Question

A low voltage supply from which one needs high currents must have very low internal resistance. Why ?

Answer 1

The current supplied by a source is given by

\[ I=\frac{E}{R+r} \]

where,

• \(E\) = emf of the source
• \(R\) = external resistance
• \(r\) = internal resistance of the source

For a low-voltage source, the emf \(E\) is small. To obtain a large current, the total resistance \((R+r)\) must be as small as possible.

If the internal resistance \(r\) is large, a significant part of the voltage is lost inside the source itself due to the drop \(Ir\). This reduces the current delivered to the external circuit.

Therefore, a low-voltage supply must have very low internal resistance so that the voltage drop inside the source is minimum and a large current can be supplied efficiently.