GCSE · Chapter 20
Magnetism, electromagnets, the motor effect and induction
By the end of this chapter you will be able to
- Draw and describe the field of a bar magnet and the Earth
- Say how to make an electromagnet stronger, and what it is used for
- Apply Fleming's left-hand rule to find the direction of the force
- Explain how a motor turns, and how a generator makes an induced voltage
- Calculate the secondary voltage or number of turns for a transformer
You should already know
- Current, pd and resistance (Chapters 5 and 6)
- Forces and their directions (Chapter 13)
Where this leads
- Mains supply and the cost of electricity (Chapter 8)
- The electromagnetic spectrum (Chapter 19)
GCSE Physics. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe magnetic fields and field lines, including the bar magnet and the Earth’s magnetic field, Distinguish permanent and induced magnets, Describe electromagnets and the factors that change their strength, and their uses, Use Fleming's left-hand rule for the force on a current-carrying conductor in a magnetic field, and describe the motor, the split-ring commutator and the loudspeaker, Describe electromagnetic induction, and the generator (idea), Use the transformer equation Vp / Vs = Np / Ns, and [H] power in = power out.
Exercise P Do you remember?
1
FWhich is the name of the end of a magnet that points north in a compass? Write the word.
north
Method. Recall how a compass lines up.
- Recall. A compass needle points to the Earth’s north, which is near the magnetic south.north
- Write the answer. north
Check. ✓
2
FWhat is 230 multiplied by 0.5? Write the number.
115
Method. Multiply.
- Multiply. 230 x 0.5 = 115.115
- Write the answer. 115
Check. ✓
3
FWhat is 500 divided by 100? Write the number.
5
Method. Divide.
- Divide. 500 / 100 = 5.5
- Write the answer. 5
Check. ✓
4
FWhat is the direction of current in a circuit, conventionally: from the positive or from the negative terminal? Write positive or negative.
positive
Method. Conventional current leaves the positive terminal.
- Recall. Conventional current flows from positive to negative.positive
- Write the answer. positive
Check. ✓
5
FWhat is the unit of current? Write the unit.
ampere
Method. Recall the unit.
- Recall. Current is measured in amperes, A.ampere (A)
- Write the answer. ampere
Check. ✓
20.1
Magnets and magnetic fields
Video lesson: Magnets and magnetic fieldsComing soon
Context. A compass on a desk turns to point north, even though nothing touches it. Its needle is a small magnet. “What is pushing it round?” asks Tom. It is the magnetic field of the Earth, which acts at a distance.
A magnet has a north pole and a south pole. Like poles repel; unlike poles attract. A magnetic field is a region where a magnetic force acts. Field lines show the direction of the field: they go from the north pole to the south pole outside the magnet. The lines are closer together where the field is stronger.
The Earth behaves like a large bar magnet, with its field pointing roughly from the geographic south towards the geographic north. A compass needle lines up with this field.
A permanent magnet stays magnetised. An induced magnet is a material, such as iron, that becomes a magnet only when it is in a magnetic field; it is always attracted to the magnet.
The field lines of a bar magnet, from north to south
Key ideas
- Like poles repel; unlike poles attract.
- Field lines: from N to S outside the magnet; closer lines mean a stronger field.
- Permanent magnets stay magnetised; induced magnets only when in a field.
- Iron is always attracted to a magnet (it becomes induced).
Worked example 20.1.1
Two bar magnets are placed with the north pole of one next to the north pole of the other. Do they attract or repel? Write attract or repel.
Method. Like poles repel.
- Look. Both ends facing each other are north poles.N and N
- Decide. Like poles repel each other.repel
- Write the answer. repel
Check. ✓
Worked example 20.1.2
Which way does a compass needle point in a magnetic field? Write the word: along the field lines or across them.
Method. Think about how a compass lines up.
- Look. A compass needle lines up with the magnetic field.lines up
- Decide. So it points along the field lines.along the field lines
- Write the answer. along the field lines
Check. ✓
Worked example 20.1.3
What is the name of a magnetic material that becomes a magnet only when it is in a magnetic field? Write the word: induced or permanent.
Method. Recall the two types of magnet.
