Electricity and magnetism. Attnom Academy

S3.10.1 Chapter 1

Current in a circuit

When the bulb gives out light, is some of the electricity used up?

Your guess

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

Your guess

A No. The same current goes all the way round the loop.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Yes. The bulb uses up some of it, so less comes back to the cell.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The current only goes out of the cell. None comes back.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain current as a flow of charge round a circuit
Explain that current is measured in amps with an ammeter

Press the switch of a torch and the bulb lights at once. The cell, the wires and the bulb make one loop. Something moves round that loop. We call it the current.

A metal wire is full of tiny particles called electrons. Each electron carries a tiny amount of charge. The cell pushes all the electrons round the loop together. A flow of charge is a current.

A model. The beads are charges in a loop of wire. The cell pushes them all round at once.

Think of a ring of beads in a tube. Push one bead and all the beads move. No bead is lost. The same number of beads pass every spot each second. A current is like that.

Draw it first

Draw the current round a loop

1 Draw a loop with a cell, a bulb and a closed switch. The bulb is lit.
2 Draw arrows inside the loop. They leave the cell by the long plate and go round the loop. Write current.
3 Write + beside the long plate and − beside the short plate. The arrows run from + round to −.

In a drawing, the current runs from the + end of the cell, round the loop, to the − end. That is the rule.

The electrons in the wire really drift the other way round. The sums come out the same either way, so we keep to the drawing rule.

Electrons are the moving charges in a metal wire. They carry a negative charge. They drift from the − end of the cell to the + end. We draw the current the other way round, from + to −. Keep the two ideas apart in your mind.

We measure current in amps. We write A. A torch bulb might take 0.3 A. An ammeter is the meter that reads the current. It must be part of the loop, so the whole current goes through it.

See it three ways

As a picture

In words

The ammeter sits in the loop, so the current goes through it. It shows how much charge flows past each second. A bigger reading means a bigger current.

In symbols

The reading is 0.3 A. That is 0.3 amps. The symbol for amps is A.

Now put an ammeter on each side of the bulb. If the bulb used up current, the second ammeter would read less. Look.

Both ammeters read 0.3 A. The current is the same on each side of the bulb.

The current is the same all round the loop. The bulb does not use it up. The bulb changes energy from the cell into light and heat. The charges keep going round.

Worked example

In the bead model, 30 beads pass spot A in 10 seconds. How many pass each second? How many pass spot B each second?

The current is how many beads pass a spot each second.
Spot A: 30 beads in 10 s. 30 ÷ 10 = 3 beads each second.
The beads are all in one loop and none is lost. The same number pass spot B each second.
Spot B: 3 beads each second.

Answer: 3 beads each second, at both spots

Try it

Count beads round a loop

You need: a string loop about 1 metre round, 12 beads, seeds or knots on the string, evenly spaced, two stones to mark spot A and spot B, a clock, or a friend who counts seconds.

Be safe: Keep the string loop off your neck and away from small children.

Fix the beads on the string loop at equal gaps. Lay the loop in a ring on the ground.
Put a stone beside the loop at spot A and another at spot B, on the other side.
Pull the string round with your fingers at a steady pace. You are the cell.
Count the beads that pass spot A in 10 seconds. Then count at spot B.

What you will see: The two counts are the same, or off by one if you lose count. Ours were 14 and 14. The string is one loop, so the same number of beads pass every spot. Your counts will differ from ours.

If you cannot try it, this is what happens.

A wrong idea many people have

The bulb uses up the current, so less current comes back to the cell.

Why it feels right: The bulb gives out light and heat, so it feels as if something has been used up.

The test that breaks it: Put an ammeter on each side of the bulb. The first reads 0.3 A. The second also reads 0.3 A.

What is true: The bulb changes energy into light and heat. The charges are not used up. The same current goes all round the loop.

Key words

current: a flow of charge round a complete circuit.
charge: a tiny amount of electricity carried by a particle.
electrons: tiny particles that carry charge. They move in a metal wire.
amps: the unit of current. We write A.
ammeter: a meter that reads the current. It goes in the loop.

Back to your guess

The answer is A: No. The same current goes all the way round the loop.

The same current goes all the way round the loop. The bulb changes energy into light and heat, but the charges are not used up. They keep going round.

Keep it

Copy this into your exercise book.

