Chapter 4 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

S3.10.4 Chapter 4

Resistance

How can I make a bulb dimmer, like the dimmer on a lamp?

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

  • Explain resistance as how much a part slows the current
  • Make a dimmer from pencil lead in a circuit
  • Explain why wires get warm when current flows

Some parts let a current through with little trouble. Others slow it down. The amount a part slows the current is its resistance.

We measure resistance in ohms. We write Ω. A part with a big number of ohms slows the current a lot. A part with a small number slows it only a little.

Two tubes of beads. The top tube is empty and the beads have long arrows pointing right. It is labelled wide and clear: low resistance. The bottom tube has hatched lumps in the way, and the beads have short arrows pointing right. It is labelled hard to push through: high resistance. Labels: wide and clear: low resistance, hard to push through: high resistance.wide and clear: low resistancehard to push through: high resistance
A model. With the same push, the beads move slowly through the tube with lumps in it.

A part with high resistance slows the current, even when the push is the same. A part with low resistance lets the current through freely.

Draw it first

Draw a dimmer and turn it up

  1. Three loops, each with a battery of two cells, an ammeter, a bulb and a variable resistor, which is a box with an arrow through it. In the first the variable resistor is set to 5 Ω and the ammeter reads 0.3 A. In the second it is set to 10 Ω and the ammeter reads 0.2 A. In the third it is 25 Ω and the ammeter reads 0.1 A. The rays round the bulb get shorter each time. Labels: A, 5 Ω, 0.3 A, 10 Ω, 0.2 A, 25 Ω, 0.1 A.A0.3 A5 Ω

    1 A variable resistor can be set to more or less resistance. At 5 Ω the current is about 0.3 A and the bulb is bright.

  2. Three loops, each with a battery of two cells, an ammeter, a bulb and a variable resistor, which is a box with an arrow through it. In the first the variable resistor is set to 5 Ω and the ammeter reads 0.3 A. In the second it is set to 10 Ω and the ammeter reads 0.2 A. In the third it is 25 Ω and the ammeter reads 0.1 A. The rays round the bulb get shorter each time. Labels: A, 5 Ω, 0.3 A, 10 Ω, 0.2 A, 25 Ω, 0.1 A.A0.2 A10 Ω

    2 Turn it up to 10 Ω. The current falls to 0.2 A, and the bulb is dimmer.

  3. Three loops, each with a battery of two cells, an ammeter, a bulb and a variable resistor, which is a box with an arrow through it. In the first the variable resistor is set to 5 Ω and the ammeter reads 0.3 A. In the second it is set to 10 Ω and the ammeter reads 0.2 A. In the third it is 25 Ω and the ammeter reads 0.1 A. The rays round the bulb get shorter each time. Labels: A, 5 Ω, 0.3 A, 10 Ω, 0.2 A, 25 Ω, 0.1 A.A0.1 A25 Ω

    3 Turn it up to 25 Ω. The current falls to 0.1 A, and the bulb is dim. This is how a lamp dimmer works.

Parts in one loop add their resistances. And the current is the voltage divided by the resistance. A bigger resistance gives a smaller current.

What makes the resistance of a wire big? A longer wire has more resistance. A thinner wire has more resistance. A short, thick wire has the least.

Three wires drawn as bars. The first is short and thick, labelled short, thick wire, with the word least. The second is long and thick, labelled long, thick wire, with the word more. The third is long and thin, labelled long, thin wire, with the word most. The words mean the amount of resistance. Labels: short, thick wire, long, thick wire, long, thin wire, least, more, most.short, thick wireleastlong, thick wiremorelong, thin wiremost
Resistance grows with the length of a wire and falls as it gets thicker.

The middle of a pencil is called lead, but it is not metal. It is mostly graphite, a form of carbon, mixed with clay. Graphite lets a current through, but with a high resistance. A longer piece of lead has more resistance.

See it three ways

As a picture

A table with the columns dimmer, total and amps. 5 Ω: 10 Ω in all, 0.3 A. 10 Ω: 15 Ω in all, 0.2 A. 25 Ω: 30 Ω in all, 0.1 A. The bulb is 5 Ω and the push is 3 V.dimmertotalamps5 Ω10 Ω0.310 Ω15 Ω0.225 Ω30 Ω0.1

In words

A bigger resistance in the loop gives a smaller current. The bulb is dimmer when the current is smaller.

In symbols

current = voltage ÷ resistance. For the first row: 3 ÷ 10 = 0.3 A.

A loop with a battery of two cells, an ammeter, a lit bulb marked 5 Ω and a variable resistor marked 10 Ω. The ammeter shows no reading. Labels: A, 5 Ω, 10 Ω.A10 Ω5 Ω
The loop for the next example. The bulb and the dimmer are in series. In our sums the bulb keeps a fixed 5 Ω. The resistance of a real bulb rises as its wire gets hot.

