Electrical Circuit
Electricity and Magnetism

Voltage around many loops

Teaching Guidance for 11-14 Supporting Physics Teaching

The closer bulb gets more

Wrong Track: The bulb closer to the battery will get more from the battery as it's closer.

Right Lines: These are two separate loops, and the battery pushes charged particles through each of them quite separately. That's why each bulb glows just like it does in the simple circuit, with one bulb and one battery.

Each bulb gets the full battery voltage

Thinking about the learning

Pupils will be familiar with the idea that when a second bulb is added in parallel to a circuit, the two bulbs become equally, normally bright.

Adding a bulb in parallel sets up a second current loop in which charged particles are set in motion. The power dissipated by both bulbs is equal to the power dissipated by one bulb. As a result of the accumulation of this action, the energy shifted from the chemical store will turn up in another store when that same quantity of charge is passed through the bulb or resistor in the circuit.

This idea can be formalised by stating that the full battery voltage is dropped across each of the two bulbs. We think that dealing with each loop separately makes this more acceptable.

Thinking about the teaching

You might start by revising the physics:

Teacher: OK, so the battery is 3 volt. So it's working at 3 watt so long as there is a current of 1 ampere in it. How many watt in the bulb in this loop with the blue wire?

Dylan: 3 watt?

Teacher: Yes that's right! Each bulb is equally bright – 3 watt of power. The energy shifted each second so long as there is 1 ampere in the bulb is 3 joule, so the voltage across the bulb in the blue loop is 3 volt. What about the bulb in the loop with the green wires?

Kay: It's the same! 3 volt.

Teacher: Exactly! 3 joule of energy are shifted a second, so long as there is 1 ampere in the bulb.

It's important that pupils are able to visualise the double flow of charge through the battery, with each loop contributing the same as if it were in a simple circuit.

Two rope loops of different colours and of different lengths can be used to great effect here. Use each loop independently to model the effects of a single resistor in each loop. Alter the resistances by how firmly you grasp the rope, so altering the slip force. Then combine the two independent loops to find the resultant current in the battery – depending on the choices that you've made for each loop.

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