Current is a flow of charge round a loop. We measure it in amps with an ammeter in the loop. It is the same all round and is not used up.

Draw: A loop with a cell, a bulb and two ammeters, both reading 0.3 A. Draw arrows from + round to −.

S3.10.2 Chapter 2

Cells and voltage

Why do some torches use two or three cells?

Your guess

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

Your guess

A Cells in a line add their push, so more current flows and the bulb is brighter.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The extra cells are spares, used only when the first one goes flat.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Two cells give the same push as one, but they last longer.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain voltage as the push that makes current flow
Predict how adding cells changes the brightness of a bulb

In the last chapter the cell pushed the charges round the loop. How hard does it push? We measure the push as voltage.

We measure voltage in volts. We write V. One torch cell gives about 1.5 V. A bigger push makes a bigger current.

A model. The same beads are pushed harder in the bottom tube, so more beads pass each second.

Voltage is the push. Current is the flow that the push makes. They are not the same thing, and they have different units.

A battery is two or more cells joined in a line. The + end of one cell touches the − end of the next. Their pushes add up.

Draw it first

Draw one, two and three cells

1 Draw a loop with 1 cell and a bulb. The push is 1.5 V. The current is 0.3 A.
2 Add a second cell. The push is now 3 V. The current is about double, 0.6 A, and the bulb is brighter.
3 Add a third cell. The push is 4.5 V. The current is about three times as much, 0.9 A, and the bulb is brightest.

The push adds as you add cells, but only when they face the same way. Each cell has a + end and a − end. Join + to − all along the line.

A cell turned the wrong way pushes against the others. It cancels one of them.

A voltmeter measures the push across a part. You join it across the part, side by side with it. It does not go in the loop.

The voltmeter is across the bulb, not in the loop. It reads the push across the bulb.

See it three ways

As a picture

In words

Each new cell adds 1.5 V. The same bulb takes more current when the push is bigger, so it is brighter. In our sums a bulb keeps the same resistance. A real filament gets hotter as the current grows. So its current grows a little less than the sums.

In symbols

volts = cells × 1.5. For this bulb, 1 cell gives 0.3 A, so 3 cells give about 3 × 0.3 = 0.9 A.

A bulb is made for a certain push. Too many cells send too much current through its thin wire, and it burns out. Use only the cells the bulb is made for.

The loop for the next example. A resistor has the same resistance whatever the current.

Worked example

Kofi's resistor takes 0.2 A on one cell. He joins 4 cells in a line, each 1.5 V. How many volts push, and how many amps flow?

Cells in a line add their volts. 4 × 1.5 = 6 V.
The push is 4 times bigger than for one cell.
For the same resistor the current grows in step. 4 × 0.2 = 0.8 A.

Answer: 6 V and 0.8 A

Try it

Make a bulb brighter with a second cell

You need: two small torch cells, 1.5 V each, a small bulb made for 2.5 V or 3 V, two strips of kitchen foil, folded long and thin, tape.

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

Tape the + end of one cell to the − end of the other. Now they are in a line.
Touch one foil strip to the bulb side and the other to the bulb tip.
Touch the free ends of the strips to the cell you are using, for 3 seconds only. Look at the bulb.
Try one cell alone, then the two cells in a line. Compare the glow.

What you will see: One cell gives a dim glow. Two cells in a line make the bulb brighter. A weak cell gives less push, so an old cell gives a dimmer light too. Your looks may differ from ours.

If you cannot try it, this is what happens.

A wrong idea many people have

Voltage and current are the same thing.

Why it feels right: Both seem to be "how much electricity" a cell gives, and both grow when you add cells.

The test that breaks it: A 1.5 V cell with a 5 Ω bulb gives 0.3 A. A 1.5 V cell with a 10 Ω bulb gives only 0.15 A. The volts are the same, but the amps are not.

What is true: Voltage is the push, in volts. Current is the flow the push makes, in amps. The same push can make a bigger or smaller current.

Key words

voltage: the push that makes a current flow round a circuit.
volts: the unit of voltage. We write V.
battery: cells joined in a line, + to −. Their volts add up.
voltmeter: a meter that reads the push across a part. It goes across the part.

Back to your guess

The answer is A: Cells in a line add their push, so more current flows and the bulb is brighter.

Cells joined in a line add their push. A bigger push makes more current flow round the bulb, so it is brighter. The extra cells are not spares.

Keep it

Copy this into your exercise book.

Voltage is the push, in volts. Cells in a line add their volts. A bigger push means a bigger current and a brighter bulb. A voltmeter goes across a part.

Draw: Three cells in a line, labelled 1.5 V each, with 4.5 V in all. Mark + and − on each cell.

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

A The lights are in parallel, so each has its own path and the full push.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Each light in a house has its own cell.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The wires in a house carry so much that nobody notices one light missing.
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 Series: draw one loop with the cell and two bulbs. The current is 0.15 A, and both bulbs are dim.
2 Parallel: draw two branches across the cell, one bulb on each. The cell gives 0.6 A, and both bulbs are bright.
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.

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

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.

Three lamps in parallel, each with its own switch. A cell stands in for the supply here.

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?

In parallel, each branch gets the full push of the cell.
So each branch takes the same current as one bulb alone: 0.2 A.
The cell gives the sum of the branch currents.
0.2 + 0.2 + 0.2 = 0.6 A.
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.

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.
Lift bulb 1 out of the loop. Look at bulb 2.
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.
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 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.

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.

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

A Put a longer piece of copper wire in the loop. That dims the bulb a lot.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Put a longer piece of pencil lead in the loop. It slows the current.
You picked B. Read on. At the end, the chapter comes back to your guess.
C A dimmer takes electrons out of the loop, so less comes back.
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.

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

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

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.

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?

The push is 2 × 1.5 = 3 V.
In one loop, the resistances add.
Total resistance = 5 + 10 = 15 Ω.
Current = voltage ÷ resistance.
Current = 3 ÷ 15 = 0.2 A.
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.

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.

Make a loop with the cells, the bulb and the foil. Leave a gap, and lay one end of the lead in the gap.
Hold a foil strip on the lead 2 cm from the other foil strip. Count 3 seconds. Look at the bulb.
Slide the strip to 6 cm, then 12 cm. Look at the bulb each time. Do not hold for more than 3 seconds.
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 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.

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.

S3.10.5 Chapter 5

Static electricity

Why does a rubbed comb pick up bits of paper?

Your guess

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

Your guess

A Rubbing makes the comb into a magnet, and paper is magnetic.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Rubbing makes the comb sticky, so the paper sticks to it.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Rubbing gives the comb an electric charge, and the charge pulls the paper.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Charge a comb by rubbing it and use it to lift paper bits
Explain that like charges repel and unlike charges attract

Pull a woolly jumper off in the dark and you may hear a crackle. Rub a plastic comb and it can pick up paper. This is static electricity. The charge stays in one place. It does not flow round a loop.

Everything is made of tiny parts. Some carry a plus charge and some carry a minus charge. Most of the time they balance, so a thing has no charge overall.

Rubbing two materials together can move electrons from one to the other. The material that gains electrons has more minus charges. We say it is negative. The material that loses electrons has more plus charges. We say it is positive.

Draw it first

Draw a comb being charged

1 Draw a comb and wool. Each has equal plus and minus: 3 and 3 on the comb, 4 and 4 on the wool.
2 Rub them. 2 electrons move from the wool to the comb. The comb now has 5 minus marks and the wool has 2. They attract.
3 Hold the comb near bits of paper. The comb pulls on the paper. The paper has equal plus and minus marks, but they move a little.

There is a rule for the pulls and pushes between charges. Charges of the same sign repel, which means they push apart. Charges of different signs attract, which means they pull together.

Two negative balloons repel. A negative and a positive balloon attract.

See it three ways

As a picture

In words

Charges of the same sign push each other away. Charges of different signs pull each other together.

In symbols

+ and + repel. − and − repel. + and − attract.

Why does a charged comb lift paper that has no charge? The comb is negative. It pulls the plus charges in the paper a little nearer. It pushes the minus charges a little away.

The paper has no charge overall. Its plus marks move to the side nearest the comb.

The plus charges are now nearer to the comb than the minus ones. A pull from nearby is stronger than a push from farther away. So the paper is pulled to the comb.

After rubbing, the comb has 2 more minus marks than plus. The wool has 2 more plus marks than minus.

Worked example

A comb has 3 plus and 3 minus charges. Rubbing moves 2 electrons onto it. How many more minus than plus charges does the comb have now?