Worked example

Mei has two cells of 1.5 V in a line. Her bulb is 5 Ω and her dimmer is set to 10 Ω, all in one loop. What is the current?

  1. The push is 2 × 1.5 = 3 V.
  2. In one loop, the resistances add.
  3. Total resistance = 5 + 10 = 15 Ω.
  4. Current = voltage ÷ resistance.
  5. Current = 3 ÷ 15 = 0.2 A.
  6. The bulb is dimmer than before. At 5 Ω the current was 3 ÷ 10 = 0.3 A.

Answer: 0.2 A

A current through a resistance makes it warm. The moving charges bump into the atoms of the metal and make them shake more. Energy goes to the thermal store. The wire gets warmer.

A piece of metal wire drawn as a box. Two rows of hatched squares are the atoms of the metal, labelled atoms of the metal. Between the rows are five circles, the moving charges, labelled moving charges, with a long arrow pointing right under the wire. Labels: atoms of the metal, moving charges.atoms of the metalmoving charges
The charges bump into the atoms, so the atoms shake more. The wire warms up.

That is how a heater works. A heater has a wire with a high resistance, so it gets hot. A bulb has a very thin wire, called the filament. It gets so hot that it glows. Ordinary wires have a low resistance, so they warm only a little.

Try it

Make a dimmer from pencil lead

You need: two torch cells of 1.5 V, taped + to −, a small bulb made for 3 V, a soft pencil with the wood shaved away along one side by an adult, three strips of kitchen foil, folded long and thin, a ruler.

Be safe: Ask an adult to shave the wood off with a knife, or use a pencil already shaved. Connect for only a few seconds. The lead and the cells can get warm, so let them cool before you touch them.

  1. Make a loop with the cells, the bulb and the foil. Leave a gap, and lay one end of the lead in the gap.
  2. Hold a foil strip on the lead 2 cm from the other foil strip. Count 3 seconds. Look at the bulb.
  3. Slide the strip to 6 cm, then 12 cm. Look at the bulb each time. Do not hold for more than 3 seconds.
  4. Let everything cool for a minute. Do not touch the lead or the foil while it may be warm.

What you will see: A longer length of lead dims the bulb more. The lead and the foil may get a little warm, because a current warms a resistance. That is why you leave them to cool. Your results may differ from ours.

If you cannot try it, this is what happens.
A table of one learner's results, with the heads length of lead and bulb. 2 cm: bright. 6 cm: dim. 12 cm: faint.leadbulb2 cmbright6 cmdim12 cmfaint

A wrong idea many people have

A resistor uses up the current, so less current comes out.

Why it feels right: The resistor gets warm, so it seems as if something has been used up in it.

The test that breaks it: Put an ammeter on each side of a 10 Ω resistor in the loop. Both read 0.2 A. The current is the same on both sides.

What is true: A resistor slows the current in the whole loop. The energy goes to the thermal store, which warms it. The current is not used up.

Key words

resistance
how much a part slows the current.
ohms
the unit of resistance. We write Ω.
graphite
the soft black stuff in a pencil. It carries a current with a high resistance.
filament
the thin wire inside a bulb. It gets so hot that it glows.

Back to your guess

The answer is B: Put a longer piece of pencil lead in the loop. It slows the current.

A longer piece of pencil lead slows the current more. Copper wire hardly slows it at all. And no electrons are taken out of the loop. The current is not used up.

Check yourself

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

  1. What does the resistance of a part tell you?

    Tap your answer.

    1. Not this one. That is the voltage, in volts. Resistance is how much a part slows the current. Go to S3.10.2. Try another.

    2. Not this one. No part uses up the current. The same current goes all round a series loop. Go to S3.10.1. Try another.

    3. Yes, that is right. Resistance is how much a part slows the current, measured in ohms.

    See the answer

    How much the part slows the current.

    Resistance is how much a part slows the current, measured in ohms.

    • If you chose A: That is the voltage, in volts. Resistance is how much a part slows the current. Go to S3.10.2.
    • If you chose B: No part uses up the current. The same current goes all round a series loop. Go to S3.10.1.

  2. Which of these three wires has the most resistance?

    Three wires drawn as bars and labelled A, B and C. Wire A is short and thick. Wire B is long and thick. Wire C is long and thin. Labels: A, B, C.ABC
    Tap your answer.

    1. Yes, that is right. A longer wire has more resistance, and so does a thinner wire. C is both long and thin.

    2. Not this one. A is short and thick, which gives the least resistance, not the most. Go to S3.10.4. Try another.

    3. Not this one. B is long, but it is as thick as A. The thin wire C is also long, so it slows the current more. Go to S3.10.4. Try another.

    See the answer

    Wire C

    A longer wire has more resistance, and so does a thinner wire. C is both long and thin.

