Question

Two plates identical in size, one of black and rough surface $\left(B_{1}\right)$ and the other smooth and polished $\left(A_{2}\right)$ are interconnected by a thin horizontal pipe with a mercury pellet at the centre. Two more plates $A_{1}$ (identical to $A_{2}$ ) and $B_{2}$ (identical to $B_{1}$ ) are heated to the same temperature and placed closed to the plates $B_{1}$, and $A_{2}$ as shown infigure. The mercury pellet

• A. moves to the right
• B. moves to the left
• C. remains stationary
• D. starts oscillating left and right

Answer 1

Answer: (C)

Smooth and polished plates are poor radiators of heat. Hence, heat coming out from $A$ is small, even though $B$ being a black and rough plate is a good absorber. Effectively the heat coming to the left of pellet $P$ is small.
Black and rough plates are good radiators of heat. Hence, plate $B_{2}$ radiates heat to a satisfactory level; however, plate $A_{2}$, being smooth and polished, is a bad absorber. Effectively, the heat coming to the right of $P$ is also small.