Before rubbing: 3 plus and 3 minus. They balance.
Rubbing adds 2 electrons. Each is a minus charge.
Minus charges now: 3 + 2 = 5.
The plus charges are still 3.
5 − 3 = 2, so the comb has 2 more minus than plus. It is negative.

Answer: 2 more minus than plus, so the comb is negative

Static electricity works best on a dry day. In damp air the charge leaks away into the moisture, so a comb loses its charge fast.

Try it

Lift paper with a comb

You need: a plastic comb, or a plastic pen or ruler, dry hair or a woollen cloth, tiny bits of paper, each the size of a fingernail, a dry day.

Tear the paper into tiny bits and spread them on a table.
Hold the comb above the bits without rubbing it. Does anything move?
Rub the comb on dry hair or wool, 20 quick strokes in one direction.
Hold the comb just above the paper, not touching. Count the bits that jump up.

What you will see: The comb that was not rubbed lifts nothing. The rubbed comb pulls up several bits. Some bits may jump off again. They take the comb charge and are pushed away. Your counts will differ from ours.

If you cannot try it, this is what happens.

A wrong idea many people have

The comb picks up paper because the comb becomes a magnet.

Why it feels right: A magnet also picks things up without touching them, so the two seem alike.

The test that breaks it: A magnet does not move bits of kitchen foil. A rubbed comb pulls them. The two kinds of pull are different.

What is true: The comb has an electric charge. It pulls the charges in the paper. A magnet works in a different way.

Key words

static electricity: charge that stays in one place and does not flow.
negative: having more minus charges than plus charges.
positive: having more plus charges than minus charges.
repel: to push apart.
attract: to pull together.

Back to your guess

The answer is C: Rubbing gives the comb an electric charge, and the charge pulls the paper.

Rubbing moves electrons onto the comb, so it has a charge. The charge pulls on the paper. Paper is not magnetic, and the comb is not sticky.

Keep it

Copy this into your exercise book.

Rubbing moves electrons. The one that gains them is negative. Like charges repel and unlike charges attract. A charged comb pulls paper. Dry days work best.

Draw: A comb with more minus marks near bits of paper, with an arrow of attract. Write + and − on both.

S3.10.6 Chapter 6

Magnetic fields

How can I see the invisible push and pull around a magnet?

Your guess

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

Your guess

A A magnet has a field all round it, and a compass or steel wool can show it.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A magnet acts only at its two ends and nowhere else.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The pull stops at the surface of the magnet, so there is nothing to see.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Map a magnet's field with a compass or bits of steel wool
Explain that the Earth has a magnetic field

A magnet can pull a pin across a table without touching it. The push or pull reaches out into the space round the magnet. That space is the magnetic field of the magnet.

You cannot see a field. But a small magnet, such as a compass needle, feels it. The needle turns until it lines up with the field.

We draw a field with field lines. They run from the north pole of the magnet round to the south pole. An arrow on each line shows the way a north pole would be pushed.

Draw it first

Draw the field of a bar magnet

1 Draw a bar magnet. Write N on one end and S on the other.
2 Draw lines that leave the N end, curve round and come into the S end. Put an arrow on each line, pointing from N to S.
3 Put a compass on a line. The black end of the needle points the way the arrow points.

The lines are closest together near the poles. That is where the field is strongest. Far from the magnet the lines are far apart, and the field is weak.

The lines crowd together near the poles, where the field is strongest.

See it three ways

As a picture

In words

A compass needle lines up with the field. Its black end points the way the field line points. Near the N pole it points away from the magnet. Near the S pole it points towards it.

In symbols

Outside a magnet, field lines run from N to S.

Two magnets make a joined field. If a N pole faces a S pole, the lines join across the gap. The magnets pull together. If two N poles face each other, the lines push apart. The magnets repel.

N facing S: the lines join across the gap and the magnets attract.

N facing N: the lines push apart. At the neutral point the field of one magnet cancels the other.

The Earth has a magnetic field too. It is as if a huge bar magnet were inside the Earth. A compass needle lines up with this field. The black end points roughly to the geographic north.

The Earth acts like a huge magnet. Almost everywhere, a compass needle points roughly north.

The end of the Earth's magnet near the geographic north is a magnetic S pole. It pulls the black north end of a compass towards it. This works in most places. Very near the magnetic poles, compasses are not reliable.