    • If you chose B: A is short and thick, which gives the least resistance, not the most. Go to S3.10.4.
    • If you chose C: B is long, but it is as thick as A. The thin wire C is also long, so it slows the current more. Go to S3.10.4.

  3. Why does a wire get warm when a current flows in it?

    Tap your answer.

    1. Not this one. The current is not used up. The same current goes all round the loop. Go to S3.10.1. Try another.

    2. Yes, that is right. Energy goes from the moving charges to the atoms. The atoms shake more, so the wire warms. The energy goes to the thermal store.

    3. Not this one. Wires joined to a buzzer or a motor warm up too, with no light at all. The warming comes from the charges bumping into the atoms. Go to S3.10.4. Try another.

    See the answer

    The moving charges bump into the atoms of the metal and make them shake.

    Energy goes from the moving charges to the atoms. The atoms shake more, so the wire warms. The energy goes to the thermal store.

    • If you chose A: The current is not used up. The same current goes all round the loop. Go to S3.10.1.
    • If you chose C: Wires joined to a buzzer or a motor warm up too, with no light at all. The warming comes from the charges bumping into the atoms. Go to S3.10.4.

  4. Mei makes a dimmer from pencil lead. She slides the foil strip farther along the lead, so more lead is in the loop. What happens to the bulb?

    Tap your answer.

    1. Not this one. More lead means more resistance, not less. More resistance slows the current. Go to S3.10.4. Try another.

    2. Not this one. Pencil lead is graphite, which has a high resistance. A longer piece slows the current more than a short one. Go to S3.10.4. Try another.

    3. Yes, that is right. The longer the lead in the loop, the more the resistance. A bigger resistance gives a smaller current, so the bulb is dimmer.

    See the answer

    It gets dimmer, because a longer piece of lead has more resistance.

    The longer the lead in the loop, the more the resistance. A bigger resistance gives a smaller current, so the bulb is dimmer.

    • If you chose A: More lead means more resistance, not less. More resistance slows the current. Go to S3.10.4.
    • If you chose B: Pencil lead is graphite, which has a high resistance. A longer piece slows the current more than a short one. Go to S3.10.4.

  5. Amina joins two cells of 1.5 V in a line. She adds a 4 Ω resistor and a 2 Ω resistor in one loop. What is the current?

    A loop with a battery of two cells, a resistor marked 4 Ω and a resistor marked 2 Ω, all in one loop. Labels: 2 cells, 4 Ω, 2 Ω.2 cells4 Ω2 Ω
    Tap your answer.

    1. Yes, that is right. The push is 2 × 1.5 = 3 V. The resistances add: 4 + 2 = 6 Ω. Current = 3 ÷ 6 = 0.5 A.

    2. Not this one. You used only the 4 Ω resistor. In one loop the resistances add, so both count: 4 + 2 = 6 Ω. Go to S3.10.4. Try another.

    3. Not this one. You multiplied the volts by the ohms. Divide instead: current = voltage ÷ resistance. Go to S3.10.4. Try another.

    See the answer

    0.5 A

    The push is 2 × 1.5 = 3 V. The resistances add: 4 + 2 = 6 Ω. Current = 3 ÷ 6 = 0.5 A.

    • If you chose B: You used only the 4 Ω resistor. In one loop the resistances add, so both count: 4 + 2 = 6 Ω. Go to S3.10.4.
    • If you chose C: You multiplied the volts by the ohms. Divide instead: current = voltage ÷ resistance. Go to S3.10.4.

  6. From an earlier chapter

    Review: one bulb alone takes 0.5 A from a cell. 3 of these bulbs are joined in parallel across the cell. How many amps does the cell give?

    Tap your answer.

    1. Not this one. That is the current for one bulb. Each branch takes its own current, and the cell gives the sum. Go to S3.10.3. Try another.

    2. Yes, that is right. Each of the 3 branches takes 0.5 A. 3 × 0.5 = 1.5 A.

    3. Not this one. You shared the current between the bulbs. In parallel each branch gets the full push and takes its full current. Go to S3.10.3. Try another.

    See the answer

    1.5 A

    Each of the 3 branches takes 0.5 A. 3 × 0.5 = 1.5 A.

    • If you chose A: That is the current for one bulb. Each branch takes its own current, and the cell gives the sum. Go to S3.10.3.
    • If you chose C: You shared the current between the bulbs. In parallel each branch gets the full push and takes its full current. Go to S3.10.3.

  7. A 6 V supply is joined to a resistor of 12 Ω. What is the current?

    See the answer

    0.5 A

    Current = voltage ÷ resistance = 6 ÷ 12 = 0.5 A.

Keep it

Copy this into your exercise book.

Resistance is how much a part slows the current, in ohms. Longer or thinner wire has more. Current = voltage ÷ resistance. Current warms a resistance.

Draw: A loop with a cell, a bulb and a variable resistor. Write 3 V, then 3 ÷ 15 = 0.2 A.

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

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