Question

Two identical slabs are welded end to end and $20cal$ of heat flows through it for $4min$. If the two slabs are now welded by placing them one above the other, and the same heat is flowing through two ends under the same difference of temperatures, the time taken is

• A. $1min$
• B. $2min$
• C. $4min$
• D. $16min$

Answer 1

The correct option is A $1min$

The rate of heat transfer is directly proportional to the area and inversely to length. So when slabs are placed on top of one another, area is doubled and length is halved, compared to the case when slabs are welded side-by-side.

So rate of heat transfer becomes $2/0.5=4$ times.

So time taken to supply same amount of heat will become $1/4$ times.

Its value becomes $4\times 1/4=1min$