UPPSC 2025 Prelims GS Paper-I · Question 104 of 150

Correct — B, (A) is true, but (R) is false. The Assertion is the standard textbook statement and it is correct: the wire in an ordinary rewirable fuse is an alloy of lead and tin. The Reason states the physics backwards. A lead-tin alloy melts at a very LOW temperature, not a high one — tin melts at about 232 degrees Celsius, lead at about 328, and their common alloys somewhere around 180 to 250 degrees. Copper melts at about 1,085 degrees Celsius and aluminium at about 660. So the fuse alloy melts far below copper and aluminium, not above them. That low melting point is the whole reason the alloy is chosen: the fuse is meant to be the deliberate weak link in the circuit, the one piece that gives way first when the current rises above the rated value, so that the copper or aluminium house wiring never gets hot enough to be damaged. Since (A) stands and (R) falls, the code is (b).

• (a)Both (A) and (R) are true, but (R) is not the correct explanation of (A) — This requires (R) to be true, and it is not. The claim that a lead-tin alloy melts at a higher temperature than copper and aluminium is factually wrong by roughly 800 degrees Celsius in the case of copper. A statement cannot be a non-explanatory truth when it is not a truth at all.
• (c)(A) is false, but (R) is true — This inverts the position exactly. The Assertion is the true half — lead-tin alloy really is the usual fuse-wire material — and the Reason is the false half. Anyone marking this option has read the two statements the wrong way round.
• (d)Both (A) and (R) are true and (R) is the correct explanation of (A) — The most seductive option, because a single word makes the difference. Had (R) read 'a LOWER melting point than copper and aluminium', it would have been both true and the correct explanation of (A), and (d) would have been the answer. As printed, (R) says 'higher', which reverses the property that makes the alloy usable, so (d) fails on the truth of (R) before the question of explanation even arises.

A fuse is a safety device deliberately built to be the weakest point of a circuit. It is a short, thin piece of wire connected in series in the live line, made of a metal or alloy chosen so that it melts at a low temperature. When current flows, heat is generated according to Joule's law, H = I squared times R times t, so the heating rises with the square of the current. If a short circuit or an overload pushes the current past the fuse's rated value, the thin low-melting element reaches its melting point long before the thick copper or aluminium conductors of the installation do; the fuse melts, the circuit is broken, and the wiring and appliances are saved. Lead-tin alloy suits the job because it combines a low melting point with an appreciable resistivity in a wire thin enough to heat quickly.

Assertion-reason questions in physics are usually decided on the Reason, and the examiner's favourite device is to state a real property with its direction reversed. Read (R) as a claim about numbers and check it against values you should already carry: copper about 1,085 degrees Celsius, aluminium about 660, tin about 232, lead about 328. Once you see that the alloy melts hundreds of degrees below both metals named, (R) collapses and only option (b) survives. Note also that this booklet prints the assertion-reason codes in a non-standard order — (a) is 'both true, R is not the explanation' and (d) is 'both true, R is the explanation' — but here that ordering does not change the outcome, because the answer rests on (R) being false.

• Fuse wire is generally an alloy of lead and tin; pure tin melts at about 232 degrees Celsius and pure lead at about 328, and their common alloys melt in the range of roughly 180 to 250 degrees
• Copper melts at about 1,085 degrees Celsius and aluminium at about 660 degrees Celsius, so both melt far above any lead-tin alloy — the exact opposite of what Reason (R) claims
• The fuse is always connected in series in the live wire, so that melting it breaks the circuit; connecting it in parallel would defeat the purpose
• Heat produced in a conductor follows Joule's law, H = I squared R t, so doubling the current quadruples the rate of heating and an overload melts a thin fuse element very quickly
• Modern domestic installations increasingly replace the rewirable fuse with a miniature circuit breaker (MCB), which trips mechanically on overload and can be reset rather than replaced

• Reading past the single reversed word in the Reason; 'higher melting point' instead of 'lower melting point' turns a correct explanation into a false statement
• Assuming a fuse works by having a very high resistance; it needs a low melting point and only an appreciable resistance in a thin element
• Thinking the fuse can be placed in parallel or in the neutral line; it belongs in series in the live wire

UPPSC states this as an assertion-reason pair or as a one-line property question about fuse material, always turning on the low melting point; UPSC has asked the same idea directly — which statement about a fuse is correct, and what solder is made of — so the tin-lead pairing is worth memorising with its approximate melting range.

• practice — not a real PYQ

  In a domestic electrical circuit, the fuse should be connected

  • (a)in parallel with the appliance
  • (b)in series in the live wire
  • (c)in series in the neutral wire only
  • (d)between the neutral and the earth wire

  Answer(b) in series in the live wire — melting the fuse then breaks the circuit and isolates the appliance from the live supply.

• practice — not a real PYQ

  According to Joule's law, the heat produced in a conductor of resistance R carrying a current I for a time t is proportional to

  • (a)I R t
  • (b)I² R t
  • (c)I R² t
  • (d)I² R² t

  Answer(b) I² R t — because the heating grows with the square of the current, even a modest overload melts a thin fuse element quickly.