AP Physics Physics: Electricity & Magnetism 3

Question 1

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:

Accepted Answer

Halves

Answer

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

Question 2

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:

Accepted Answer

Is constant and opposes the change

Answer

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

Question 3

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?

Accepted Answer

On the side of −Q, farther from +2Q

Answer

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.

Question 4

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

Accepted Answer

½LI²

Answer

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

Question 5

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

Accepted Answer

½Cε²

Answer

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

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