Q30. A 20 Ω resistance wire is cut into two equal lengths and the parts are placed in parallel. Calculate the resulting resistance.
[UPSC NDA and Naval Academy — 2020 (Combined Examination)]
(A) 5 Ω
(B) 10 Ω
(C) 20 Ω
(D) 40 Ω
Answer: (A) 5 Ω
Explanation: Cutting the uniform wire in half produces two 10 Ω pieces. Connecting these two equal resistances in parallel yields 10/2 = 5 Ω. Because both shortening and adding a branch reduce resistance, the total becomes one-quarter of the original.
Q29. What occurs in the electric current when a circuit is shorted?
[UPSC NDA and Naval Academy — 2020 (Combined Examination)]
(A) Current decreases greatly
(B) Current suddenly becomes very large
(C) Current necessarily reverses direction
(D) Current becomes exactly zero
Answer: (B) Current suddenly becomes very large
Explanation: An accidental low-resistance path bypasses the intended load, producing an excessive current from the supply. The resulting I²R heating may damage cables or start a fire. This is why distribution circuits include protective fuses or breakers.
Q28. Which electrical instrument detects the presence of small currents?
[UPSC NDA and Naval Academy — 2020 (Combined Examination)]
(A) Galvanometer
(B) Transformer
(C) Electroscope
(D) Dynamo
Answer: (A) Galvanometer
Explanation: A sensitive galvanometer detects small electric currents through the deflection of a moving coil or other indicator. With suitable added resistances, its operating range can be adapted to measure larger currents or potential differences as an ammeter or voltmeter.
Q27. What is the name of the mechanism by which moving conducting fluid in Earth’s outer core generates electric currents and magnetism?
[RPSC Senior Teacher Grade II (Science) — 4 July 2019]
(A) Joule effect
(B) Seebeck effect
(C) Thomson effect
(D) Dynamo effect
Answer: (D) Dynamo effect
Explanation: The geodynamo operates as molten, electrically conducting iron moves through magnetic fields in Earth’s outer core. Induced currents reinforce the magnetic field, allowing it to persist. Joule heating and thermoelectric effects describe different physical processes.
Q26. An electrical safety fuse wire should have which combination of properties?
[UPSC NDA — 21 April 2019]
(A) Low melting point and suitable relatively high resistivity
(B) High melting point and zero resistivity
(C) Very high melting point and zero resistance
(D) No conductivity whatsoever
Answer: (A) Low melting point and suitable relatively high resistivity
Explanation: When a fault causes excessive current, the fuse element must heat and melt before the protected wiring becomes unsafe. Suitable resistivity allows sufficient resistive heating, while a low melting point ensures prompt interruption. A conductor that never melts cannot serve this purpose.
Q25. Does Ohm’s law apply to every conducting material under every condition?
[UPSC NDA — 2019-II]
(A) Yes, at all temperatures and voltages
(B) No, only materials with linear voltage-current behaviour obey it
(C) Yes, because all resistances are zero
(D) Only magnetic materials obey it
Answer: (B) No, only materials with linear voltage-current behaviour obey it
Explanation: Ohm’s law describes conductors whose current varies linearly with applied voltage under fixed physical conditions. Diodes, filament bulbs across changing temperatures, and many electrolytes can show nonlinear characteristics. Hence the voltage-to-current ratio need not remain constant in all materials.
Q24. Current produced by convection of metallic liquid inside Earth helps create its magnetic field. What is this called?
[RPSC Senior Teacher Grade II (Science) — 1 November 2018]
(A) Seebeck effect
(B) Thomson effect
(C) Dynamo effect
(D) Meissner effect
Answer: (C) Dynamo effect
Explanation: Earth’s flowing molten core is conductive. Its motion through an existing magnetic field induces electric currents which in turn generate magnetic fields. This self-sustaining feedback is the dynamo mechanism that explains much of the planet’s global magnetism.
Q23. Which SI unit measures electrical resistance?
[RRB Group D — 31 October 2018]
(A) Ampere
(B) Ohm
(C) Coulomb
(D) Farad
Answer: (B) Ohm
Explanation: The ohm is the SI derived unit of electrical resistance. One ohm is the resistance through which one ampere flows under one volt. Amperes measure current, coulombs measure charge, and farads are units of capacitance.
Q22. Which is the SI unit of electrical potential difference?
[RRB Group D — 26 October 2018]
(A) Watt
(B) Volt
(C) Ampere
(D) Ohm
Answer: (B) Volt
Explanation: Potential difference is energy transferred per unit charge and has the SI unit volt. One volt equals one joule per coulomb. Unlike the ampere, it does not express the rate of flow of charge through a conductor.
Q21. Which law relates heat generated in an electrical conductor to I²Rt?
[RRB Group D — 11 October 2018]
(A) Joule’s law of heating
(B) Newton’s law of cooling
(C) Coulomb’s law
(D) Faraday’s second law only
Answer: (A) Joule’s law of heating
Explanation: Joule established that heat released in a resistor grows with the square of current, its resistance and the duration of current flow. The equation H = I²Rt is crucial when sizing heaters, fuses, and wiring against thermal damage.
