Chapter 3 of 8

All 8 chapters
  1. 1Current in a circuit, readunderstood
  2. 2Cells and voltage, readunderstood
  3. 3Series and parallel circuits, readunderstood
  4. 4Resistance, readunderstood
  5. 5Static electricity, readunderstood
  6. 6Magnetic fields, readunderstood
  7. 7Electromagnets, readunderstood
  8. 8Electricity safety at home, readunderstood

S3.10.3 Chapter 3

Series and parallel circuits

Why does one light going out not put all the lights in a house out?

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 series circuit and a parallel circuit with two bulbs
  • Explain why the lights in a house are wired in parallel

Two bulbs and one cell can be joined in two ways. The way you join them changes how bright they are. It changes what happens when one goes out.

In a series circuit the parts are in one loop, one after the other. There is only one path. The same current goes through every part.

Draw it first

Draw two bulbs in series, then in parallel

  1. Three circuits, each with a cell, an ammeter and two bulbs. The first is in series, with both bulbs in one loop, and the ammeter reads 0.15 A. The second is in parallel, with each bulb on its own branch, and the ammeter reads 0.6 A. In the third, labelled one branch switched off, one branch has an open switch, its bulb is dark and the other bulb is lit, and the ammeter reads 0.3 A. Labels: cell, bulb, A, 0.15 A, 0.6 A, 0.3 A, series, parallel, one branch switched off.cellA0.15 Abulbbulbseries

    1 Series: draw one loop with the cell and two bulbs. The current is 0.15 A, and both bulbs are dim.

  2. Three circuits, each with a cell, an ammeter and two bulbs. The first is in series, with both bulbs in one loop, and the ammeter reads 0.15 A. The second is in parallel, with each bulb on its own branch, and the ammeter reads 0.6 A. In the third, labelled one branch switched off, one branch has an open switch, its bulb is dark and the other bulb is lit, and the ammeter reads 0.3 A. Labels: cell, bulb, A, 0.15 A, 0.6 A, 0.3 A, series, parallel, one branch switched off.cellA0.6 Abulbbulbparallel

    2 Parallel: draw two branches across the cell, one bulb on each. The cell gives 0.6 A, and both bulbs are bright.

  3. Three circuits, each with a cell, an ammeter and two bulbs. The first is in series, with both bulbs in one loop, and the ammeter reads 0.15 A. The second is in parallel, with each bulb on its own branch, and the ammeter reads 0.6 A. In the third, labelled one branch switched off, one branch has an open switch, its bulb is dark and the other bulb is lit, and the ammeter reads 0.3 A. Labels: cell, bulb, A, 0.15 A, 0.6 A, 0.3 A, series, parallel, one branch switched off.cellA0.3 Abulbbulbone branch switched off

    3 Put a switch in one branch and open it. That bulb goes out. The other bulb stays lit.

In series, the two bulbs share the push of the cell. Each gets only part of it, so each is dimmer than one bulb alone. If one bulb is taken out, or a switch opens, the loop has a gap. All the bulbs go out.

A series circuit with a cell, an open switch and two bulbs in one loop. Both bulbs are dark. Labels: cell, bulb, switch.cellbulbbulb
One gap in the loop stops the current everywhere. Both bulbs are dark.

In a parallel circuit the current has more than one path. Each path is a branch. Each branch is joined straight across the cell, so each bulb gets the full push.

Each bulb is as bright as one bulb alone. A switch can turn one branch off while the others stay on. The cell gives more current in all, because each branch takes its own share. So the cell runs down sooner.

See it three ways

As a picture

A table with the columns circuit, amps and bulbs. one bulb: 0.3 A, bright. series: 0.15 A, dim. parallel: 0.6 A, bright. Here amps is the current from the cell and bulbs says how bright each bulb is.circuitampsbulbsone bulb0.3brightseries0.15dimparallel0.6bright

In words

Two bulbs in series take about half the current of one bulb, so they are dim. Two bulbs in parallel each take the full current. The cell gives double. In our sums a bulb keeps the same resistance. A real filament changes a little as it heats.

In symbols

Parallel: 0.3 A + 0.3 A = 0.6 A from the cell. Series: about 0.15 A, which is 0.3 A ÷ 2.

This is why houses are wired in parallel. Each light and socket is on its own branch. It gets the full push, so it works at full strength. You can switch one off without touching the others.

