Gaokao Exam Gaokao Physics: Electromagnetism 3 — Questions and Answers
Question 1: Coulomb's law states that the force between two point charges is proportional to:
- The product of the charges divided by the square of the distance (Correct answer)
- The product of the charges divided by the distance
- The sum of the charges divided by the square of the distance
- The product of the charges multiplied by the distance
Correct answer: The product of the charges divided by the square of the distance
Coulomb's law: F = kq₁q₂/r², force proportional to charge product and inversely proportional to distance squared.
Question 2: The magnetic force on a current-carrying conductor of length L carrying current I in a field B at angle θ is:
- F = BIL sinθ (Correct answer)
- F = BIL cosθ
- F = BIL tanθ
- F = BIL/sinθ
Correct answer: F = BIL sinθ
The Ampere force is F = BIL sinθ, maximum when current is perpendicular to the field.
Question 3: A 10 μF capacitor is charged to 100 V. The energy stored in the capacitor is:
- 0.05 J (Correct answer)
- 0.10 J
- 0.005 J
- 0.20 J
Correct answer: 0.05 J
U = ½CV² = ½ × 10×10⁻⁶ × 100² = 0.05 J.
Question 4: When the distance between two parallel plates of a capacitor (connected to a battery) is doubled, the capacitance:
- Halves (Correct answer)
- Doubles
- Stays the same
- Quadruples
Correct answer: Halves
C = ε₀A/d; doubling d halves C.
Question 5: In an AC circuit, the root-mean-square (rms) voltage across a 10 Ω resistor is 20 V. The average power dissipated is:
- 40 W (Correct answer)
- 20 W
- 80 W
- 10 W
Correct answer: 40 W
P = V_rms²/R = 400/10 = 40 W.
Question 6: An alpha particle (charge +2e) moves at 2×10⁶ m/s perpendicular to a 0.3 T magnetic field. The magnitude of the force on it is: (e = 1.6×10⁻¹⁹ C)
- 1.92×10⁻¹³ N (Correct answer)
- 9.6×10⁻¹⁴ N
- 3.84×10⁻¹³ N
- 4.8×10⁻¹⁴ N
Correct answer: 1.92×10⁻¹³ N
F = qvB = 2 × 1.6×10⁻¹⁹ × 2×10⁶ × 0.3 = 1.92×10⁻¹³ N.
Question 7: Kirchhoff's voltage law (KVL) states that:
- The algebraic sum of all voltages around any closed loop is zero (Correct answer)
- The sum of currents entering a junction equals the sum leaving
- Voltage across resistors in series is equal
- Current through parallel branches is equal
Correct answer: The algebraic sum of all voltages around any closed loop is zero
KVL is based on conservation of energy: the sum of voltage rises and drops around any closed loop equals zero.
Gaokao Exam Gaokao Physics: Electromagnetism 3
Coulomb's law states that the force between two point charges is proportional to: