A potentiometer wire of length 1 m is connected to a driver cell of emf 3 V. When a cell of emf 1.5V is used in the secondary circuit, the balance points is found to be 60 cm. On replacing this cell with a cell of inknown emf, the balance points shifts to 80 cm.

(i) Calculate unknown emf of the cell. (ii) Explain with reason, whether the circuit works, if the drivere cell is replaced with a cell of emf 1 V. (iii) Does the high resistance R, used in the secondary circuit affect the balance point? Justify your answer.

Explanation

`epsilon_(1)/epsilon_(2) = l_(1)/l_(2)`
or `epsilon_(2) = epsilon_(1) xx l_(2)/l_(1) = 1.5 xx 80/60 =2.0V`
(ii) When the driver cell of emf 2 V is replaced by a cell emf 1 V, the circuit will not work because the fall of potential on the entire potentiometer wire will become less than the emf of cell 1.5 V to be balanced on the wire. Due to it, balance point cannot be obtained on potentiometer wire. (iii) When the high resistance R is used in the secondary circuit, the position of balance point on the potentiometer wire will not be affected bacause no current flows through the cell at the balance point.