- Look. A permanent magnet stays magnetised all the time.stays magnetised
- Decide. A material that is only magnetised in a field is an induced magnet.induced
- Write the answer. induced
Check. ✓
Watch out
Watch out: magnetic field lines are not real lines; they show the direction and strength of the field. They never cross each other.
Exam tip
Exam tip. A field line goes from north to south outside a magnet. Draw arrows on the lines to show the direction.
Exercise 20A Magnets and fields
1
FWhat happens when the north pole of one magnet is placed near the south pole of another: attract or repel? Write the word.
attract
Method. Unlike poles attract.
- Recall. Unlike poles attract each other.attract
- Write the answer. attract
Check. ✓
2
FWhat happens when two south poles are brought together: attract or repel? Write the word.
repel
Method. Like poles repel.
- Recall. Like poles repel each other.repel
- Write the answer. repel
Check. ✓
3
FIn which direction do field lines point outside a bar magnet? Write the words: from north to south.
north to south
Method. Recall the direction.
- Recall. Field lines point from the north pole to the south pole outside the magnet.north to south
- Write the answer. from north to south
Check. ✓
4
FWhat does a closer spacing of field lines show? Write one word: stronger or weaker.
stronger
Method. Closer lines mean a stronger field.
- Recall. Where the field lines are closer, the field is stronger.stronger
- Write the answer. stronger
Check. ✓
5
FName the magnetic material that is always attracted to a magnet. Write the name.
iron
Method. Recall the material.
- Recall. Iron is attracted to a magnet; it becomes an induced magnet.iron
- Write the answer. iron
Check. Accept steel or nickel. ✓
6
FWhat type of magnet is a bar of steel that stays magnetised? Write the word: permanent or induced.
permanent
Method. It stays magnetised.
- Recall. A magnet that stays magnetised is a permanent magnet.permanent
- Write the answer. permanent
Check. ✓
7
PSWhich way does the Earth’s magnetic field point at the surface, roughly? Write the words: south to north or north to south.
south to north
Method. The Earth acts like a large bar magnet.
- Recall. The field points roughly from the geographic south to the geographic north.south to north
- Write the answer. south to north
Check. ✓
8
PSA compass is held near a magnet and its needle points along the field. What does the north-seeking end of the needle show? Write the word: direction or strength.
direction
Method. The needle shows the direction of the field.
- Recall. The compass needle shows the direction of the magnetic field at that point.direction
- Write the answer. direction
Check. ✓
9
RExplain in one sentence why a paper clip is attracted to a magnet on one side. Accept any sentence that says the magnet induces a magnetic effect in the clip, which is then attracted.
Accept any sentence that says the magnet induces a magnetic effect in the clip, which is then attracted.
Method. Think about the clip.
- Induce. The magnet induces magnetism in the iron of the clip.induces
- Attract. The induced pole is attracted to the magnet.attracted
- Write the answer. Accept any sentence that says the magnet induces a magnetic effect in the clip, which is then attracted.
Check. ✓
10
RA pupil says “magnetic field lines cross each other at the poles.” Explain why this is wrong. Accept any answer that says field lines never cross, because a point can have only one field direction.
Accept any answer that says field lines never cross, because a point can have only one field direction.
Method. Think about one point in the field.
- One. At any point the field has only one direction.one direction
- Cross. So field lines never cross each other.never cross
- Write the answer. Accept any answer that says field lines never cross, because a point can have only one field direction.
Check. ✓
20.2
Electromagnets
Video lesson: ElectromagnetsComing soon
Context. A scrap yard uses a large magnet on a crane to lift cars. The magnet can be switched off to drop the metal. How can a magnet be turned on and off? The answer is an electromagnet.
A current in a wire makes a magnetic field around it. If the wire is wound into a coil (a solenoid), the fields add up and the coil acts like a bar magnet. Putting an iron core inside the coil makes the magnet much stronger. The magnet is switched on and off by switching the current on and off, and its strength can be changed.
The strength of an electromagnet increases with: the current, the number of turns on the coil, and the presence of an iron core.
Electromagnets are used in scrap yards, relays, door bells, and in electric motors and loudspeakers.
An electromagnet: a coil with an iron core, switched by a current
Key ideas
- Electromagnet: a coil carrying current, often with an iron core.
