A brilliant student of physics developed a magnetic balance to weight objects. The mass `m` to be measured is hung from the center the bar. Bar is kept in a uniform magnetic field of `1.5 T` directed into the plane of the figure. Battery voltage can be adjusted to vary the current in the circuit. The horizontal bar shown is `60cm` long and is made of extremly light weight material. It is connected to the battery via a resistance. There is no tension in the supporting wired. The magnetic force only supports the hanging weight.

Which point of battery is positive?

Options

A. (a) `A`
B. (b) `B`
C. (c ) either `A or B`
D. (d) cannot be found because time of rotation is not given.

Answer

A

Explanation

(a) Current should enter the bar from `P` so that magnetic force is upward.
`ilB = mg or (V)/(5)lB = mg`
or `m = (150 xx 0.6 xx 1.5)/(5 xx 10) = 2.7 kg`