Rohan, a Science student purchased a few cells from a shop keeper for his torch. He noticed that the torch was not working will with these new cells. He thought of checking the emf of each cell. Using a potentiometer he measured their emfs and noticed that the emf of each cell was less than the value of oblige him. He then lodged a complaint with consumer forum and got theredressal.
Read the above passage and answer the following question:
(i) What are the basic values displayed by Rohan?
(ii) Why rohan used potentiometer instead of voltmeter to measure emf of the cells?
(iii) If the lenght of potentiometer wire is (a) doubled and (b) halved, what will be the effect on the position of zero deflection in a potentiometer.

Explanation

(i) Rohan used a potentionmeter for checking the emf of cells. It shows that his knowledge in this respect is good. Instead of fighting with the shopkeeper, he approached the consumer forum for redressel of his
(ii) EMF is the maximum potential differnce potential difference between two electrodes two electrodes of a cell in open circuit. while measuring the emf of a cell using potentiometer method, a potentiometer does not draw any current from the cell whose emf is to be determined, whereas a voltmeter always draws some little current. Therefore, emf measured by voltmeter is slightly less than actual value of emf measured by potentionmeter.
(iii) (a) when the lenght of wire is doubled, the potential gradient across the potentiometer wire will decrease. Now the position of zero deflection will occur at longer lenght.
(b) When lenght of wire is halved, the reverse will true.