- Stronger when: more current, more turns, iron core.
- Switched on and off by the current.
- Uses: scrap yard cranes, relays, bells, motors, loudspeakers.
Worked example 20.2.1
Give two ways to make an electromagnet stronger. Write two words, separated by a comma.
Method. Recall the factors.
- Look. A stronger electromagnet needs more current or more turns on the coil.current and turns
- Add. An iron core also makes the magnetic field much stronger.iron core
- Write the answer. current, turns
Check. Accept iron core, too. ✓
Worked example 20.2.2
Why is an electromagnet useful for a scrap yard crane? Write one reason in one sentence. Accept any sentence that says it can be switched off to drop the metal.
Method. Think about switching.
- Switch. The magnetism stops when the current is switched off.switched off
- Use. So the load can be dropped at the right place.dropped
- Write the answer. Accept any sentence that says it can be switched off to drop the metal.
Check. ✓
Worked example 20.2.3
What is a long coil of wire carrying a current called? Write the word.
Method. Recall the name.
- Look. The coil acts like a bar magnet while the current flows.bar magnet
- Recall. This coil is called a solenoid.solenoid
- Write the answer. solenoid
Check. ✓
Watch out
Watch out: an electromagnet needs a current. It is strong only while the current flows, and it has no field when the current is off (unless it is a permanent magnet with an iron core that has been magnetised).
Exam tip
Practical safety. Use a low-voltage supply with a teacher. Do not leave a coil on for long, because it heats up.
Exercise 20B Electromagnets
1
FWhat does a current in a coil of wire produce around it? Write the word: magnetic field or heat.
magnetic field
Method. Recall the effect of a current.
- Recall. A current in a wire produces a magnetic field.magnetic field
- Write the answer. magnetic field
Check. ✓
2
FWhat is a coil of wire with a current called, when it acts like a bar magnet? Write the word.
solenoid
Method. Recall the name.
- Recall. A long coil carrying a current is a solenoid.solenoid
- Write the answer. solenoid
Check. Accept electromagnet. ✓
3
FWhat material is placed in the coil to make an electromagnet stronger? Write the material name.
iron
Method. Recall the core.
- Recall. An iron core makes the magnetic field much stronger.iron
- Write the answer. iron
Check. ✓
4
FHow can the strength of an electromagnet be increased without changing the coil? Write one way: current or colour.
current
Method. Recall the factor.
- Recall. A bigger current makes the electromagnet stronger.current
- Write the answer. current
Check. Accept more turns, or an iron core. ✓
5
FName one use of an electromagnet in a school or home. Write the use.
bell
Method. Recall a use.
- Recall. An electric bell uses an electromagnet to strike the gong.bell
- Write the answer. bell
Check. Accept a relay or loudspeaker. ✓
6
FIs an electromagnet strong with no current? Write yes or no.
no
Method. It needs a current.
- Recall. Without a current it has no magnetic field, so it is not a magnet.no
- Write the answer. no
Check. ✓
7
PSA coil with 200 turns has a current of 2 A. A second coil with 400 turns has the same current. Which is stronger? Write the number of turns of the stronger coil.
400
Method. More turns, same current, is stronger.
- Compare. The coil with 400 turns is stronger, because it has more turns with the same current.400
- Write the answer. 400 turns
Check. ✓
8
RExplain why adding an iron core makes an electromagnet stronger, in one sentence. Accept any sentence that says the iron is magnetised by the coil and adds to its field.
Accept any sentence that says the iron is magnetised by the coil and adds to its field.
Method. Think about the iron.
- Iron. The iron is magnetised by the coil.magnetised
- Field. So its field adds to the field of the coil, making it stronger.adds
- Write the answer. Accept any sentence that says the iron is magnetised by the coil and adds to its field.
Check. ✓
9
RA pupil says “an electromagnet always stays magnetic.” Explain why this is wrong. Accept any answer that says it is magnetic only while the current flows.
Accept any answer that says it is magnetic only while the current flows.
Method. Think about the current.
- Current. An electromagnet needs a current to produce its field.current
- Switch. When the current is switched off, the magnetism stops.stops
- Write the answer. Accept any answer that says it is magnetic only while the current flows.
