Electrical Circuit
Electricity and Magnetism

Understanding parallel circuits

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

This demonstration activity offers a way of helping pupils come to understand how parallel circuits work.

What to Prepare

  • 12 volt, 24 watt bulbs
  • 12 volt direct current power supply
  • demonstration ammeter with a large, easy to read display

Using 12 volt, 24 watt car headlamp bulbs, a 12 volt lab-pack supply and a demonstration analogue meter (0–5 ampere direct current) you are likely to measure currents of 1.4 ampere in each loop.

What Happens During this Activity

Part 1: lighting two bulbs from one electrical supply

To start with, the teacher gathers the pupils around and, by way of review, demonstrates the familiar points that:

  • One bulb connected to the supply is of normal brightness.
  • Two identical bulbs connected in series are equally dim.

Then poses the question:

Teacher: How might you connect two bulbs to one supply such that both bulbs are of normal brightness?

Pupils are likely to suggest: Just connect up each bulb to the battery to make two circuits (and if they don't, you might offer: here's an easy way to do it).

Connect up the circuit and switch on. Both bulbs light to equal, normal brightness.

Seeing parallel circuits as two loops

At this point, with the bulbs there in front of the pupils' eyes, you should draw attention to the apparently odd nature of this circuit.

Teacher: So! Both bulbs light to normal brightness. I haven't added more batteries or anything else like that and yet we get two bulbs lit, twice the energy out. How can that be? It's the Yorkshire-man's dream: summat for nowt! What's going on here?

As a starting point to finding out what's going on, suggest measuring the currents in each of the circuit loops:

Teacher: Well, let's look at the currents in each of these loops. The big loop to the left and the small loop to the right.

Part 2: seeing parallel circuits as two loops

The next step is to re-organise the circuit so that it begins to look more like the standard parallel circuit format:

Teacher: Suppose I just lift this big loop across so that it fits around the small loop.

Focus on the current, then on the standard parallel circuit

Teacher: We have a current of 1.4 ampere in the small circuit coming from the supply, and 1.4 ampere in the big circuit, also coming from the supply. What, do you think, will be the total current from the supply?

The easiest way to measure the total current in the supply is to use two additional leads, from and to the supply, and to connect these to both loops. Measure the current in each of the two leads to the supply:

Teacher: So, the current from the battery here is 2.8 ampere and back to the battery here is 2.8 ampere.

The demonstration circuit now looks like the standard circuit diagram format for parallel circuits:

All that remains is to piece the explanation together.

Teacher: OK, so with one battery and one bulb, we can picture the charged particles moving through the battery around to the bulb and energy is shifted by each charge. When a second bulb is added in parallel, an extra loop is provided around which the charged particles move. The number of charged particles passing through the battery each second is therefore doubled, and the same amount of energy is shifted by each charge. So with the second loop the charged particles shift energy from the store associated with the battery at twice the rate and the battery will flatten more quickly.

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