You need three things in order to make a complete circuit:

• a conductor (e.g. wire)
• a power source (e.g. wall outlet or battery)
• a resistor (e.g. light bulb or motor).

The conductor runs a circular path from the power source, through the resistor, and back to the power source. The power source moves the existing electrons in the conductor around the circuit. This is called a current. Electrons move through a wire from the negative end to the positive end. The resistor uses the energy of the electrons around the wire and slows down the flow of electrons.

A battery is one way to generate electric current. Inside the battery, chemical reactions take place. One reaction (at the negative end of the battery) creates loose electrons; the other (at the positive end) uses them up. To recharge the battery, the chemical reactions must be reversed to move the electrons in the opposite direction.

In this activity:

• students are the electrons
• energy provided by the battery is represented by smarties.
• current is the amount of charge (electrons) moving in the circuit per unit time, measured in amperes.

In order to increase the electrical current, we must speed up the movement of electrons; we do this in the model by adding extra energy in the form of extra smarties.

Students will feel warmer as they speed up, which mimics what takes place along a wire in a real circuit. This physical reaction can be used to build in a safety feature in a circuit: if there is a sudden surge of charge and the wire heats up to a certain temperature, a wire could melt, stopping the current. This is basically how a fuse works.

A fuse uses a metal wire that melts at a certain temperature, corresponding to the pre-determined limit for the circuit.

The problem with fuses is that they can only be used once. Once the metal within it has melted, it has to be replaced.

Most new homes use circuit breakers to provide a safety mechanism to an electrical circuit. The excess heat will either bend a strip of metal or activate an electromagnet that physically opens the circuit so that no more current can get through. To turn the current back on, a person goes to the circuit breaker box and flips the switch back to its original position, closing the circuit and allowing current to flow.

If this safety feature were not included in a household circuit, a surge of electricity could heat up its wires to the point that it would melt its surrounding insulation, potentially igniting the wood framing to which the wires are stapled.

### Objectives

• Describe the components required to complete an electric circuit.

### Materials

• Per Group:
students
stool, chair or box
box of Smarties (or suitable small, nut free candy)

### Key Questions

• How could we increase the current (in other words, how can we make the electrons move faster)?
• When we increased the current did you feel yourself get warmer?
• Can you think of some real examples where increasing a current and heating up a wire could be useful?

### What To Do

1. Students form a circle to represent the wire. It may help to tape a circle on the floor (or use a circle marked on the gym floor).
2. Explain that the students are electrons. There are always electrons in the wire, and they are always moving randomly, a little bit in every direction.
3. Choose one of the students to be the power source (battery). This student holds a box of Smarties – the Smarties represent “energy”. The closest student to the battery moves forward to get a Smartie. As soon as the electrons start moving in one place, they start moving everywhere.
4. As the electrons pass the battery, they get energy
5. Next pick someone to be a switch. The switch, when off, will completely stop the electron movement. The switch could either put up their hand, or turn to the side to represent “off”
6. Have the circuit practice on and off a couple times. Note that when the switch is off, all the electrons stop at once (they don’t pile up somewhere).Now put a stool (or a chair or a box) in the circle. This represents a resistance. The electrons have to climb over the stool to move forward. The whole electron chain will slow down, showing that the current slows down when there is a resistance.
7. How could we convince the electrons to move faster through the resistance? We could pass out more smarties! This represents a greater voltage (more energy per electron).
8. When the box of Smarties is empty, the battery is “dead” and the current will stop. Note that the battery gets used up faster if you pass out more energy per electron.

### Extensions

• Investigate how a circuit breaker prevents electrical fires.
• Look around the classroom. How many items can you find that add resistance to a circuit?
• Take apart some everyday appliances e.g. toasters, hairdryers and identify the different parts of the circuits inside. Which part of the toaster heats up and darkens the bread? How does the toast pop back up? Which part of the hairdryer moves the air? Which parts make the air hot?

### Other Resources

BC Hydro | Exploring Simple Circuits

BC Hydro | Electrical Safety

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