Electrical Circuit
Electricity and Magnetism

Calculating the current in whole circuit loops

Physics Narrative for 14-16 Supporting Physics Teaching

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 VR tells I what to be.

Written more conventionally:

I = VR

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.
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