Electricity and Magnetism MCQs
Physics: Electricity & Magnetism MCQs
1. If the distance between two equal point charges is doubled, the electrostatic force between them becomes:
- A) Half of the original force
- B) Double the original force
- C) One-fourth of the original force
- D) Four times the original force
View Answer & Explanation
Answer: C
Explanation: According to Coulomb’s Law, F ∝ 1/r². If r is doubled, the new force becomes F/(2)² = F/4.
2. The electric field intensity inside a hollow charged spherical conductor is:
- A) Infinite
- B) Zero
- C) Uniform but non-zero
- D) Dependent on the radius of the sphere
View Answer & Explanation
Answer: B
Explanation: Charges reside only on the outer surface of a conductor. Therefore, by Gauss’s Law, the net charge inside is zero, making the electric field zero.
3. The work done in moving a charge of 5 Coulombs from one point to another on an equipotential surface is:
- A) 5 Joules
- B) 25 Joules
- C) Zero
- D) Infinite
View Answer & Explanation
Answer: C
Explanation: On an equipotential surface, the potential difference (ΔV) between any two points is zero. Work done W = q(ΔV) = 0.
4. Three capacitors of capacitances 2 µF, 3 µF, and 6 µF are connected in series. The equivalent capacitance is:
- A) 11 µF
- B) 1 µF
- C) 1.2 µF
- D) 36 µF
View Answer & Explanation
Answer: B
Explanation: For capacitors in series: 1/Cs = 1/C1 + 1/C2 + 1/C3. Therefore, 1/Cs = 1/2 + 1/3 + 1/6 = 3/6 + 2/6 + 1/6 = 6/6 = 1. Cs = 1 µF.
5. If a uniform wire of resistance R is stretched to double its original length, its new resistance will be:
- A) R
- B) 2R
- C) 4R
- D) R/2
View Answer & Explanation
Answer: C
Explanation: Volume remains constant. If length is doubled (l’ = 2l), area becomes half (A’ = A/2). Resistance R = ρ(l/A). New resistance R’ = ρ(2l / (A/2)) = 4 ρ(l/A) = 4R.
6. Kirchhoff’s First Law (Junction Rule) is a consequence of the conservation of:
- A) Energy
- B) Momentum
- C) Charge
- D) Mass
View Answer & Explanation
Answer: C
Explanation: The junction rule states that total current entering a junction equals the total current leaving it, which is the principle of conservation of electric charge.
7. The drift velocity of free electrons in a conductor is directly proportional to:
- A) The square of the electric field
- B) The applied electric field
- C) The volume of the conductor
- D) The reciprocal of the electric field
View Answer & Explanation
Answer: B
Explanation: Drift velocity (Vd) = (eE/m)τ, where E is the applied electric field. Thus, Vd ∝ E.
8. An ideal voltmeter should have:
- A) Zero resistance
- B) Very low resistance
- C) Very high but finite resistance
- D) Infinite resistance
View Answer & Explanation
Answer: D
Explanation: An ideal voltmeter draws no current from the circuit so it does not alter the potential difference it is measuring. Therefore, it must have infinite resistance.
9. A charged particle moves parallel to a uniform magnetic field. The magnetic force on the particle is:
- A) Maximum
- B) Zero
- C) Dependent on the particle’s mass
- D) Circular
View Answer & Explanation
Answer: B
Explanation: Magnetic force F = qvB sin(θ). If the particle moves parallel to the field, θ = 0°, so sin(0°) = 0, and the force is zero.
10. The path of a charged particle entering a uniform magnetic field at a right angle is:
- A) Straight line
- B) Parabola
- C) Circle
- D) Helix
View Answer & Explanation
Answer: C
Explanation: When velocity is perpendicular to the magnetic field, the magnetic force acts as a centripetal force, causing the particle to move in a circular path.
11. How can a galvanometer be converted into an ammeter?
- A) By connecting a high resistance in series
- B) By connecting a high resistance in parallel
- C) By connecting a low resistance in series
- D) By connecting a low resistance in parallel
View Answer & Explanation
Answer: D
Explanation: A low resistance (shunt) connected in parallel allows most of the current to bypass the galvanometer, thereby increasing its measuring range and converting it into an ammeter.
12. The magnetic susceptibility of a diamagnetic substance is:
- A) Small and positive
- B) Large and positive
- C) Small and negative
- D) Zero
View Answer & Explanation
Answer: C
Explanation: Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field, resulting in a small, negative magnetic susceptibility.
