Physics: Electricity & Magnetism Flashcards

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?

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.

Two long parallel wires carry currents in the same direction. The magnetic force between them is:

Answer: Attractive

Parallel currents in the same direction create magnetic fields that cause the wires to attract each other.

A proton moves horizontally to the right through a uniform magnetic field directed into the page. The magnetic force on the proton is directed:

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.

Which of the following correctly describes the electric field inside a conductor in electrostatic equilibrium?

Answer: It is zero

In electrostatic equilibrium, free charges redistribute until the net electric field inside the conductor is exactly zero.

A solenoid with N turns, length L, and cross-sectional area A carries current I. Its self-inductance is:

Answer: μ₀N²A/L

The self-inductance of a solenoid is L = μ₀N²A/ℓ, derived from the flux linkage NΦ = N·(μ₀NI/ℓ)·A.

Three resistors of 2 Ω, 4 Ω, and 6 Ω are connected in parallel. What is the equivalent resistance?

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.

Gauss's Law states that the net electric flux through any closed surface equals:

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.