Check. ✓
10
RWhy is an electromagnet preferred to a permanent magnet in a scrap yard? Write one reason in one sentence. Accept any sentence that says it can be switched off to release the metal.
Accept any sentence that says it can be switched off to release the metal.
Method. Think about the control.
- Control. A permanent magnet cannot be switched off.cannot switch off
- Release. An electromagnet can be switched off to drop the load.switched off
- Write the answer. Accept any sentence that says it can be switched off to release the metal.
Check. ✓
20.3
The motor effect and the electric motor
Video lesson: The motor effect and the electric motorComing soon
Context. A fan, a drill and a loudspeaker all have a moving part. Each uses the same effect: a current-carrying wire in a magnetic field feels a force. This is the motor effect.
When a conductor carrying a current is placed in a magnetic field, it experiences a force. The force is greatest when the conductor is at right angles to the field. The size of the force increases with the current and the strength of the field. The direction of the force is given by Fleming’s left-hand rule:
- First finger: the field (from north to south).
- Second finger: the current (from positive to negative).
- Thumb: the direction of the force (motion).
In an electric motor, a coil carries a current in a magnetic field. The two sides of the coil feel opposite forces, so the coil turns. A split-ring commutator reverses the current in the coil every half turn, so the turning effect continues in the same direction. A loudspeaker uses the same effect: a coil carrying a changing current moves a cone in and out.
Fleming’s left-hand rule: thumb force, first finger field, second finger current
A simple motor: the coil turns because the forces on its two sides are opposite
Key ideas
- Force on a conductor: greatest when at right angles to the field.
- Force increases with current and with field strength.
- Fleming's left-hand rule: thumb = force, first finger = field, second finger = current.
- Split-ring commutator: reverses the current every half turn so the motor keeps turning in one direction.
Worked example 20.3.1
Which two quantities does Fleming’s left-hand rule link to the direction of the force? Write two words, separated by a comma.
Method. Recall what the first and second fingers show.
- First finger. The first finger shows the magnetic field.field
- Second finger. The second finger shows the current.current
- Thumb. The thumb then shows the direction of the force.force
- Write the answer. field, current
Check. ✓
Worked example 20.3.2
Name the part of an electric motor that reverses the current in the coil every half turn. Write the name.
Method. Recall the part.
- Look. The part connects the coil to the supply through two half-rings.half-rings
- Recall. This is the split-ring commutator.commutator
- Write the answer. split-ring commutator
Check. ✓
Worked example 20.3.3
State one change that would make a motor turn faster. Write one change in a few words. Accept any answer such as more current or a stronger magnet.
Method. Think about the force.
- Force. The force on the coil increases with current and field strength.force increases
- Change. So more current, or a stronger magnet, makes the motor faster.more current
- Write the answer. Accept any answer such as more current or a stronger magnet.
Check. ✓
Watch out
Watch out: in Fleming’s left-hand rule use the left hand, and the second finger is the conventional current from positive to negative.
Exam tip
Exam tip. Use the left hand, fingers at right angles, and name each finger: thumb force, first finger field, second finger current.
Exercise 20C The motor effect
1
FIn Fleming’s left-hand rule, which finger shows the magnetic field? Write the finger: first, second or thumb.
first
Method. Recall the rule.
- Recall. The first finger shows the field, from north to south.first
- Write the answer. first finger
Check. ✓
2
FIn Fleming’s left-hand rule, which finger shows the current? Write the finger: first, second or thumb.
second
Method. Recall the rule.
- Recall. The second finger shows the current, from positive to negative.second
- Write the answer. second finger
Check. ✓
3
FIn Fleming’s left-hand rule, which digit shows the force? Write the digit: thumb, first or second.
thumb
Method. Recall the rule.
- Recall. The thumb shows the direction of the force.thumb
- Write the answer. thumb
Check. ✓
4
PSWhat is the force on a wire carrying a current parallel to a magnetic field? Write the number of newtons, or none, as the word zero.
zero
Method. The force is greatest at right angles and zero when parallel.
- Recall. A conductor parallel to the field experiences no force.zero
- Write the answer. zero
Check. ✓
5
FWhat is the effect on the force of doubling the current in a conductor in a magnetic field? Write the word: doubles, halves or stays the same.
doubles
Method. The force increases with current.
