UPSC Prelims 1998 · General Studies

Question 128: Science and Technology

Assertion (A): The temperature of a metal wire rises when an electric current is passed through it. Reason (R): Collision of metal atoms with each other releases heat energy.

aBoth A and R are true, and R is the correct explanation of A

bBoth A and R are true, but R is NOT a correct explanation of A

cA is true, but R is false

dA is false, but R is true

AnswerThe answer is (c).

A current heats a metal wire (Joule heating), so A is true. R is false as stated: the heat is produced when the moving free electrons collide with the vibrating metal ions of the lattice and give up energy to them, not by collisions of the metal atoms with each other. The heat produced is I squared R t.

A. A current raises the temperature of a metal wire (Joule heating).
R. The heat comes from electrons colliding with the lattice, not atoms colliding with each other.

How you could have got this

If you were sure statement 1 is correct, (d) goes. Then judge the second statement; if it is correct too, decide whether it explains the first.

This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.

Which options go is worked out from UPSC’s key and the options, not by a model.

Why each option is right or wrong

(a) R is false, so it cannot explain A.
(b) R is false, not merely a poor explanation.
(c) Right: A is true; R is false because the electrons, not the atoms, collide.
(d) A is true, so this fails.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 1998, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.