AP Physics Physics: Electricity & Magnetism 3 — Questions and Answers
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:
- Doubles
- Halves (Correct answer)
- Stays the same
- Quadruples
Correct answer: Halves
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:
- Is zero since the rate is constant
- Is constant and opposes the change (Correct answer)
- Increases over time
- Depends on the loop's resistance
Correct 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.
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?
- On the side of +2Q, farther from −Q
- On the side of −Q, farther from +2Q (Correct answer)
- Midway between the charges
- Nowhere outside the pair
Correct 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.
Question 4: What is the energy stored in an inductor with inductance L carrying current I?
- LI
- ½LI² (Correct answer)
- LI²
- ½LI
Correct answer: ½LI²
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:
- Cε
- ½Cε² (Correct answer)
- Cε²
- ½Cε
Correct answer: ½Cε²
Energy stored in a capacitor is U = ½CV² = ½Cε² when fully charged to voltage V = ε.
Question 6: According to Lenz's Law, the direction of the induced current in a loop is such that it:
- Enhances the change in magnetic flux
- Opposes the change in magnetic flux (Correct answer)
- Is always clockwise when viewed from above
- Maximizes the power dissipated
Correct 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.
Question 7: 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:
- kQ/R²
- Zero (Correct answer)
- 2kQ/R²
- k(2Q)/R²
Correct 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.
AP Physics Physics: Electricity & Magnetism 3
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: