Chapter 6 of 8

All 8 chapters
  1. 1What magnets pull, readunderstood
  2. 2North and south poles, readunderstood
  3. 3Magnetic force through things, readunderstood
  4. 4Making a compass, readunderstood
  5. 5Electricity: safe and unsafe, readunderstood
  6. 6A simple circuit, readunderstood
  7. 7Conductors, insulators and switches, readunderstood
  8. 8Circuit drawings, readunderstood

S2.8.6 Chapter 6

A simple circuit

Why will my bulb not light with just one wire?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Build a circuit with a cell, wires and a bulb
  • Explain why a circuit must be a complete loop

A torch has a cell and a bulb. Press the switch and the bulb lights.

You can light a bulb with a cell and two wires. But you must join them the right way.

A cell has two ends. One end has a small knob. The other end is flat.

A cell standing up. A small knob on its top is labelled knob. The body is labelled cell. Its bottom is labelled flat end, with a short line to it. Labels: knob, cell, flat end.knobcellflat end
The two ends of a cell.

Electricity goes out of one end of the cell. It must come back to the other end.

Draw it first

Draw a cell lighting a bulb

  1. A cell on the left, labelled cell, and a bulb on the right, labelled bulb. One wire, labelled wire, runs over the top from the knob of the cell to the metal side of the bulb. A second wire runs under the bottom from the flat end of the cell to the tip of the bulb. Short lines round the bulb show it is lit.cellbulb

    1 Draw a cell and a bulb. Write cell and bulb.

  2. A cell on the left, labelled cell, and a bulb on the right, labelled bulb. One wire, labelled wire, runs over the top from the knob of the cell to the metal side of the bulb. A second wire runs under the bottom from the flat end of the cell to the tip of the bulb. Short lines round the bulb show it is lit.cellbulbwire

    2 Draw a wire from the cell knob to the bulb side. Write wire.

  3. A cell on the left, labelled cell, and a bulb on the right, labelled bulb. One wire, labelled wire, runs over the top from the knob of the cell to the metal side of the bulb. A second wire runs under the bottom from the flat end of the cell to the tip of the bulb. Short lines round the bulb show it is lit.cellbulbwire

    3 Draw a second wire from the flat end to the bulb tip.

  4. A cell on the left, labelled cell, and a bulb on the right, labelled bulb. One wire, labelled wire, runs over the top from the knob of the cell to the metal side of the bulb. A second wire runs under the bottom from the flat end of the cell to the tip of the bulb. Short lines round the bulb show it is lit.cellbulbwire

    4 Draw short lines round the bulb to show it is lit.

A circuit is a path that goes all the way round. It leaves one end of the cell. It goes through the wires and the bulb. It comes back to the other end.

A bulb has two contact points. A contact point is a place where a wire must touch. One is the metal side. The other is the tip at the bottom.

Electricity goes in at one contact point. It goes out at the other. On the way it passes through a thin wire inside the bulb. That thin wire glows.

See it three ways

As a picture

A bulb with a round glass top and a metal base. A line points to the metal base, labelled side. A line points to the bottom of the base, labelled tip.sidetip

In words

A bulb has two contact points. One is the metal side. The other is the tip. Both must touch wires.

In symbols

A cell has 2 ends. A bulb has 2 contact points. All 4 must be in the loop.

A circuit must be a complete loop. A gap is a break in the loop. A gap anywhere stops the electricity. The bulb stays dark.

A cell on the left, labelled cell, and a bulb on the right, labelled bulb. A wire runs over the top from the cell to the bulb. A second wire starts at the cell but stops short of the tip of the bulb. The break is labelled gap. The bulb is dark.cellbulbgap
One wire stops short. The loop has a gap. The bulb is dark.

Put two bulbs in one loop, one after the other. Both light, but each is dimmer than one bulb alone. The first bulb does not use up the electricity. The same electricity goes all the way round.

A loop with a cell on the left, labelled cell, a switch on the right, labelled switch, and two bulbs along the bottom, each labelled bulb. The switch is closed. Both bulbs are lit, with short rays round them. Labels: cell, switch, bulb, bulb.bulbbulbcellswitch
Two bulbs in one loop. Both light, and each is dim.

Try it

Light a bulb with a cell

You need: a torch cell marked 1.5 V, a small torch bulb, marked 1.5 V if you can, two strips of kitchen foil, folded long and thin.

