## Calculating the current in whole circuit loops

Physics Narrative for 14-16

#### Words follow actions

To make things memorable and to emphasise the physics, just follow what you do.

Talk through your action in building a circuit. The relationship between potential difference, resistance and current for the circuit may then make more sense.

Choose a cell, then choose a lamp. Build them into a loop circuit and find that the current in the lamp, and in the wires, and in the cell, is set by these choices.

So *V**R* tells *I* what to be.

Written more conventionally:

*I* = *V**R*

Now perform two quick checks:

As *V* gets closer to zero, so *I* gets closer to zero.

As *R* gets closer to zero, so *I* becomes very large.

These simple checks show that the relationship correctly represents what you observe in the circuits.

What you can choose:

- A potential difference: select the cell.
- A resistance: choose a resistor or lamp.

What you cannot choose:

- A current – that is the result of your first two choices.