Gaokao Exam Gaokao Physics: Electromagnetism 1

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?

Accepted Answer

0.10 m from the +2 μC charge

Answer

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)

Accepted Answer

0.157 m

Answer

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?

Accepted Answer

628 V

Answer

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:

Accepted Answer

Counterclockwise to oppose the increasing flux

Answer

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?

Accepted Answer

50 Ω

Answer

Z = √(R² + (X_L − X_C)²) = √(40² + 30²) = √(1600 + 900) = 50 Ω.

Gaokao Exam Practice Test

Gaokao Exam Practice Test

Gaokao Exam Gaokao Physics: Electromagnetism 1