13. What is the value of the angle of dip at the Earth’s magnetic poles?
- A) 0°
- B) 45°
- C) 90°
- D) 180°
View Answer & Explanation
Answer: C
Explanation: At the magnetic poles, the Earth’s magnetic field lines point vertically downwards or upwards, making an angle of 90° with the horizontal.
14. Lenz’s Law of electromagnetic induction strictly adheres to the principle of conservation of:
- A) Charge
- B) Mass
- C) Momentum
- D) Energy
View Answer & Explanation
Answer: D
Explanation: Lenz’s Law states the induced current opposes the change in magnetic flux. The mechanical work done against this opposing force is converted into electrical energy, conserving total energy.
15. The core of a transformer is laminated to reduce energy losses due to:
- A) Flux leakage
- B) Copper loss
- C) Eddy currents
- D) Hysteresis
View Answer & Explanation
Answer: C
Explanation: Laminating the core breaks the conducting path, significantly increasing electrical resistance and thereby reducing the magnitude of eddy currents and the resulting heat loss.
16. The SI unit of magnetic flux is the:
- A) Tesla
- B) Weber
- C) Henry
- D) Gauss
View Answer & Explanation
Answer: B
Explanation: Tesla is the unit of magnetic field strength (B), while Weber (Wb) is the unit of magnetic flux (Φ).
17. The power factor of a purely inductive AC circuit is:
- A) 1
- B) 0.5
- C) 0
- D) Infinite
View Answer & Explanation
Answer: C
Explanation: In a purely inductive circuit, the phase difference (φ) between voltage and current is 90° (π/2). The power factor is cos(φ) = cos(90°) = 0.
18. At resonance in an LCR series circuit, the phase difference between voltage and current is:
- A) π/2
- B) π
- C) π/4
- D) 0
View Answer & Explanation
Answer: D
Explanation: At resonance, inductive reactance equals capacitive reactance (Xl = Xc). The circuit behaves as a purely resistive circuit, meaning voltage and current are in phase (angle = 0).
19. Electromagnetic waves are produced by:
- A) Static charges
- B) Charges in uniform motion
- C) Accelerating charges
- D) Uncharged particles
View Answer & Explanation
Answer: C
Explanation: A static charge produces only an electric field. A charge in uniform motion produces both electric and magnetic fields but no waves. Accelerating charges emit electromagnetic radiation.
20. The magnetic field at the center of a circular current-carrying coil of radius R and current I is given by:
- A) μ₀I / 2πR
- B) μ₀I / 2R
- C) μ₀I / 4πR
- D) μ₀I / R
View Answer & Explanation
Answer: B
Explanation: According to the Biot-Savart law, the magnetic field at the center of a single circular loop is B = μ₀I / 2R. (μ₀I / 2πR is for a straight infinite wire).
21. In a purely capacitive AC circuit, the relationship between current and voltage is:
- A) Current lags voltage by π/2
- B) Current leads voltage by π/2
- C) Current and voltage are in phase
- D) Current leads voltage by π
View Answer & Explanation
Answer: B
Explanation: In a capacitor, current leads the voltage by a phase angle of 90 degrees (π/2 radians). ICE is a helpful mnemonic (Current ‘I’ leads Voltage ‘E’ in a Capacitor ‘C’).
22. The Biot-Savart Law in magnetism plays a role analogous to which law in electrostatics?
- A) Gauss’s Law
- B) Faraday’s Law
- C) Coulomb’s Law
- D) Ampere’s Law
View Answer & Explanation
Answer: C
Explanation: Coulomb’s law calculates the electric field produced by an infinitesimal charge element, whereas the Biot-Savart law calculates the magnetic field produced by an infinitesimal current element.
23. Which of the following electromagnetic waves has the highest frequency?
- A) X-rays
- B) Ultraviolet rays
- C) Gamma rays
- D) Microwaves
View Answer & Explanation
Answer: C
Explanation: On the electromagnetic spectrum, Gamma rays possess the shortest wavelength and thus the highest frequency and highest energy.
24. The cyclotron frequency of a charged particle moving in a magnetic field is independent of its:
- A) Mass
- B) Charge
- C) Speed
- D) Magnetic field strength
View Answer & Explanation
Answer: C
Explanation: The cyclotron frequency is f = qB / (2πm). It depends on charge, magnetic field, and mass, but is completely independent of the particle’s speed (or radius of its path).
25. In an AC circuit, the average power dissipated over a complete cycle is given by:
- A) V_rms * I_rms
- B) V_rms * I_rms * sin(φ)
- C) V_rms * I_rms * cos(φ)
- D) Zero
View Answer & Explanation
Answer: C
Explanation: True power in an AC circuit is given by P = V_rms * I_rms * cos(φ), where cos(φ) is known as the power factor.
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