Gaokao Exam Gaokao Physics: Electromagnetism 6 — Questions and Answers
Question 1: The electric field inside a conductor in electrostatic equilibrium is:
- Zero (Correct answer)
- Equal to the surface field
- Maximum at the centre
- Proportional to the charge density
Correct answer: Zero
Free charges in a conductor redistribute until the internal field is zero; all excess charge resides on the surface.
Question 2: A negatively charged particle enters a magnetic field directed into the page moving to the right. The particle experiences a force directed:
- Downward (Correct answer)
- Upward
- Into the page
- Out of the page
Correct 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.
Question 3: Two identical resistors are first connected in series, then in parallel, to the same battery. The ratio of total power dissipated (parallel/series) is:
- 4 (Correct answer)
- 2
- 1/2
- 1/4
Correct 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.
Question 4: The resonance frequency of an LC circuit with L = 4 mH and C = 100 pF is:
- ~252 kHz (Correct answer)
- ~25.2 kHz
- ~2.52 MHz
- ~252 Hz
Correct answer: ~252 kHz
f = 1/(2π√(LC)) = 1/(2π√(4×10⁻³ × 10⁻¹⁰)) ≈ 252 kHz.
Question 5: The electric flux through a closed surface surrounding a charge of +Q is (in SI units):
- Q/ε₀ (Correct answer)
- Qε₀
- Q/(4πε₀)
- 4πQ/ε₀
Correct answer: Q/ε₀
Gauss's law: Φ_E = Q_enc/ε₀.
Question 6: When the frequency of an AC source in a series RLC circuit equals the resonance frequency, the impedance equals:
- R (Correct answer)
- X_L
- X_C
- X_L + X_C
Correct answer: R
At resonance X_L = X_C, so Z = √(R² + 0²) = R.
Question 7: 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:
- 0 Wb (Correct answer)
- 0.01 Wb
- 0.02 Wb
- 0.04 Wb
Correct 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.
Gaokao Exam Gaokao Physics: Electromagnetism 6
The electric field inside a conductor in electrostatic equilibrium is: