Electrical Circuit
Electricity and Magnetism

Adding a bulb reduces the current

Teaching Guidance for 11-14 Supporting Physics Teaching

Adding a bulb prevents things happening

Wrong Track: The first bulb grabs the current, so that there's none left for the second.

Wrong Track: The first bulb slows down the current, and the second one slows it even more.

Right Lines: The current is the same everywhere in the loop. The bulbs both act together to reduce the flow. Ther really isn't a first and second.

One more bulb, reduced current

Thinking about the learning

The fact that adding a second identical bulb in series results in the two bulbs being dimmer makes intuitive good sense to most pupils. Their underlying reasoning is that the single source (or battery) is now being shared between the two bulbs. This idea needs both development and some refining, so that it increases in precision.

The key idea for the pupils to get hold of here is that adding an extra bulb introduces more resistance to the circuit, and this has the effect of slowing down the passage of charge around the whole circuit. The number of charged particles passing any point in the circuit is reduced; in other words, the current is reduced.

Thinking about the teaching

The rope loop teaching model is very useful in getting over the idea that the electric current goes down when a second bulb is added to the circuit.

Teacher: OK! Now I want both Julia and Anita to loosely hold the rope. I'm pulling the rope round with the same force and it's obvious that Anita's extra grip or resistance has slowed down the whole of the rope loop.

Having talked through the teaching model, attention is returned to the electric circuit model:

Teacher: When a second bulb is added, extra resistance is introduced to the circuit. This has the effect of slowing down the flow of charge all around the circuit. In other words the current is reduced.

It is worth emphasising that it is the filament of the bulb that offers the resistance to the flow of charge:

Teacher: The filament of the bulb is made from very thin tungsten wire. The filament is designed so that it is difficult for the charge to pass through and, as they do, they interact with the tungsten atoms and the whole lot heats up until the filament glows white-hot. This is rather different from what happens in the connecting leads. These are made from relatively thick lengths of copper wire that have a very low resistance and so very little heating occurs in the connecting leads.

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