Question

Derive an expression for current flowing through a circuit when an external resistance is connected to a real e.m.f. device.

Answer 1

i. If a current (I) flows through an emf device, there is an internal resistance (r) and the emf (E) differs from the potential difference across its two terminals (V).

V = E – Ir ……… (1)

ii. The negative sign is due to the fact that the current I flows through the emf device from the negative terminal to the positive terminal.

iii. By the application of Ohm’s law,

V = IR …….(2)

From equations (1) and (2),

IR = E – Ir

∴ \(\frac{E}{R+r}\)