e.m.f and internal resistance - Physics form 5

e.m.f., Ɛ is the energy transferred or work done by an electrical source to move one coulomb of charge in a complete circuit.

For a dry cell with 1.5 V, the e.m.f. for the cell is 1.5 V or 1.5 J C^-1.

Both e.m.f. and potential difference have the same S.I. unit which is V (volt).

For a dry cell, the value of potential difference is slightly less than the value of e.m.f..

Internal resistance, r is the resistance caused by electrolyte in the dry cell.

The unit for internal resistance is Ω (ohm).

Electromotive force = Potential difference produced by cell + Voltage drop

1.5 V = 1.48 V + 0.02 V

Based on experiment 3.5 (text book page 118), the voltage drop increases as the current, I increases.

It is impossible to get I = 0 A, but we can extrapolate the graph.

The value of Ɛ is obtained from the y-intercept.

Note that a straight line is produced in the graph of V against I.

The gradient of the straight line is represent the value of internal resistance, r of the dry cell.

From the graph, we can have the equation which is V = −r + Ɛ

The equation is actually resemble y = mx + c

The value of r is negative since the gradient of the straight line is negative.