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.