# Define Cell And Potential Difference Of Resistor Terms Of Work Done In Moving Unit Charge

Define the e.m.f. (E) of a cell and the potential difference (V) of a resistor R in … Hence obtain an expression for the current i in…

E. M. F. of a cell is generally defined as the amount of work done (or the energy spent) in taking a unit positive charge around the complete circuit of the cell (i.e., in the wire outside the cell and the electrolyte within the cell). It is also defined as the potential difference between the terminals of a cell when no current is drawn from it (or when the cell is in open circuit).

Potential difference (p.d.) or the terminal voltage of a cell is generally defined as the amount of work done in carrying a unit positive charge round the circuit connected across the terminals of the cell. If W is the amount of work done (in J) in moving a test charge q between the terminals of a cell through a resistor R, then the p.d. i.e., work done in moving a unit positive charge across the terminals of the cell.
Relation between E, V (p.d.), external resistance R and the internal resistance r’,

Applying ohm’s  law to external resistance only

I = V/R   ….(i)

Again applying Ohm’s law to complete circuit

I = E/R + r   …(ii)

Comparing I in (i) and (ii), We have

V/R = E/(R + r)