Simple circuit

What is it about?

A bulb only lights up in a closed circuit: there must be a continuous path of conductors from one terminal of the battery through the bulb to the other terminal. A switch opens or closes this path. If it is open, the circuit is broken (open circuit) and no current flows.

Electrons: Metals contain free, negatively charged electrons. They are present everywhere in the wires – even when no current flows. When the circuit is closed, they all start moving at the same time: outside the battery from the negative terminal (−) to the positive terminal (+). The battery drives them like a pump; they are not used up in the bulb – just as many electrons flow out as flow in.

Conventional current: This direction was defined before electrons were known. Outside the battery it points from the positive to the negative terminal – opposite to the motion of the electrons. Circuit diagrams usually show the conventional current.

Circuit diagram: Instead of drawing the real parts, we use standard circuit symbols (DIN EN 60617) and draw the wires as straight lines:

• Battery: long thin line = positive terminal, short thick line = negative terminal
• Bulb: circle with a cross; electric motor: circle with an M
• Switch: a line that breaks the conductor (open) or closes it
• Fuse: rectangle with the conductor running through it lengthwise

Conductors and insulators: Metals and graphite conduct electric current (conductors). Wood, plastic, rubber and glass do not (insulators). Insulators protect us, e.g. as the plastic coating around cables.

Short circuit: If the two terminals of the bulb – and therefore the terminals of the battery – are connected by a conductor with almost no resistance, the current bypasses the bulb. The bulb goes out and the current rises sharply: in the simulation from about $0.3\,\text{A}$ to about $6\,\text{A}$. The wire and the battery get hot – there is a fire hazard. A fuse protects against this: its thin wire melts if the current is too large and breaks the circuit.

Try it

1. Tap the switch. Watch the electrons: do they start moving at the battery or everywhere at the same time?
2. Compare the set-up and the circuit diagram. Which symbol belongs to which part? Move the mouse over the parts or tap them.
3. Replace the bulb with a motor or a bell. Motor · Bell
4. Switch to conventional current and compare it with the motion of the electrons. Conventional current
5. Tap the loose wire so that it lies across the bulb. What happens to the bulb and to the current? Short circuit
6. Add a fuse and then make the short circuit. With a fuse
7. Test all the objects in the test gap: which ones conduct? Conductor test

Tasks

Task 1: Drawing a circuit diagram

Draw the circuit diagram of a circuit with a battery, a switch and an electric motor. Then compare it with the simulation.

Task 2: Conductor or insulator?

First predict which of the eight objects conduct electricity. Then test them in the test gap. Load task

Task 3: Where do the electrons come from?

Lena says: “When I close the switch, the electrons set off from the battery. It takes a moment until they reach the bulb.” What is wrong with this?

Task 4: Short circuit

Why does the bulb go out when a wire connects its two terminals – although the circuit is closed? Why is this dangerous? Load task

Task 5: Fuse

The fuse has melted. Tom wants to replace it with a piece of wire “because that conducts too”. What do you tell him?

Ideas for the classroom

• Use: introduction of the electric circuit, circuit symbols and diagrams, conductors and insulators, short circuit and fuse (Years 7/8, depending on the curriculum also earlier). Set-up and diagram are shown side by side; hovering over a part highlights it in both. With view=plan or view=real only one view is shown; with flow=none you only see the effects of the current – as in the real experiment.
• Model: $4.5\,\text{V}$ flat battery with $0.7\,\Omega$ internal resistance, $4.5\,\text{V}\,/\,0.3\,\text{A}$ bulb, motor and bell simplified as resistors, $1\,\text{A}$ fuse. The speed of the dots only indicates the current; real electrons drift far more slowly. What happens inside the battery is not shown. The short-circuit current of real batteries depends on their age and type.
• Common stumbling blocks:
  • “The current is used up in the bulb.” – Just as many electrons flow back; it is energy that is converted.
  • “The electrons come out of the battery.” – They are present everywhere in the conductor.
  • “One wire to the bulb is enough.” – You need a closed loop with a wire there and a wire back.
  • “A short circuit means a short wire.” – What matters is the missing resistance between the terminals.
• Safety: Experiments with mains sockets are life-threatening. Rechargeable batteries (e.g. lithium batteries) must never be short-circuited either.