- Recall. The force is proportional to the current, so it doubles.doubles
- Write the answer. doubles
Check. ✓
6
FName the device in which the split-ring commutator is used. Write the name.
electric motor
Method. Recall the device.
- Recall. The split-ring commutator is part of an electric motor.electric motor
- Write the answer. electric motor
Check. ✓
7
FIn a loudspeaker, what moves in response to the changing current? Write the word: cone or magnet.
cone
Method. The cone is the part that moves.
- Recall. The coil moves the cone in and out, making the air vibrate.cone
- Write the answer. cone
Check. Accept the coil or the cone. ✓
8
RIn a motor, why is the split-ring commutator needed? Write one reason in one sentence. Accept any sentence that says it reverses the current every half turn so the coil keeps turning the same way.
Accept any sentence that says it reverses the current every half turn so the coil keeps turning the same way.
Method. Think about the forces on the coil.
- Forces. The forces on the sides of the coil would reverse as it turns.reverse
- Commutator. The commutator reverses the current, so the coil keeps turning in one direction.one direction
- Write the answer. Accept any sentence that says it reverses the current every half turn so the coil keeps turning the same way.
Check. ✓
9
RA wire carrying a current is at right angles to a magnetic field. Explain why it feels a force, in one sentence. Accept any sentence that says the current and the magnetic field interact to produce a force.
Accept any sentence that says the current and the magnetic field interact to produce a force.
Method. Think about the two fields.
- Interact. The current produces its own magnetic field.current field
- Force. The two fields interact, which gives a force on the wire.force
- Write the answer. Accept any sentence that says the current and the magnetic field interact to produce a force.
Check. ✓
10
RA pupil says “the force on the wire is largest when the wire is parallel to the field.” Is the pupil right? Write yes or no.
no
Method. The force is greatest at right angles.
- Check. The force is zero when parallel and greatest at right angles.right angles
- Write the answer. no
Check. ✓
11
RA coil in a motor has its two sides in a field of opposite direction. Explain why the forces on the two sides are in opposite directions, in one sentence. Accept any sentence that says the current flows in opposite directions in the two sides, so the forces are opposite.
Accept any sentence that says the current flows in opposite directions in the two sides, so the forces are opposite.
Method. Think about the current in each side.
- Current. The current flows in opposite directions in the two sides of the coil.opposite current
- Force. So the forces on the two sides act in opposite directions, producing a turning effect.turning
- Write the answer. Accept any sentence that says the current flows in opposite directions in the two sides, so the forces are opposite.
Check. ✓
20.4
Electromagnetic induction and the generator
Video lesson: Electromagnetic induction and the generatorComing soon
Context. A bicycle dynamo lights the lamp when the wheel turns. No battery is needed: the moving magnet makes a voltage in a coil. This is electromagnetic induction.
When a magnet moves relative to a coil, or a coil moves through a magnetic field, a voltage is induced in the coil. If the circuit is complete, a current flows. The induced voltage is bigger when the magnet moves faster, when the field is stronger, and when the coil has more turns.
In an AC generator, a coil is turned steadily in a magnetic field. The induced voltage changes direction every half turn, so the output is alternating current. A dynamo works in the same way.
An AC generator: a turning coil in a magnetic field gives an alternating voltage
Key ideas
- Electromagnetic induction: a changing magnetic field induces a voltage in a coil.
- Induced voltage increases with faster movement, a stronger field and more turns.
- A generator turns a coil in a field, to give an alternating voltage.
- A dynamo is a small generator used on bicycles.
Worked example 20.4.1
A magnet is moved into a coil and a voltage is induced. What happens to the induced voltage if the magnet is moved faster? Write the word: bigger or smaller.
Method. Think about the rate of change of the field.
- Look. A faster movement changes the magnetic field more quickly.faster
- Decide. So the induced voltage is bigger.bigger
- Write the answer. bigger
Check. ✓
Worked example 20.4.2
Name the type of current produced by an AC generator. Write the name.
Method. Recall the output.
- Look. The voltage reverses direction every half turn of the coil.reverses
- Recall. So the output is alternating current.AC
- Write the answer. AC
Check. ✓
Worked example 20.4.3
What does a faster turning generator do to the frequency of the output? Write the word: increases or decreases.
