A model of internal resistance in terms of energy
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An electric cell is a part of the circuit in which there is current. Therefore there must be a current in the cell, as well as in the wires in the circuit. Remember that putting a cell into a circuit starts the charge moving everywhere–including charge in the cell.
Real cells are made of materials with a resistance
Just like a wire, a cell isn't perfect. Both are special arrangements of atoms through which the charged particles move while there is a current. Both have a resistance, which is usually ignored in our models of the circuit. However, cells, like wires, can get very hot as a result of large currents in the cell. Even a small resistance, and therefore a small potential difference, can result in a large power in the pathway if the current is large. The energy dissipated in the cell as a result of this action is then not available to the rest of the circuit. And a cell that provides less energy for each charge is a cell with a smaller potential difference. So drawing large currents through cells significantly drops the potential difference across the cell.
This resistance of the cell is called the internal resistance, and it can usually be safely ignored in models that describe circuits with moderate currents. A car starter motor draws 30 ampere–not a moderate current. You will notice the headlights dimming when you start the car as a result of this reduced potential difference.
The output coil of a transformer acts like a small cell when the magnetic effect through the coil changes. This
cell may contain a lot of wire, so it too has an internal resistance. This could be put to use, designing sources with high internal resistance, so that the current that can be drawn is limited.
In fact, all sources of potential difference have some internal resistance, and this is a feature that is often considered in choosing a particular source. One reason why the domestic mains supply is to be treated with respect is that its effective internal resistance is rather small–it has to be, or the sockets and supply wiring would get rather warm.