −Q are separated by distance d. Where on the line through the charges (outside the pair) is the electric field zero?

Answer: On the side of −Q, farther from +2Q

The weaker −Q charge requires a closer distance to match the field of +2Q, so the null point is beyond −Q on its far side.

What is the energy stored in an inductor with inductance L carrying current I?

Answer: ½LI²

The energy stored in an inductor is U = ½LI², analogous to ½kx² for a spring.

A capacitor with capacitance C is connected to a battery of EMF ε. The energy stored in the capacitor is:

Answer: ½Cε²

Energy stored in a capacitor is U = ½CV² = ½Cε² when fully charged to voltage V = ε.

According to Lenz's Law, the direction of the induced current in a loop is such that it:

Answer: Opposes the change in magnetic flux

Lenz's Law states that induced current flows in a direction to oppose the change in flux that caused it, consistent with energy conservation.

A point charge Q is at the center of a spherical shell of radius R with surface charge −Q. The electric field just outside the shell is:

Answer: Zero

The total enclosed charge is Q + (−Q) = 0, so by Gauss's Law the net flux and thus the electric field just outside is zero.