Method. More turns per second means more cycles per second.
- Look. A faster turning coil completes more cycles per second.more cycles
- Decide. So the frequency increases.increases
- Write the answer. increases
Check. ✓
Watch out
Watch out: induction needs change. A steady magnetic field in a coil that is not moving induces no voltage.
Exam tip
Exam tip. For induction, say what changes (movement, field, or turns), and say the result (bigger voltage or current).
Exercise 20D Induction and the generator
1
FWhat is the name of the effect in which a changing magnetic field induces a voltage? Write the name.
electromagnetic induction
Method. Recall the name.
- Recall. A changing magnetic field induces a voltage: electromagnetic induction.electromagnetic induction
- Write the answer. electromagnetic induction
Check. ✓
2
FWhat type of current does a generator produce? Write the type: AC or DC.
AC
Method. The output reverses direction.
- Recall. A generator produces alternating current.AC
- Write the answer. AC
Check. ✓
3
FWhich device is a small generator used on bicycles? Write the name.
dynamo
Method. Recall the device.
- Recall. A bicycle dynamo is a small generator.dynamo
- Write the answer. dynamo
Check. ✓
4
FDoes a magnet held still next to a coil induce a voltage? Write yes or no.
no
Method. Induction needs a change.
- Recall. A steady field gives no change, so no voltage is induced.no
- Write the answer. no
Check. ✓
5
FWhat is the effect on the induced voltage of using more turns on the coil? Write the word: bigger or smaller.
bigger
Method. More turns give more voltage.
- Recall. More turns in the coil increase the induced voltage.bigger
- Write the answer. bigger
Check. ✓
6
RExplain why a generator needs to be turned to produce electricity, in one sentence. Accept any sentence that says turning the coil changes the magnetic field through it, which induces a voltage.
Accept any sentence that says turning the coil changes the magnetic field through it, which induces a voltage.
Method. Think about the change.
- Change. Turning the coil changes the field through it.changes
- Voltage. That change induces a voltage in the coil.induces
- Write the answer. Accept any sentence that says turning the coil changes the magnetic field through it, which induces a voltage.
Check. ✓
7
PSA generator has a coil that turns at 25 turns per second. What is the frequency of the output, if one turn gives one cycle? Write the number of hertz.
25
Method. One turn gives one cycle.
- Recall. One turn per cycle gives 25 cycles per second, which is 25 Hz.25
- Write the answer. 25 Hz
Check. ✓
8
RA pupil says “a steady magnet and a steady coil always produce a current.” Explain why this is wrong, in one sentence. Accept any answer that says there must be a change in the field through the coil.
Accept any answer that says there must be a change in the field through the coil.
Method. Think about the need for change.
- Change. Induction needs a changing field through the coil.changing field
- Write the answer. Accept any answer that says there must be a change in the field through the coil.
Check. ✓
9
RExplain how a dynamo lights a bicycle lamp without a battery, in one sentence. Accept any sentence that says the wheel turns a magnet near a coil, inducing a voltage that lights the lamp.
Accept any sentence that says the wheel turns a magnet near a coil, inducing a voltage that lights the lamp.
Method. Follow the energy.
- Motion. The turning wheel moves a magnet near a coil.turning
- Voltage. This induces a voltage, which drives a current through the lamp.induces
- Write the answer. Accept any sentence that says the wheel turns a magnet near a coil, inducing a voltage that lights the lamp.
Check. ✓
10
RExplain in one sentence how a bicycle dynamo produces a voltage. Accept any sentence that says a turning magnet moves past a coil, inducing a voltage.
Accept any sentence that says a turning magnet moves past a coil, inducing a voltage.
Method. Think about the magnet and the coil.
- Move. The turning wheel moves a magnet past a coil.moves
- Induce. The changing magnetic field induces a voltage in the coil.induces
- Write the answer. Accept any sentence that says a turning magnet moves past a coil, inducing a voltage.
Check. ✓
20.5
Transformers
Video lesson: TransformersComing soon
Context. The electricity from a power station is stepped up to a very high voltage for long-distance transmission, and stepped down to 230 V for the home. The device that does both is the transformer.
