Gaokao Exam Gaokao Physics: Electromagnetism 1 — Questions and Answers
Question 1: Two point charges of +2 μC and +8 μC are separated by a distance of 0.30 m in vacuum. At what point along the line joining them is the electric field zero?
- 0.10 m from the +2 μC charge (Correct answer)
- 0.10 m from the +8 μC charge
- 0.15 m from each charge
- 0.20 m from the +8 μC charge
Correct answer: 0.10 m from the +2 μC charge
The field is zero where kQ₁/r² = kQ₂/(d−r)²; solving gives r = 0.10 m from the smaller charge.
Question 2: A proton moves with velocity 3×10⁶ m/s perpendicular to a uniform magnetic field of 0.20 T. What is the radius of the circular orbit? (mp = 1.67×10⁻²⁷ kg, e = 1.6×10⁻¹⁹ C)
- 0.157 m (Correct answer)
- 0.314 m
- 0.628 m
- 0.078 m
Correct answer: 0.157 m
r = mv/(qB) = (1.67×10⁻²⁷ × 3×10⁶)/(1.6×10⁻¹⁹ × 0.20) ≈ 0.157 m.
Question 3: A rectangular coil of 100 turns with area 0.04 m² rotates at 50 rev/s in a uniform magnetic field of 0.5 T. What is the maximum EMF induced?
- 628 V (Correct answer)
- 314 V
- 200 V
- 1256 V
Correct answer: 628 V
EMF_max = NBAω = 100 × 0.5 × 0.04 × 2π × 50 ≈ 628 V.
Question 4: According to Lenz's law, when a bar magnet's north pole approaches a horizontal conducting loop from above, the induced current in the loop, viewed from above, flows:
- Counterclockwise to oppose the increasing flux (Correct answer)
- Clockwise to oppose the increasing flux
- In no specific direction
- Alternating rapidly
Correct answer: Counterclockwise to oppose the increasing flux
The induced current creates a magnetic field opposing the increasing upward flux, which by the right-hand rule requires counterclockwise current viewed from above.
Question 5: In a series RLC circuit, R = 40 Ω, X_L = 70 Ω, X_C = 40 Ω. What is the impedance of the circuit?
- 50 Ω (Correct answer)
- 70 Ω
- 110 Ω
- 30 Ω
Correct answer: 50 Ω
Z = √(R² + (X_L − X_C)²) = √(40² + 30²) = √(1600 + 900) = 50 Ω.
Question 6: Two parallel plate capacitors, each with capacitance C, are connected in series to a battery of EMF ε. The energy stored in the combination is:
- Cε²/4 (Correct answer)
- Cε²/2
- Cε²
- 2Cε²
Correct answer: Cε²/4
Series capacitance is C/2; energy = ½(C/2)ε² = Cε²/4.
Question 7: A conducting rod of length 0.5 m moves at 4 m/s perpendicular to a uniform magnetic field of 0.8 T. The induced EMF is:
- 1.6 V (Correct answer)
- 0.8 V
- 3.2 V
- 0.4 V
Correct answer: 1.6 V
EMF = BLv = 0.8 × 0.5 × 4 = 1.6 V.
Gaokao Exam Gaokao Physics: Electromagnetism 1
Two point charges of +2 μC and +8 μC are separated by a distance of 0.30 m in vacuum.
At what point along the line joining them is the electric field zero?