# Resource 5: How to model electric circuits

Teacher resource for planning or adapting to use with pupils

## Teacher instructions for role play

Here are two role plays you could use to model what happens in an electric circuit with a small group (for use with Activity 3).

Note: In the descriptions, the different parts of the role play are explained (‘this person is the battery’, ‘this is charge moving round the circuit’, etc.). When you use these models with students, you might decide not to tell them all of this, but just say that this is a role play to model what happens in an electric circuit. Tell everyone in the role-play what to do, then ask them questions like, ‘Who is the battery?’ ‘What represents the moving charges?’ ‘What represents the resistance?’ or ‘How does this show that energy is transferred?’

## Sweets and cups

What you need: a packet of wrapped sweets, two boxes, and some paper cups.

What to do:

• Start with everyone except one in a circle. The one outside the circle is an observer.
• One person (the battery) has a box with some wrapped sweets in it: they pass one sweet every second to the person on their right, who immediately passes each sweet to the person on their right, and so on. (It may help to have someone outside the circle keep time for this by tapping the table once a second.)
• One person in the circle has a cup. They represent a lamp or a resistor. When a sweet arrives, they hold it in the cup for a second before they pass it on. Soon, all the sweets in the box are moving steadily around the circle. The observer stands behind the person on the left of the ‘battery’ and claps every time the person they are standing behind passes a sweet back to the battery. The rate the sweets are moving around is the current. Allow the sweets to go round several times, so that everyone settles into the rhythm before you make any changes.
• Now give a cup to a second person, so there are now two lamps/resistors in the circuit. What happens to the rate that sweets pass round the circuit (how often the observer claps) now?
• Now give someone else in the group a box, and half of the sweets. They also pass one sweet a second, so now there are two people passing sweets to the rest of the circle, so there are two sweets a second being passed). This increases the rate that sweets pass round the circle, and the observer claps twice as fast.

This model is good because you can see that the number of charges moving around stays the same. It is also good because you can see that increasing resistance reduces the current. Adding another battery increases the current as you would expect. There is a risk, however, that students will think that adding batteries adds more charges, although you have not got any more sweets moving round than before: focus attention on the rate at which sweets pass the observer. (Alternatively, just keep one person as the battery, but tell them to pass round sweets at twice the rate, i.e., pass a sweet every half second. This is harder to keep up: if you have someone keeping time, then get them to clap at twice the rate they did before.)

Ideally, the sweets would get a little bit smaller every time they passed the ‘lamp’, representing the transfer of energy to the lamp. Eventually, the sweets would be used up – representing the battery running out of energy. This is one feature of an electric circuit that is not represented very well in this model: the transfer of energy from the circuit to the lamp. The second model is better in this respect as students can ‘feel’ the energy as heat is generated by friction.

## Rope

What you need: a (large) loop of rope, ideally with a pattern or marks on it every metre, so you can see how fast it is moving round.

What to do:

• Everyone in the group stands in a circle, so that the rope loop is not pulled too tightly, but does not sag anywhere either.
• One person is the battery: they pull the rope around steadily, i.e. with a steady amount of pull. When they pull, the rope should start to move round, and everyone in the circle should feel it move at the same time. The moving rope represents moving charge: charges around the circuit are all moving at the same time.
• Everyone else is the resistance: they grip the rope very lightly as it moves round, to slow it down. As the rope moves through their hands, their hands will be warmed by friction with the rope; and the more tightly the ‘resistances’ grip the rope as it goes round, the more energy is transferred to their hands (beware of sore hands and friction burns caused by people tugging the rope). More grip is meant to slow the rope down, to model how increased resistance gives a smaller current. (This is not a tug of war game: the ‘battery’ is meant to give a constant amount of pull, and should not start pulling harder and harder against the resistance.)

This model is good because it shows that when the current flows around the circuit, the charges are all moving round the circuit at the same time. It also links resistance with energy transfer, and shows that bigger resistance gives a smaller current. However, if the ‘battery’ starts to pull harder to move the rope round, then students might think that adding more resistance will make the battery work harder to keep the current the same.

For each model you should ask the class:

• What forms the circuit in this model?
• What represents the charge moving round the circuit?
• What represents energy in the circuit?
• Where does the current collect energy?
• Where does it give up energy?
• In what ways is this model similar to your own ideas about electricity? In what ways is it different?
• Which model is better?

Resource 4: Information on circuits