Q80. An electric bell mainly operates by which effect of electric current?
[SSC CGL — 16 September 2025, Shift 1]
(A) Chemical effect
(B) Magnetic effect
(C) Photoelectric effect
(D) Cooling effect
Answer: (B) Magnetic effect
Explanation: When current passes through the bell’s coil, the soft-iron core becomes an electromagnet. It attracts the armature and moves the striker; the contact repeatedly breaks and reconnects. The resulting rapid vibration produces the ringing sound.
Q79. A circuit needs which arrangement for a steady electric current?
[RRB NTPC Undergraduate — 29 August 2025, Shift 1]
(A) An open wire without a source
(B) A closed conducting path with a source
(C) Only a resistor
(D) Only an insulating switch
Answer: (B) A closed conducting path with a source
Explanation: Current persists when there is a complete conducting loop and an energy source maintaining a potential difference. An open switch interrupts the path, stopping ordinary conduction. A resistor alone cannot sustain a continuous current without an applied source.
Q78. An electric motor primarily uses which effect of current, rather than the heating effect?
[RRB NTPC Undergraduate — 29 August 2025, Shift 1]
(A) Magnetic effect
(B) Evaporation effect
(C) Radioactive effect
(D) Capillary effect
Answer: (A) Magnetic effect
Explanation: Electric motors exploit the forces experienced by current-carrying conductors in magnetic fields. These forces produce torque and rotate the shaft. Some resistive heat is unavoidably produced in the windings, but heating is not the motor’s useful primary action.
Q77. A charge of 100 C passes a cross-section in 50 s. What is the mean current?
[RRB NTPC Undergraduate — 29 August 2025, Shift 1]
(A) 0.5 A
(B) 2 A
(C) 5 A
(D) 50 A
Answer: (B) 2 A
Explanation: Average current is the quantity of charge transported per second. Dividing Q = 100 C by t = 50 s gives I = 2 A. Larger charge transferred in the same time corresponds to greater current.
Q76. How much potential difference corresponds to 24 joules of work transferring 2 coulombs of charge?
[UPSC NDA — 2025-I]
(A) 6 V
(B) 12 V
(C) 24 V
(D) 48 V
Answer: (B) 12 V
Explanation: Potential difference is work done per unit charge, V = W/Q. Dividing 24 joules by 2 coulombs gives 12 volts. This is the energy transferred to each coulomb as it moves between the two circuit points.
Q75. Three identical 1 Ω resistors are connected in parallel. Find their equivalent resistance.
[UPSC NDA — 2025-I]
(A) 3 Ω
(B) 1 Ω
(C) 1/3 Ω
(D) 1/2 Ω
Answer: (C) 1/3 Ω
Explanation: Reciprocals of resistances add for parallel branches. Here 1/R = 1 + 1 + 1 = 3 per ohm, so R = 1/3 Ω. Adding parallel pathways always decreases the equivalent resistance.
Q74. What is the SI unit used to express specific electrical resistance or resistivity?
[SSC CGL — 26 September 2024]
(A) Ohm
(B) Ohm metre
(C) Ampere per metre
(D) Volt per ampere
Answer: (B) Ohm metre
Explanation: The resistivity relation is ρ = RA/L. Resistance is measured in ohms, area in square metres and length in metres; consequently the derived SI unit is ohm metre, Ω m. Resistivity is not measured in plain ohms.
Q73. Which is the SI base unit of electric current?
[SSC CGL — 13 September 2024]
(A) Coulomb
(B) Volt
(C) Ampere
(D) Watt
Answer: (C) Ampere
Explanation: The ampere is an SI base unit and measures electric current. A coulomb is the derived unit of charge, equal to one ampere-second. The volt measures potential difference, whereas the watt measures the rate of energy transfer.
Q72. Which rule determines the direction of the magnetic field surrounding a straight current-carrying wire?
[SSC JE — 7 June 2024]
(A) Right-hand thumb rule
(B) Pascal’s law
(C) Hooke’s law
(D) Archimedes’ principle
Answer: (A) Right-hand thumb rule
Explanation: Point the right thumb in the conventional current direction; the curled fingers indicate the magnetic field lines surrounding the conductor. This is the right-hand thumb rule, frequently used to establish field orientation in electromagnetism.
Q71. Approximately how many electrons pass a point in a wire each second when the current is 1 A?
[SSC JE — 5 June 2024]
(A) 6.25 × 10¹²
(B) 6.25 × 10¹⁵
(C) 6.25 × 10¹⁸
(D) 6.25 × 10²¹
Answer: (C) 6.25 × 10¹⁸
Explanation: One ampere transports one coulomb of charge per second. The magnitude of the elementary charge is approximately 1.6 × 10⁻¹⁹ C, so 1/(1.6 × 10⁻¹⁹) yields about 6.25 × 10¹⁸ electrons each second.
