Question

why is potentiometer preferred over voltmeter to measure emf

Answer 1

While a voltmeter draws a small current from the cell for its operation.So it measures the terminal p.d. in a closed circuit which is less than the emf of a cell.That is why a potentiometer is preferred over a voltmeter for measuring the emf of a cell.

Answer 2

Hey !!

Potentiometer does not draw any (net) current from the cell. Voltmeter draws some current fom cell, when connected across it, hence it measures terminal voltage.

DETAILED EXPLANATION

Potentiometer is preferred over voltmeter because voltmeter needs a current to pass through it so as to measure the potential difference. The potential difference being measured by the voltmeter is given as

                  V = E - IR

where,

                     V = voltage measured

                      E = emf of battery

                      R = resistance of voltmeter

                       I = current passing through voltmeter

Hence, the voltage V measured is less than emf of a battery E. Potentiometer does not draw any current from the cell and hence will not cause such problem and can measure the emf directly.

GOOD LUCK !!