Gaokao Physics: Electromagnetism Flashcards

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?

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.

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)

Answer: 0.157 m

r = mv/(qB) = (1.67×10⁻²⁷ × 3×10⁶)/(1.6×10⁻¹⁹ × 0.20) ≈ 0.157 m.

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?

Answer: 628 V

EMF_max = NBAω = 100 × 0.5 × 0.04 × 2π × 50 ≈ 628 V.

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:

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.

In a series RLC circuit, R = 40 Ω, X_L = 70 Ω, X_C = 40 Ω. What is the impedance of the circuit?

Answer: 50 Ω

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

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:

Answer: Cε²/4

Series capacitance is C/2; energy = ½(C/2)ε² = Cε²/4.

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:

Answer: 1.6 V

EMF = BLv = 0.8 × 0.5 × 4 = 1.6 V.