A transformer has two coils wound on an iron core. An alternating current in the primary coil makes a changing magnetic field in the core, which induces an alternating voltage in the secondary coil. The voltages are related to the numbers of turns:
Vp / Vs = Np / Ns
A step-up transformer has more turns on the secondary, so Vs is bigger than Vp. A step-down transformer has fewer turns on the secondary. Transformers work only with alternating current, because a changing field is needed to induce a voltage.
A transformer: the primary and secondary coils are linked by an iron core
| Type | Relationship | Example |
|---|---|---|
| step-up | Vs is bigger than Vp, so Ns is bigger than Np | at a power station, to reduce losses in the cables |
| step-down | Vs is smaller than Vp, so Ns is smaller than Np | a phone charger or the transformer near a house |
Step-up and step-down transformers
Key ideas
- Vp / Vs = Np / Ns (voltage ratio equals turns ratio).
- Step-up: more turns on the secondary, so Vs > Vp.
- Step-down: fewer turns on the secondary, so Vs < Vp.
- Transformers need an alternating current (a changing field).
Worked example 20.5.1
A transformer has 100 turns on the primary and 500 turns on the secondary. The primary voltage is 20 V. Find the secondary voltage. Write the number of volts.
Method. Use Vs = Vp x Ns / Np.
- Take the numbers. Vp = 20 V, Np = 100 and Ns = 500.Vp = 20, Np = 100, Ns = 500
- Calculate. Vs = 20 x 500 / 100 = 100 V.20 x 500 / 100 = 100
- Write the answer. 100 V
Check. ✓
Worked example 20.5.2
A step-down transformer has 2000 primary turns and 100 secondary turns. The primary is 230 V. Find the secondary voltage. Write the number of volts.
Method. Use Vs = Vp x Ns / Np.
- Take the numbers. Vp = 230 V, Np = 2000 and Ns = 100.Vp = 230, Np = 2000, Ns = 100
- Calculate. Vs = 230 x 100 / 2000 = 11.5 V.230 x 100 / 2000 = 11.5
- Write the answer. 11.5 V
Check. ✓
Worked example 20.5.3
[H] A transformer has an ideal power output equal to its input. The primary has 230 V and 0.5 A. The secondary has 11.5 V. Find the secondary current. Write the number of amperes.
Method. Use power in = power out, so Vp x Ip = Vs x Is.
- Power in. Power in = 230 x 0.5 = 115 W.115 W
- Current. Is = 115 / 11.5 = 10 A.115 / 11.5 = 10
- Write the answer. 10 A
Check. 11.5 x 10 = 115 W. ✓
Watch out
Watch out: check which coil is the primary and which the secondary. The ratio is Vp / Vs = Np / Ns, so the voltage goes with the number of turns on the same coil.
Exam tip
Exam tip. Write Vp / Vs = Np / Ns, put in the four values, and cross-multiply. Check whether the answer is step-up or step-down.
Exercise 20E Transformers
1
FWhat type of current must be used in a transformer? Write the word: AC or DC.
AC
Method. A changing field is needed.
- Recall. A changing magnetic field is needed to induce a voltage, so AC is used.AC
- Write the answer. AC
Check. ✓
2
FWhat is the material of the core in a transformer? Write the name.
iron
Method. Recall the core.
- Recall. A transformer core is made of iron.iron
- Write the answer. iron
Check. ✓
3
FA transformer has 50 turns on the primary and 150 on the secondary. Is it step-up or step-down? Write the word.
step-up
Method. More turns on the secondary means a bigger voltage.
- Recall. The secondary has more turns, so Vs is bigger: step-up.step-up
- Write the answer. step-up
Check. ✓
4
PSA transformer has 100 primary turns and 25 secondary turns with 230 V input. Calculate the output voltage. Write the number of volts.
57.5
Method. Vs = Vp x Ns / Np.
- Calculate. Vs = 230 x 25 / 100 = 57.5 V.230 x 25 / 100 = 57.5
- Write the answer. 57.5 V
Check. ✓
5
PSA step-up transformer has 40 primary turns and 800 secondary turns, with 12 V input. Find the output voltage. Write the number of volts.
240
Method. Vs = Vp x Ns / Np.