Try it

Map a magnet's field

You need: a bar magnet, a plain sheet of paper, tiny bits of steel wool, cut small by an adult with scissors, a compass, if you have one.

Be safe: Steel wool is sharp. Ask an adult to cut it. Keep it away from your eyes and mouth, and wash your hands after.

Lay the magnet flat on the table. Put the paper over it.
Sprinkle the bits of steel wool over the paper. Tap the paper gently.
Look at the pattern. Which way do the lines curve? Where are they thickest?
With a compass, move it slowly round the magnet. Watch the needle turn.

What you will see: The steel wool lines up in curves from one end of the magnet to the other. It is thickest near the ends of the magnet, where the field is strongest. The compass needle turns as you move it, always along the lines. Your pattern will look like this, but not exactly.

If you cannot try it, this is what happens.

A wrong idea many people have

A magnet acts only at its two ends.

Why it feels right: Pins stick to the ends of a magnet. The ends seem to be the only places that pull.

The test that breaks it: Put a compass above the middle of a bar magnet. The needle turns to line up with the magnet. The field is there, too.

What is true: The field is all round the magnet. It is strongest near the poles, but it is not zero anywhere near the magnet.

Key words

magnetic field: the space round a magnet where it pushes or pulls.
field lines: lines that show a field. Outside a magnet they run from N to S.
compass: a small magnet on a pivot. It lines up with a field.

Back to your guess

The answer is A: A magnet has a field all round it, and a compass or steel wool can show it.

A magnet has a magnetic field all round it. The field is strongest near the poles, but it spreads out in every direction. A compass or bits of steel wool can show it.

Keep it

Copy this into your exercise book.

A magnet has a field all round it. Field lines run from N to S, closest near the poles. A compass lines up with them. The Earth has a field too.

Draw: A bar magnet marked N and S. Three curved lines with arrows from N to S. A compass on one line.

S3.10.7 Chapter 7

Electromagnets

Can I make a magnet that I can switch on and off?

Your guess

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

Your guess

A No. A magnet is always a magnet and cannot be switched off.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Yes. A current in a coil of wire makes a magnet, and it stops when the current stops.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Yes. Rubbing a nail on wool makes a magnet that you can switch off.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Make an electromagnet with a nail, wire and a cell
Test how more turns of wire change its strength

A wire that carries a current has a magnetic field round it. The field of one wire is weak. Wind the wire into a coil and the fields of all the turns add up.

A coil with a current acts like a bar magnet. It has a N pole at one end and a S pole at the other. Turn the current off and the field goes.

A bar magnet, standing in for the coil. A coil with a current has a field like this one. Reverse the current and the N and S ends swap.

Put an iron nail inside the coil. The nail is the core. Iron becomes a magnet when it is in a field. The nail adds its own field to that of the coil. Together they are much stronger.

An electromagnet is a coil with a current in it, usually wound round an iron core. It is a magnet that you can switch on and off.

Draw it first

Draw an electromagnet and switch it on

1 Wind wire round an iron nail and join it to a cell and a switch. Leave the switch open. No current flows, so no clips stick.
2 Close the switch. A current flows in the coil and the nail becomes a magnet. With 10 turns it holds 2 clips.
3 Wind more turns. With 20 turns, the same cell holds 3 clips. The magnet is stronger.

When the switch is opened, the current stops. The iron nail loses nearly all of its magnetism, and the clips fall. A steel nail may keep a little magnetism.

One learner's test, 20 turns and one cell. The iron nail makes the coil much stronger.

Two things make an electromagnet stronger. One is more turns of wire. The other is a bigger current, which you get from more cells. The results below are one learner's. Yours will differ.

See it three ways

As a picture

In words

More turns, or more cells, lift more clips. Clip counts are rough, so do not expect them to grow exactly in step.

In symbols

5 turns: 1 clip. 20 turns: 3 clips. More turns, more clips.

The results for the next example. One cell each time.

Worked example

One learner's electromagnet picks up 1 clip with 5 turns and 3 clips with 20 turns. How many times as many turns is that, and how many times as many clips?

Read the table: 5 turns pick up 1 clip. 20 turns pick up 3 clips.
The turns grew 20 ÷ 5 = 4 times.
The clips grew 3 ÷ 1 = 3 times.
More turns picked up more clips. Clip counts are rough, so they do not grow exactly in step.

