Q70. A magnetic field can be produced by which of the following?
[SSC GD Constable — 1 March 2024]
(A) A moving electric charge
(B) An isolated stationary neutron
(C) A wooden block at rest
(D) A stationary neutral molecule only
Answer: (A) A moving electric charge
Explanation: Moving electrical charges constitute current and produce magnetic fields. A stationary electric charge creates an electric field, whereas motion contributes a magnetic field. This connection underlies the magnetic force around current-carrying wires and the action of electromagnets.
Q69. A circuit contains 5 Ω and 7 Ω in series, connected in parallel with 36 Ω. What is the equivalent resistance?
[UPSC NDA — 2024-II]
(A) 9 Ω
(B) 12 Ω
(C) 18 Ω
(D) 48 Ω
Answer: (A) 9 Ω
Explanation: The first branch has series resistance 5 + 7 = 12 Ω. Combining it in parallel with 36 Ω gives R = (12 × 36)/(12 + 36) = 9 Ω. The result is lower than either branch resistance.
Q68. In an alternating-current supply of frequency 50 Hz, how often does the current reverse direction?
[UPSC NDA — 2024-II]
(A) Every 0.02 s
(B) Every 0.01 s
(C) Every 0.05 s
(D) Every 1 s
Answer: (B) Every 0.01 s
Explanation: A 50 Hz waveform completes fifty cycles per second, so one full period lasts 1/50 = 0.02 second. Polarity and current direction reverse after each half-cycle, meaning reversals occur every 0.01 second.
Q67. Which among electric current, current density, power and wattless current is a vector?
[WBCS Mains (Science) — 2023]
(A) Electric current
(B) Current density
(C) Power
(D) Wattless current
Answer: (B) Current density
Explanation: Current density includes the direction of charge transport through a surface and is therefore a vector quantity. Circuit current is treated as a signed scalar, while electrical power has magnitude without direction. Current density is measured in amperes per square metre.
Q66. Who experimentally discovered the magnetic effect of electric current?
[WBCS Mains (Science) — 2023]
(A) Michael Faraday
(B) Hans Christian Oersted
(C) André-Marie Ampère
(D) Niels Bohr
Answer: (B) Hans Christian Oersted
Explanation: In 1820 Oersted observed that a nearby magnetic compass needle deflected when current flowed through a wire. This demonstrated a connection between electricity and magnetism. Faraday later discovered electromagnetic induction, while Ampère developed mathematical descriptions of magnetic forces.
Q65. Which is NOT an application of the chemical effect of an electric current?
[SSC CHSL — 16 August 2023, Shift 1 (memory-based)]
(A) Electrolysis
(B) Electroplating
(C) Electromagnetism
(D) Galvanization
Answer: (C) Electromagnetism
Explanation: Electromagnetism concerns magnetic fields generated by moving charges, so it is the magnetic rather than chemical effect of current. Electrolysis and electroplating involve reactions driven by electrical energy. Some industrial galvanizing methods also use electrochemical zinc deposition.
Q64. Which description correctly identifies an electrical conductor?
[SSC MTS — 19 June 2023]
(A) A substance that allows electric charge to flow
(B) A material that prohibits all electron motion
(C) A body that always has zero temperature
(D) A substance that creates electricity without energy
Answer: (A) A substance that allows electric charge to flow
Explanation: Conductors contain mobile charged particles and allow electrical current when a potential difference is applied. Metals possess mobile electrons, while electrolyte solutions conduct through ions. Insulators have relatively few freely mobile carriers under normal conditions.
Q63. An electric heater rated 220 V draws 0.5 A. What current would it draw at 120 V if its resistance stays unchanged?
[UPSC NDA — 2023-II]
(A) 0.27 A
(B) 0.50 A
(C) 0.92 A
(D) 1.10 A
Answer: (A) 0.27 A
Explanation: The heater resistance at rated voltage is R = 220/0.5 = 440 Ω. At the reduced voltage, Ohm’s law predicts I = 120/440 ≈ 0.273 A. The approximation assumes resistance does not change with temperature.
Q62. Which alloy is commonly chosen as a heating element in an electric iron?
[UPSC NDA — 2023-II]
(A) Solder
(B) Nichrome
(C) Brass
(D) Soft iron
Answer: (B) Nichrome
Explanation: Nichrome combines high electrical resistivity with a high melting point and resistance to oxidation. Consequently it can produce heat without melting rapidly. Soft iron is useful for electromagnet cores, whereas low-melting solder is unsuited for a heating coil.
Q61. A wire is stretched to twice its original length without loss of material. How does its resistance change?
[UPSC NDA — 2023-I]
(A) Becomes one-fourth
(B) Becomes one-half
(C) Doubles
(D) Becomes four times
Answer: (D) Becomes four times
Explanation: If the wire’s volume stays constant, doubling length halves cross-sectional area. Since R = ρL/A, both changes multiply resistance, making it four times the original. With constant applied voltage, the resulting current falls to one-quarter.