A circuit with a supply and three branches in parallel. Each branch has a lamp and its own switch, all closed, and all three lamps are lit. Labels: supply, switch.supplyswitchswitchswitch
Three lamps in parallel, each with its own switch. A cell stands in for the supply here.
A parallel circuit with a cell and an ammeter on the supply wire. Three branches each have a lit bulb and an ammeter. None of the ammeters shows a reading. Labels: A.AAAA
An ammeter in each branch and one on the supply wire. The next example finds the readings.

Worked example

One bulb alone takes 0.2 A from a 1.5 V cell. Three of these bulbs are joined in parallel across the cell. How many amps does the cell give?

  1. In parallel, each branch gets the full push of the cell.
  2. So each branch takes the same current as one bulb alone: 0.2 A.
  3. The cell gives the sum of the branch currents.
  4. 0.2 + 0.2 + 0.2 = 0.6 A.
  5. That is 3 times the current for one bulb, so the cell runs down sooner.

Answer: 0.6 A

Try it

Join two bulbs two ways

You need: one torch cell, 1.5 V, two small torch bulbs of 1.5 V, six strips of kitchen foil, folded long and thin, tape.

Be safe: Join for only a few seconds. Foil and cells can get warm, so let them cool before you touch them. Never let the two strips from the cell touch each other.

  1. Series: join the cell, bulb 1 and bulb 2 in one loop with foil strips. Hold the joins for 3 seconds. Look at both bulbs.
  2. Lift bulb 1 out of the loop. Look at bulb 2.
  3. Parallel: join each bulb across the cell with its own two strips. Each bulb has its own loop to the cell. Look at both bulbs.
  4. Lift bulb 1 away. Look at bulb 2.

What you will see: In series both bulbs glow dimly, and lifting one puts the other out. In parallel both glow more brightly, and lifting one leaves the other lit. If you have no bulbs, draw the two circuits and trace each path with your finger. Your looks may differ from ours.

If you cannot try it, this is what happens.
A table of what one learner saw, with the heads bulbs, series and parallel. all on: series dim, parallel bright. lift one: series all out, parallel one lit. Lift one means one bulb is lifted out.bulbsseriesparallelall ondimbrightlift oneall outone lit

A wrong idea many people have

Two bulbs in parallel are dimmer, because the cell has to share its push between them.

Why it feels right: In series that is true. Two bulbs in a loop do share the push.

The test that breaks it: Join one bulb across the cell: the cell gives 0.3 A. Join a second bulb in parallel: each branch still takes 0.3 A, and the cell gives 0.6 A. Both bulbs are as bright as one alone.

What is true: Each branch is joined straight across the cell, so it gets the full push. The cell gives more current in all, so it runs down sooner.

Key words

series
parts joined in one loop, one after the other.
parallel
parts joined on separate branches across the cell.
branch
one of the paths a current can take in a parallel circuit.

Back to your guess

The answer is A: The lights are in parallel, so each has its own path and the full push.

The lights are wired in parallel. Each light is on its own branch, joined straight across the supply. One branch can be off while the others stay on.

Check yourself

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

  1. Two bulbs are in series with one cell. Ravi unscrews one bulb. What happens to the other?

    Tap your answer.

    1. Not this one. That is how a parallel circuit works. In series there is only one path, and unscrewing a bulb breaks it. Go to S3.10.3. Try another.

    2. Yes, that is right. A series circuit is one loop. A bulb taken out leaves a gap, so the current stops everywhere.

    3. Not this one. With the loop broken, no current flows at all. In series every part is in the same loop. Go to S3.10.3. Try another.

    See the answer

    It goes out, because the loop is broken.

    A series circuit is one loop. A bulb taken out leaves a gap, so the current stops everywhere.

    • If you chose A: That is how a parallel circuit works. In series there is only one path, and unscrewing a bulb breaks it. Go to S3.10.3.
    • If you chose C: With the loop broken, no current flows at all. In series every part is in the same loop. Go to S3.10.3.

  2. Bulbs A and B are in parallel. The switch on bulb A is open. What happens to bulb B?

    A parallel circuit with a cell and two branches. Bulb A is on the top branch with an open switch. Bulb B is on the other branch. Labels: A, B.AB
    Tap your answer.

    1. Not this one. Only the branch with the open switch is broken. Bulb B is on its own branch, so the current still goes round it. Go to S3.10.3. Try another.

    2. Not this one. Each branch gets the full push from the cell. B does not share its push with A. Go to S3.10.3. Try another.

    3. Yes, that is right. Bulb B is on its own branch, joined straight across the cell. It gets the full push, so it stays lit.

    See the answer

    It stays lit at full brightness.