Answer: 4 times as many turns and 3 times as many clips

Electromagnets are used in scrapyard cranes, door locks and electric bells. They are useful because they can be switched off. A crane drops its load when the current stops.

Try it

Make an electromagnet

You need: a long iron nail, about 8 cm, about 1 metre of thin wire, with the ends bared by an adult, one or two torch cells of 1.5 V, tape, small paper clips or pins.

Be safe: The wire and the cell can get hot. Connect for only a few seconds, then let go. Let them cool before you touch them. Keep the nail tip and the pins away from your face.

Wind the wire in 10 neat turns round the nail. Leave a tail at each end. An adult bares the ends, with care, as the blade is sharp.
Tape one end of the wire to one end of the cell. Hold the other end on the other end of the cell for 3 seconds. Count the clips the nail tip lifts.
Lift the wire off the cell. Do the clips fall? Let the wire and the cell cool for a minute.
Now wind 20 turns and count again. Then try two cells in a line.

What you will see: With more turns the nail holds more clips. With two cells it holds more again. When you lift the wire off the cell, most clips fall. Our 20 turns and one cell held 3 clips. Your counts will differ.

If you cannot try it, this is what happens.

A wrong idea many people have

An electromagnet is a permanent magnet that stays on once it is made.

Why it feels right: A nail with a coil looks like any other magnet, and a bar magnet never switches off.

The test that breaks it: Lift some clips with the electromagnet. Lift the wire off the cell and the clips drop. Join it again and they stick again.

What is true: An electromagnet works only while a current flows. An iron core loses nearly all its magnetism when the current stops.

Key words

coil: wire wound round and round.
turns: the loops of wire in a coil.
core: the iron nail inside a coil.
electromagnet: a coil with a current in it, usually round an iron core.

Back to your guess

The answer is B: Yes. A current in a coil of wire makes a magnet, and it stops when the current stops.

A current in a coil of wire makes a magnetic field. Switch the current off and the field goes. That is an electromagnet.

Keep it

Copy this into your exercise book.

A current in a coil makes a magnet. An iron core makes it stronger. More turns or more cells make it stronger. Switch off and an iron core loses most of its magnetism.

Draw: An iron nail with a coil of wire. A cell and a switch. Paper clips under the tip.

S3.10.8 Chapter 8

Electricity safety at home

Why must I never touch a switch with wet hands?

Your guess

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

Your guess

A Water makes the mains electricity stronger.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Clean water cannot carry electricity, so wet hands are safe.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Water with things in it carries current, and wet skin lets far more current through.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain why water and electricity are a dangerous mix
Explain how a fuse protects a home circuit

The electricity in a home socket is called the mains. It is very different from a torch cell. A cell gives 1.5 V. The mains gives about 110 to 240 V, depending on where you live.

Mains electricity can kill. A torch cell is safe to touch, but never touch the mains. This chapter is about staying safe and about the part that guards a home circuit, the fuse.

The current through your body depends on the voltage and on the resistance of your body. Current = voltage ÷ resistance. Dry skin has a high resistance. Wet skin has a much lower one.

Water with anything dissolved in it carries a current. Tap water, sweat and rain all have things dissolved in them. Water on your skin lowers the resistance of your body.

Worked example

Take the mains as 200 V. Where you live it may be nearer 110 V or 240 V. Dry skin has about 10 000 Ω and wet skin about 1 000 Ω. How much more current passes through wet skin?

Current = voltage ÷ resistance.
Dry skin: 200 ÷ 10 000 = 0.02 A.
Wet skin: 200 ÷ 1 000 = 0.2 A.
Wet skin against dry skin: 0.2 ÷ 0.02 = 10 times as much current.

Answer: 10 times as much current

A made up sum. A milliamp, mA, is 0.001 A, so 20 mA is 0.02 A. Real skin varies, but wet skin always lets far more current through.

A current of 0.02 A can lock your muscles, so you cannot let go. More than about 0.05 A through the body can kill. With wet skin the current in the sum is far above both. Even with dry hands the mains can kill. At mains voltage the resistance of the skin breaks down.

So never touch a switch, a plug or an appliance with wet hands. Never use electric things near water.

