Question

Two thin bars, PQ and RS stick to each other as shown in the figure.

Which of the following can you conclude from it?

A. PQ and RS are both magnets.

B. PQ is definitely a magnet. RS is most probably an iron bar that is attracted to the magnet.

C. Either PQ or RS is a magnet. We cannot be sure wether the other one is a magnet or not.

D. Nothing can be said with certainty about either of them.​

Answer 1

Answer:

A. PQ and RS are both magnets.

B. PQ is definitely a magnet. RS is most probably an iron bar that is attracted to the magnet.

C. Either PQ or RS is a magnet. We cannot be sure wether the other one is a magnet or not.

D. Nothing can be said with certainty about either of them.

Answer 2

Let the temperature of the junction be T.

∴ rate of heat transfer =dtdQ=L2KA(T−10)=LKA(400−T)

⇒ 2(T−10)=400−T

or, T=1400C

Now, for the wire PQ, let us imagine a small length Δx at a distance x from the junction.

∴ ΔxΔT=1140−10=130

So, temperature at distance x:

T=10+130x

or, T−10=130x

Increase in length of the small element Δx is expressed as:

dxdy=αΔT=α(T−10)

or, dxdy=α×130x

Integrating both sides, we get:

∫0ΔLdy=130α∫0Lxdx

∵L=1 m  (Given)

or, ΔL=2130αx2=0.78130×1.2×10−5