Gaokao Physics: Electromagnetism Flashcards

The electric field inside a conductor in electrostatic equilibrium is:

Answer: Zero

Free charges in a conductor redistribute until the internal field is zero; all excess charge resides on the surface.

A negatively charged particle enters a magnetic field directed into the page moving to the right. The particle experiences a force directed:

Answer: Downward

F = qv × B; for negative charge moving right (v = +x̂) in field into the page (B = −ẑ): v × B = x̂ × (−ẑ) = +ŷ, then F = (−e)(+ŷ) = −ŷ, i.e. downward.

Two identical resistors are first connected in series, then in parallel, to the same battery. The ratio of total power dissipated (parallel/series) is:

Answer: 4

P = V²/R_eq. Series R_eq = 2R; parallel R_eq = R/2. P_par/P_ser = (R/2)⁻¹/(2R)⁻¹ = 2R/(R/2) = 4.

The resonance frequency of an LC circuit with L = 4 mH and C = 100 pF is:

Answer: ~252 kHz

f = 1/(2π√(LC)) = 1/(2π√(4×10⁻³ × 10⁻¹⁰)) ≈ 252 kHz.

The electric flux through a closed surface surrounding a charge of +Q is (in SI units):

Answer: Q/ε₀

Gauss's law: Φ_E = Q_enc/ε₀.

When the frequency of an AC source in a series RLC circuit equals the resonance frequency, the impedance equals:

Answer: R

At resonance X_L = X_C, so Z = √(R² + 0²) = R.

A rectangular wire loop of area 0.02 m² is placed in a uniform magnetic field of 0.5 T with its plane parallel to the field. The magnetic flux through the loop is:

Answer: 0 Wb

Φ = BA cosθ; when the plane is parallel to B, the normal to the loop is perpendicular to B, so θ = 90° and Φ = 0.