Gaokao Physics: Electromagnetism Flashcards

A proton and an electron are placed in the same uniform electric field. Which statement is correct?

Answer: They experience equal and opposite forces but different accelerations

F = qE; since |q_proton| = |q_electron|, forces are equal in magnitude but opposite in direction. However masses differ, so accelerations differ (a = F/m).

The energy stored in an inductor with inductance L = 0.5 H carrying a current of 4 A is:

Answer: 4 J

U = ½LI² = ½ × 0.5 × 16 = 4 J.

A square loop of side 0.1 m rotates at 100 rad/s in a magnetic field of 2 T. The peak EMF is:

Answer: 2 V

EMF_max = NBAω = 1 × 2 × (0.1)² × 100 = 2 V.

In the photoelectric effect experiment, increasing the intensity of light while keeping frequency constant causes:

Answer: More electrons to be emitted but with the same maximum kinetic energy

Higher intensity means more photons per second, so more electrons emitted; maximum KE depends only on photon frequency, not intensity.

Two point charges +3 μC and −3 μC are 0.30 m apart. The force between them is: (k = 9×10⁹ N·m²/C²)

Answer: 0.9 N attractive

F = kq₁q₂/r² = 9×10⁹ × 9×10⁻¹²/0.09 = 0.9 N; opposite charges attract.

Eddy currents in a metal plate moving through a magnetic field result in:

Answer: A braking force opposing the motion

By Lenz's law, induced eddy currents create a force that opposes the motion causing them, acting as electromagnetic braking.

The skin effect in AC conductors refers to:

Answer: Current concentration near the conductor surface at high frequencies

At high frequencies, electromagnetic effects cause current to concentrate in a thin surface layer (skin depth decreases with frequency), increasing effective resistance.