    Bulb B is on its own branch, joined straight across the cell. It gets the full push, so it stays lit.

    • If you chose A: Only the branch with the open switch is broken. Bulb B is on its own branch, so the current still goes round it. Go to S3.10.3.
    • If you chose B: Each branch gets the full push from the cell. B does not share its push with A. Go to S3.10.3.

  3. Each of 2 bulbs takes 0.4 A when joined alone to a cell. The 2 bulbs are joined in parallel across the cell. How many amps does the cell give?

    Tap your answer.

    1. Yes, that is right. Each branch takes 0.4 A. The cell gives 0.4 + 0.4 = 0.8 A.

    2. Not this one. You may have shared the current between the bulbs. Each branch takes its full current, and the cell gives the sum. Go to S3.10.3. Try another.

    3. Not this one. That is the current of one bulb. A second branch adds its current to it. Go to S3.10.3. Try another.

    See the answer

    0.8 A

    Each branch takes 0.4 A. The cell gives 0.4 + 0.4 = 0.8 A.

    • If you chose B: You may have shared the current between the bulbs. Each branch takes its full current, and the cell gives the sum. Go to S3.10.3.
    • If you chose C: That is the current of one bulb. A second branch adds its current to it. Go to S3.10.3.

  4. Why are the lights in a house wired in parallel?

    Tap your answer.

    1. Not this one. Parallel branches each take their own current, so the supply gives more in all. The reason is that each light works on its own. Go to S3.10.3. Try another.

    2. Yes, that is right. Each light is on its own branch across the supply. It gets the full push and works alone, even when others are off.

    3. Not this one. Parallel needs a pair of wires to each branch. The reason is that each light works on its own, at full strength. Go to S3.10.3. Try another.

    See the answer

    So that each light gets the full push and can be switched off alone.

    Each light is on its own branch across the supply. It gets the full push and works alone, even when others are off.

    • If you chose A: Parallel branches each take their own current, so the supply gives more in all. The reason is that each light works on its own. Go to S3.10.3.
    • If you chose C: Parallel needs a pair of wires to each branch. The reason is that each light works on its own, at full strength. Go to S3.10.3.

  5. Two identical bulbs are joined in series with one cell. How do they glow, compared with one of these bulbs alone on the cell?

    Tap your answer.

    1. Not this one. No bulb uses up the current. The same current goes through both bulbs in the loop, so they glow alike. Go to S3.10.1. Try another.

    2. Not this one. That is how bulbs in parallel glow. In series the two bulbs share the push of the cell, so each gets only part of it. Go to S3.10.3. Try another.

    3. Yes, that is right. In series the two bulbs share the push, so each gets about half of it. The same current goes through both, so they glow alike, and less brightly than one bulb alone.

    See the answer

    Both are dimmer than one bulb alone, and they glow alike.

    In series the two bulbs share the push, so each gets about half of it. The same current goes through both, so they glow alike, and less brightly than one bulb alone.

    • If you chose A: No bulb uses up the current. The same current goes through both bulbs in the loop, so they glow alike. Go to S3.10.1.
    • If you chose B: That is how bulbs in parallel glow. In series the two bulbs share the push of the cell, so each gets only part of it. Go to S3.10.3.

  6. From an earlier chapter

    Review: 3 cells of 1.2 V are joined in a line, + to −. What is the push in all?

    Tap your answer.

    1. Yes, that is right. Each cell adds 1.2 V. 3 × 1.2 = 3.6 V.

    2. Not this one. You may think volts do not add. Cells joined in a line add their volts. Go to S3.10.2. Try another.

    3. Not this one. You added the number of cells to the volts. Multiply the cells by the volts of each cell. Go to S3.10.2. Try another.

    See the answer

    3.6 V

    Each cell adds 1.2 V. 3 × 1.2 = 3.6 V.

    • If you chose B: You may think volts do not add. Cells joined in a line add their volts. Go to S3.10.2.
    • If you chose C: You added the number of cells to the volts. Multiply the cells by the volts of each cell. Go to S3.10.2.

  7. One resistor alone takes 0.6 A from a cell. Two of these resistors are joined in series with the cell. How many amps flow in the loop?

    See the answer

    0.3 A

    The two resistors share the push, so the current is half. 0.6 ÷ 2 = 0.3 A.

Keep it

Copy this into your exercise book.

Series: one loop, the same current everywhere, and one gap puts all out. Parallel: each branch has the full push, and the cell gives more. Houses are parallel.

Draw: A cell with two bulbs in series, then a cell with two bulbs on two branches. Write series and parallel.

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

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