Be safe: Never let the two strips touch each other. That joins the two ends of the cell. The foil gets hot fast.

  1. Fold each foil strip long and thin. Each is about 15 cm.
  2. Hold one strip on the knob end of the cell.
  3. Hold the other strip on the flat end of the cell.
  4. Touch the first strip to the metal side of the bulb.
  5. Touch the other strip to the tip of the bulb. Watch the bulb.
  6. Lift the second strip off the tip. What happens?
  7. Put the second strip back on the glass, not the tip. What happens?

What you will see: Strips on the side and the tip light the bulb. Lift one strip and the bulb goes out. On the glass it stays dark. The glass is not a contact point. If it stays dark, press harder. Check that the bulb suits the cell. No cell or bulb? Switch a torch on and off. Do not open it. Inside, the cells, the switch and the bulb make one loop. The switch opens a gap in the loop.

If you cannot try it, this is what happens.
A cell on the left, labelled cell, and a bulb on the right, labelled bulb. One wire, labelled wire, runs from the knob of the cell to the side of the bulb. A second wire runs from the flat end of the cell to the tip of the bulb. The loop is complete and short lines round the bulb show it is lit.cellbulbwire

A wrong idea many people have

One wire from the cell to the bulb is enough.

Why it feels right: A torch seems to have no second wire. The cell seems to send electricity to the bulb.

The test that breaks it: Join one foil strip from the cell knob to the bulb side. The bulb stays dark. Add a second strip from the flat end to the tip. Now it lights. Lift the second strip and it goes out again.

What is true: Electricity must go round a full loop. The bulb needs a wire at each of its two contact points.

Key words

circuit
a path round from one end of the cell to the other.
contact point
a place where a wire must touch. A bulb has two.
gap
a break in the loop. It stops the electricity.

Back to your guess

The answer is B: The bulb must be in a full loop with the cell.

The cell, the wires and the bulb must make a full loop. One wire between them leaves a gap, so the bulb stays dark.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Look at the picture. The bulb is dark. What is missing?

    A cell on the left, labelled cell, and a bulb on the right, labelled bulb. One wire runs over the top from the cell to the side of the bulb. There is no second wire. The bulb is dark.cellbulb
    Tap your answer.

    1. Not this one. A bigger cell cannot help. The loop has a gap, so nothing goes round. Try another.

    2. Not this one. The length does not matter. The loop is not complete. Try another.

    3. Yes, that is right. The loop needs a wire from the flat end to the tip.

    See the answer

    A second wire, from the flat end to the tip

    The loop needs a wire from the flat end to the tip.

    • If you chose A: A bigger cell cannot help. The loop has a gap, so nothing goes round.
    • If you chose B: The length does not matter. The loop is not complete.

  2. Amina has a cell, a bulb and two wires. Which way lights the bulb?

    Tap your answer.

    1. Yes, that is right. Each end of the cell joins one contact point of the bulb. That makes a full loop.

    2. Not this one. Both wires touch the same contact point. The loop skips the bulb. The wires get hot. Try another.

    3. Not this one. The flat end has no wire. The loop has a gap. Try another.

    See the answer

    One wire from the knob to the side. One from the flat end to the tip.

    Each end of the cell joins one contact point of the bulb. That makes a full loop.

    • If you chose B: Both wires touch the same contact point. The loop skips the bulb. The wires get hot.
    • If you chose C: The flat end has no wire. The loop has a gap.

  3. Ravi puts two bulbs in one loop with one cell. What happens?

    Tap your answer.

    1. Not this one. The first bulb does not use up the electricity. It goes all the way round. Try another.

    2. Yes, that is right. The same electricity goes through both bulbs. The cell has to light two, so each is dimmer.

    3. Not this one. The loop is complete and has a cell. So both bulbs light. Try another.

    See the answer

    Both bulbs light, but they are dim.

    The same electricity goes through both bulbs. The cell has to light two, so each is dimmer.

    • If you chose A: The first bulb does not use up the electricity. It goes all the way round.
    • If you chose C: The loop is complete and has a cell. So both bulbs light.

  4. Why does a bulb stay dark when the loop has a gap?

    Tap your answer.

