Question

why emf has more value than p.d.?​

Answer 1

I believe the question you mean to ask is, why is the Electromotive Force (EMF) of a cell larger than its Terminal Voltage.

If that is the case, then read on.

An electric cell comprises of electrodes placed in an electrolyte. The electrolyte is most commonly an acidic susbtance which is a poor conductor of electricity. Is function is to provide free electrons which can move around courtesy the electric field difference between the electrodes. The point is essentially, that the battery has a non zero, finite resistance, which is called internal resistance. In most cases, the internal resistance is quite small. The potential difference between the two terminals of a cell when the cell is NOT supplying any current is called EMF. The EMF is independent of the value of the internal resistance.

If a cell of emf E and internal resistance r is directly connected across a Voltmeter, you can measure its EMF.

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Answer 2

it is because emf dosent includes internal resistance

that is it an open circuit, no current is drawn from the cell

while in p.d it is a closed circuit hence internal resistance of cell comes into play

since u know that resistance decreases current so emf has more value compared to p.d

hope this helps and u understand well :))