Resource 3: Two models for teaching about electric circuits

Sweets and cups

What you need

A packet of wrapped sweets, two boxes and some paper cups. Put half the wrapped sweets in one box and half in the other.

This model works well if you have, say, 20 sweets and ten people in the circle, plus an observer and someone to read the questions. If you use a bigger group and more sweets, it will take too long before all the sweets are going round the circle.

What to do

Before you start, choose one person from the group to read out the instructions and the questions.

• Start with everyone except one in a circle. The one outside the circle is an observer.
• One person has a box with half of the 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. 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 person with the box, and claps every time the person they are standing behind passes a sweet back to the person with the box. 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. What happens to the rate that sweets pass round the circuit (i.e. how often the observer claps) now?
• Now give someone else in the group a box, and the other 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.

Questions

• What does the person giving out sweets represent?
• What do the sweets represent?
• What do the cups represent?
• What does adding a second person with sweets represent and what is the effect?

Answers and comments

• The person giving out the sweets is like a battery pushing the charge round the circuit. (This model may suggest, incorrectly, that the battery supplies charge. The battery only makes the charge move.)
• The sweets are the charge. You can see that the same number of sweets move. The rate at which sweets are moving around is the current. The faster the observer claps, the bigger the current in the circuit. The observer is like an ammeter, counting the rate of flow of charge.
• The cups slow down the flow of sweets. They are acting like resistors or lamps. (This is where energy leaves a real circuit, but it is harder to see this connection in this model.)
• The second person with sweets is like another battery. Adding another battery increases the current: sweets are moving past the observer more frequently now. The problem with adding someone with more sweets is that it looks as though adding the battery added more charge, when the charge moving around should be the same. It is just the speed it moves at that increases.

Strengths

This model is good for showing that the charge moving around the circuit stays the same. No sweets leave the group, and adding resistors reduces the current.

Limitations

The model suggests that batteries are a source of charge and that it takes a while for all the charge to be in motion round the circuit. The model does identify clearly where energy transfer occurs.

Rope model

What you need

A (large) loop of lightweight rope, ideally with a pattern or marks on it every metre, so you can see how fast it is moving round. The longer the rope, the more people you can have in the group doing the role play.

What to do

Before you start, choose one person from the group to read out the instructions and the questions.

• 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 pulls the rope around steadily, i.e. with a steady amount of pull.
• Everyone else should grip the rope very lightly as it moves round.
• One person should grip more tightly than the others and notice what happens. Note that they should not grip too tightly – this is not a tug of war game! The person pulling is meant to give a constant amount of pull, and should not start pulling harder and harder.

Questions

• What does the person pulling the rope represent in this model?
• What does the moving rope represent?
• When someone grips the rope more tightly, what does this represent?

Answers and comments

• The person pulling the rope is the battery. When the person pulls the rope, this supplies energy to the circuit.
• The moving rope is the charge moving round the circuit.
• When someone grips the rope more tightly, they feel their hands getting warmer, and the rope gets harder to pull. The extra grip is increased resistance. The person’s hands being warmed is like energy being transferred out of the circuit. The person gripping the rope is like a bulb or a resistor.

Strengths

This model shows that all the charge is moving around the circuit at the same time, and it makes a link between resistance and energy transfer.

Limitations

If the person pulling the rope starts to pull harder when someone grips the rope more tightly, it could suggest that the battery works harder to keep the current the same when the resistance increases.