Define cell, e.m.f., terminal potential difference and internal resistance of a cell.
A cell is a device which provides the necessary potential difference to an electric circuit to maintain a continuous flow of current in it.
A cell consists of two rods or plates called electrodes which are dipped in a chemical solution called electrolyte. The pictorial symbol of a cell is $\dashv \vdash$.
Longer vertical line shows +ve terminal and shorter vertical line shows -ve terminal of the cell.
(a) EMF (Electromotive force) :
E.M.F. of a cell may be defined as the potential difference between the terminals of the cell when no current is drawn from the cell.
In a cell, the positive charges are driven towards an electrode making it positive and the negative charges towards the other electrode making it negative. Thus, a potential difference develops between +ve and -ve electrodes i.e. terminals of the cell.
E.M.F. of a cell can also be defined as:
The work per unit charge done by a cell in moving the charge around a closed loop is called the electromotive force or e.m.f. That is, $e = \frac{W}{Q}$.
Therefore, e.m.f. of a cell is the energy spent by a cell on a unit positive charge to make it to complete the electric circuit.
S.I. unit of E.M.F. is joule per coulomb or volt (V), which is also the unit of electric potential and potential difference.
Factors on which e.m.f. of a cell depends : E.M.F. of a cell depends on
(i) Nature of electrodes of the cell
(ii) Nature and concentration of electrolyte used in the cell.
(iii) Temperature of the electrolyte.
Important facts :
The name electromotive force (E.M.F. or e.m.f.) is misleading because it is not a force but work done per unit charge.
E.M.F. of a cell is independent of the
(i) size of the electrodes of the cell
(ii) distance between the electrodes of the cell and
(iii) quantity of the electrolyte in a cell.
(b) Terminal Potential Difference :
Terminal potential difference of a cell is defined as the potential difference between its terminals in a closed circuit (i.e., when current is drawn from the cell).
SI unit of terminal potential difference is Volt (V).
(c) Internal Resistance of a Cell :
Internal resistance of a cell is defined as the opposition offered by the electrolyte and electrodes of a cell to the flow of current through it. It is denoted by $r$.
S.I. unit of internal resistance is ohm.
$V = \varepsilon_{\text{eq}} - I r_{\text{eq}}$
Factors on which internal resistance of a cell depends :
(i) Distance (d) between the electrodes of the cell.
(ii) Nature of the electrolyte used in the cell.
(iii) Nature of the electrodes used in the cell.
(iv) Area (A) of the electrodes dipped in electrolyte.