Physics Electricity and Magnetism 2 — Questions and Answers
Question 1: Two parallel plates are separated by 0.01 m with a voltage of 100 V across them. What is the electric field strength between the plates?
- 1,000 V/m
- 10,000 V/m (Correct answer)
- 100 V/m
- 0.1 V/m
Correct answer: 10,000 V/m
Electric field E = V/d = 100 V / 0.01 m = 10,000 V/m.
Question 2: A magnetic field exerts no force on a charged particle when the particle moves:
- Perpendicular to the field
- At 45° to the field
- Parallel to the field (Correct answer)
- Opposite to the field direction
Correct answer: Parallel to the field
The magnetic force F = qv×B is zero when velocity is parallel (or anti-parallel) to B, since sin(0°) = 0.
Question 3: Which law relates the line integral of the magnetic field around a closed loop to the current enclosed by that loop?
- Faraday's Law
- Gauss's Law
- Biot-Savart Law
- Ampere's Law (Correct answer)
Correct answer: Ampere's Law
Ampere's Law states ∮B·dl = μ₀I_enclosed.
Question 4: A capacitor fully charged to 12 V is connected to a resistor. After one time constant τ, the voltage across the capacitor is approximately:
- 0 V
- 4.4 V (Correct answer)
- 7.6 V
- 12 V
Correct answer: 4.4 V
After one time constant, V = V₀·e⁻¹ ≈ 12 × 0.368 ≈ 4.4 V.
Question 5: The energy stored in an inductor carrying current I with inductance L is:
- LI
- ½LI² (Correct answer)
- LI²
- ½L²I
Correct answer: ½LI²
The energy stored in an inductor is U = ½LI².
Question 6: What happens to the capacitance of a parallel-plate capacitor if the plate area is doubled while the separation is also doubled?
- Capacitance doubles
- Capacitance halves
- Capacitance stays the same (Correct answer)
- Capacitance quadruples
Correct answer: Capacitance stays the same
C = ε₀A/d; doubling both A and d gives C = ε₀(2A)/(2d) = ε₀A/d, unchanged.
Question 7: In an AC circuit, the impedance of a pure capacitor with capacitance C at angular frequency ω is:
- ωC
- 1/(ωC) (Correct answer)
- ωL
- R
Correct answer: 1/(ωC)
Capacitive reactance X_C = 1/(ωC), which is the impedance magnitude for a pure capacitor.
Physics Electricity and Magnetism 2
Two parallel plates are separated by 0.01 m with a voltage of 100 V across them.
What is the electric field strength between the plates?