Now the fuse. Many things can send too much current round a circuit. Too many appliances on one socket do it. A fault does it too. Too much current makes wires hot, and hot wires can start a fire.

A fuse is a thin wire in the live wire of the circuit. It is made to melt when the current is too big. The wire melts and breaks the circuit before the other wires get hot.

The fuse is in the live wire. All the current for the heater goes through it.

Draw it first

Draw a home circuit and a blown fuse

1 working: the fuse is whole. The lamp is on.
2 blown: the fuse wire has melted. The circuit is broken, and the lamp is off.
3 switched off: the fuse is whole. The switch is open, so the lamp is off.

Appliances on one circuit are in parallel. Each takes its own current, and the fuse carries the total. Look at a socket circuit with a heater, a kettle and a lamp.

A model of a socket circuit with a 3 A fuse. Two cells stand in for the mains, and the currents are small model ones. The fuse carries the current of all three.

Worked example

A 3 A fuse guards the circuit. The heater takes 1.2 A, the kettle 1 A and the lamp 0.6 A. Ravi plugs in a fourth appliance that takes 0.6 A. What is the new total, and does the fuse blow?

In parallel, the currents add up.
Before: 1.2 + 1 + 0.6 = 2.8 A.
With the fourth: 2.8 + 0.6 = 3.4 A.
The fuse is 3 A. 3.4 A is more than 3 A, so the fuse wire melts.

Answer: 3.4 A, so the fuse blows

A circuit breaker does the same job as a fuse. It is a switch that flips off when the current is too big. You can switch it back on once the fault is fixed. A fuse wire must be replaced.

See it three ways

As a picture

In words

Keep water away from electric things. Keep your hands dry. Tell an adult about a damaged wire or a hot plug. Never touch a power line that has fallen.

In symbols

Wet skin: resistance low, current large. Fuse: current too large, wire melts, circuit breaks.

Try it

Survey the places water and electricity meet

You need: paper and a pencil, a walk round your home, looking only.

Be safe: This activity uses no electricity. Look from a safe distance and do not touch plugs, sockets, switches or wires. If you see a damaged wire, tell an adult.

Walk round your home with the paper. Do not touch any plug, switch or wire.
Look for each place where water and electricity are close. Think of the sink, the bathroom, a tap near a socket, and a cable outside in the rain.
Draw each place. Mark where the water is and where the electric thing is. Keep to a safe distance as you draw.
For each place write one habit that keeps them apart. Tell an adult about any damaged wire.

What you will see: Most homes have several places where water and electricity are close. A habit, such as drying your hands or keeping plugs off the wet floor, keeps each one safe. Your places will differ from ours.

If you cannot try it, this is what happens.

A wrong idea many people have

It is safe to touch a switch with wet hands if the water looks clean.

Why it feels right: Clean water looks harmless, and the water does not change the switch.

The test that breaks it: Do not test it with electricity. Use the sum instead. With dry skin, 0.02 A passes. With wet skin, 0.2 A passes, which is 10 times as much. Dry hands are not safe on the mains.

What is true: Water on your skin lowers its resistance. Even clean-looking water has things dissolved in it. Dry your hands before you touch any electric thing.

Key words

mains: the electricity supply to a home. It can kill.
fuse: a thin wire that melts when too much current flows.
circuit breaker: a switch that flips off when too much current flows. It can be reset.

Back to your guess

The answer is C: Water with things in it carries current, and wet skin lets far more current through.

Water with anything dissolved in it carries a current. Wet skin lets far more current through the body. The mains is not stronger, but the body lets more current pass.

Keep it

Copy this into your exercise book.

Mains can kill. Wet skin lets far more current through, so keep hands dry. A fuse is a thin wire that melts when the current is too big. A circuit breaker can be reset.

Draw: A home circuit with a supply, a fuse, a switch and a lamp. Mark the fuse wire.

Draw the current round a loop

Count beads round a loop

Draw one, two and three cells

Make a bulb brighter with a second cell

Draw two bulbs in series, then in parallel

Join two bulbs two ways

Draw a dimmer and turn it up

Make a dimmer from pencil lead

Draw a comb being charged

Lift paper with a comb

Draw the field of a bar magnet

Map a magnet's field

Draw an electromagnet and switch it on

Make an electromagnet

Draw a home circuit and a blown fuse

Survey the places water and electricity meet

Key words, A to Z