**Objectives:**

- Identify and understand that a source of e.m.f. has an internal resistance
- To understand the term terminal p.d. and how it involves ‘lost volts’
- Be able to select and use the equations , and

The power supplied by the cell must equal the power delivered to the resistor R and the power wasted in the cell due its internal resistance;

Since two of the equations for power are; and , we can write;

by squaring everything we get;

Multiplying both sides by R gives;

Since , the right hand side of this equation is the power used by resistor R;

And because of this the peak of the ‘power verses resistance’ curve is at R = r, as can be seen below;

Maximum power is delivered when the load resistance (of the component) is equal to the internal resistance (When the load is matched to the source).

Further reading:

- Isaac Physics – The electrical cell – This is a reading resource