AP Physics Physics: Electricity & Magnetism 2 — Questions and Answers
Question 1: A parallel-plate capacitor is fully charged and then disconnected from the battery. If the plate separation is doubled, what happens to the voltage across the capacitor?
- It halves
- It doubles (Correct answer)
- It stays the same
- It quadruples
Correct answer: It doubles
With the battery disconnected, charge Q is constant; since C = ε₀A/d and V = Q/C, doubling d halves C and therefore doubles V.
Question 2: Two long parallel wires carry currents in the same direction. The magnetic force between them is:
- Repulsive
- Attractive (Correct answer)
- Zero
- Dependent only on their resistance
Correct answer: Attractive
Parallel currents in the same direction create magnetic fields that cause the wires to attract each other.
Question 3: A proton moves horizontally to the right through a uniform magnetic field directed into the page. The magnetic force on the proton is directed:
- Into the page
- Out of the page
- Downward
- Upward (Correct answer)
Correct answer: Upward
Using F = qv × B with v to the right (+x) and B into the page (−z), the cross product gives force in the +y (upward) direction.
Question 4: Which of the following correctly describes the electric field inside a conductor in electrostatic equilibrium?
- It equals the surface charge density divided by ε₀
- It is zero (Correct answer)
- It points radially outward from the center
- It equals the external applied field
Correct answer: It is zero
In electrostatic equilibrium, free charges redistribute until the net electric field inside the conductor is exactly zero.
Question 5: A solenoid with N turns, length L, and cross-sectional area A carries current I. Its self-inductance is:
- μ₀NA/L
- μ₀N²A/L (Correct answer)
- μ₀NI/L
- μ₀N²I/L
Correct answer: μ₀N²A/L
The self-inductance of a solenoid is L = μ₀N²A/ℓ, derived from the flux linkage NΦ = N·(μ₀NI/ℓ)·A.
Question 6: Three resistors of 2 Ω, 4 Ω, and 6 Ω are connected in parallel. What is the equivalent resistance?
- 12 Ω
- 1.09 Ω (Correct answer)
- 4 Ω
- 0.92 Ω
Correct answer: 1.09 Ω
1/R_eq = 1/2 + 1/4 + 1/6 = 6/12 + 3/12 + 2/12 = 11/12, so R_eq = 12/11 ≈ 1.09 Ω.
Question 7: Gauss's Law states that the net electric flux through any closed surface equals:
- The total charge enclosed divided by ε₀ (Correct answer)
- The product of electric field and surface area
- The surface charge density times the area
- Zero for any closed surface
Correct answer: The total charge enclosed divided by ε₀
Gauss's Law: Φ_E = Q_enc/ε₀, where Q_enc is the total charge enclosed by the Gaussian surface.
AP Physics Physics: Electricity & Magnetism 2
A parallel-plate capacitor is fully charged and then disconnected from the battery.
If the plate separation is doubled, what happens to the voltage across the capacitor?