    1. Not this one. A bulb holds none. The cell pushes electricity round a full loop. Try another.

    2. Not this one. Thin wires still work. The gap is the problem. Try another.

    3. Not this one. A weak cell lights a bulb dimly. With a gap, nothing goes round at all. Try another.

    4. Yes, that is right. A gap breaks the loop. With no way round, nothing goes through the bulb.

    See the answer

    Electricity cannot go all the way round.

    A gap breaks the loop. With no way round, nothing goes through the bulb.

    • If you chose A: A bulb holds none. The cell pushes electricity round a full loop.
    • If you chose B: Thin wires still work. The gap is the problem.
    • If you chose C: A weak cell lights a bulb dimly. With a gap, nothing goes round at all.

  5. From an earlier chapter

    Which of these can kill?

    Tap your answer.

    1. Not this one. A small torch cell is gentle and safe to use. The mains is the danger. Go to S2.8.5. Try another.

    2. Not this one. A clock cell is small and gentle. The mains is the danger. Go to S2.8.5. Try another.

    3. Yes, that is right. The mains pushes at least 60 times harder than a torch cell. It can kill.

    See the answer

    Mains electricity from a wall socket

    The mains pushes at least 60 times harder than a torch cell. It can kill.

    • If you chose A: A small torch cell is gentle and safe to use. The mains is the danger. Go to S2.8.5.
    • If you chose B: A clock cell is small and gentle. The mains is the danger. Go to S2.8.5.

  6. Draw a cell, a bulb and two wires so the bulb lights. Show where each wire touches.

    See the answer

    One wire runs from the knob to the side of the bulb. The other runs from the flat end to the tip.

    Both cell ends and both contact points are in one loop.

  7. Hana joins a cell to a bulb with two foil strips. One is on the side. The other is on the glass. What happens?

    Tap your answer.

    1. Yes, that is right. The loop needs wires on the side and on the tip. The glass leaves a gap.

    2. Not this one. A bulb has two contact points, the side and the tip. The glass is not one of them. Try another.

    3. Not this one. It does not light at all. The strip on the glass does not touch a contact point. Try another.

    See the answer

    The bulb stays dark. The glass is not a contact point.

    The loop needs wires on the side and on the tip. The glass leaves a gap.

    • If you chose B: A bulb has two contact points, the side and the tip. The glass is not one of them.
    • If you chose C: It does not light at all. The strip on the glass does not touch a contact point.

  8. Which of these is a complete circuit that lights a bulb?

    Tap your answer.

    1. Not this one. The tip touches nothing, so the loop has a gap. Both contact points must be in the loop. Try another.

    2. Not this one. Nothing pushes the electricity round without a cell. The bulb stays dark. Try another.

    3. Not this one. Both wires touch one contact point. The loop skips the other one, so the bulb stays dark. Try another.

    4. Yes, that is right. A cell pushes electricity round a complete loop. Both contact points of the bulb are in it.

    See the answer

    A cell, a bulb and two wires in one full loop.

    A cell pushes electricity round a complete loop. Both contact points of the bulb are in it.

    • If you chose A: The tip touches nothing, so the loop has a gap. Both contact points must be in the loop.
    • If you chose B: Nothing pushes the electricity round without a cell. The bulb stays dark.
    • If you chose C: Both wires touch one contact point. The loop skips the other one, so the bulb stays dark.

  9. A cell has 2 ends. A bulb has 2 contact points. How many of them must be in the loop?

    Tap your answer.

    1. Not this one. You missed one. The cell has 2 ends and the bulb has 2 contact points. That is 4. Try another.

    2. Not this one. That counts only the cell ends, or only the contact points. Count both. Try another.

    3. Yes, that is right. The cell has 2 ends. The bulb has 2 contact points. Together that is 4, and all 4 must be in the loop.

    See the answer

    4

    The cell has 2 ends. The bulb has 2 contact points. Together that is 4, and all 4 must be in the loop.

    • If you chose A: You missed one. The cell has 2 ends and the bulb has 2 contact points. That is 4.
    • If you chose B: That counts only the cell ends, or only the contact points. Count both.

Keep it

Copy this into your exercise book.

A circuit is a path all the way round. A bulb lights in a complete loop of cell, wires and bulb. A gap stops it. Both contact points of the bulb must be in the loop.

Draw: A cell and a bulb. Two wires make a loop. Draw short lines round the bulb.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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