- Calculate. Vs = 12 x 800 / 40 = 240 V.12 x 800 / 40 = 240
- Write the answer. 240 V
Check. ✓
6
PSA transformer has 600 turns on the primary and an output of 12 V from a 240 V input. How many turns are on the secondary? Write the number.
30
Method. Rearrange to Ns = Np x Vs / Vp.
- Calculate. Ns = 600 x 12 / 240 = 30 turns.600 x 12 / 240 = 30
- Write the answer. 30 turns
Check. ✓
7
PSA transformer with a turns ratio Ns / Np of 5 is supplied with 20 V. Find the output voltage. Write the number of volts.
100
Method. Vs = Vp x (Ns / Np).
- Calculate. Vs = 20 x 5 = 100 V.20 x 5 = 100
- Write the answer. 100 V
Check. ✓
8
RExplain why a transformer cannot work with a steady direct current, in one sentence. Accept any sentence that says there is no changing magnetic field, so no voltage is induced in the secondary.
Accept any sentence that says there is no changing magnetic field, so no voltage is induced in the secondary.
Method. Think about induction.
- Change. A steady current gives a steady field in the core.steady
- Induce. With no change, no voltage is induced in the secondary.no voltage
- Write the answer. Accept any sentence that says there is no changing magnetic field, so no voltage is induced in the secondary.
Check. ✓
9
RWhy does the national grid use step-up transformers before sending electricity along the cables? Write one reason in one sentence. Accept any sentence that says a higher voltage means a smaller current, so less energy is lost as heat in the cables.
Accept any sentence that says a higher voltage means a smaller current, so less energy is lost as heat in the cables.
Method. Think about the current in the cables.
- Current. A higher voltage means a smaller current for the same power.smaller current
- Loss. Less energy is lost as heat in the cables.less heat
- Write the answer. Accept any sentence that says a higher voltage means a smaller current, so less energy is lost as heat in the cables.
Check. ✓
10
PS[H] A step-down transformer has an ideal 230 V input with a current of 2 A, and an output of 23 V. Find the output current. Write the number of amperes.
20
Method. Power in = power out: 230 x 2 = 23 x Is.
- Power. Power in = 230 x 2 = 460 W.460 W
- Current. Is = 460 / 23 = 20 A.460 / 23 = 20
- Write the answer. 20 A
Check. ✓
Chapter summary
| Skill | Method |
|---|---|
| Magnets | Like poles repel; unlike attract; field lines N to S outside; closer lines = stronger |
| Electromagnet | Current in a coil; stronger with more current, more turns, iron core; switchable |
| Motor effect | Force on a current-carrying wire in a field; Fleming's left-hand rule; greatest at right angles |
| Induction and generator | Changing field induces voltage; faster, stronger, more turns = bigger voltage; AC from generator |
| Transformers | Vp / Vs = Np / Ns; step-up or step-down; [H] power in = power out |
Can you…?
- I can describe a magnetic field and field lines
- I can say how to make an electromagnet stronger
- I can apply Fleming's left-hand rule
- I can explain how a motor and a generator work
- I can use Vp / Vs = Np / Ns, and [H] power in = power out
Key terms
- magnetic field
- a region where a magnetic force acts
- electromagnet
- a coil carrying a current, which acts as a magnet only while the current flows
- solenoid
- a long coil of wire carrying a current
- Fleming's left-hand rule
- a rule giving the direction of the force on a current-carrying wire in a field
- commutator
- a split ring that reverses the current in a motor coil every half turn
- electromagnetic induction
- the production of a voltage by a changing magnetic field
- transformer
- a device that changes the voltage of an alternating supply using two coils and an iron core
Exercise 20Z Check-up
1
Which pole of a magnet repels another north pole? Write the word: north or south.
north
Method. Like poles repel.
- Recall. A north pole repels another north pole.north
- Write the answer. north
Check. ✓
2
What is the name of the rule for the direction of the force on a current-carrying wire? Write the name.
Fleming's left-hand rule
Method. Recall the rule.
- Recall. The direction of the force is given by Fleming’s left-hand rule.Fleming's left-hand rule
- Write the answer. Fleming's left-hand rule
Check. ✓
3
What is the effect of an iron core on an electromagnet: stronger or weaker? Write the word.
stronger
Method. The core is magnetised by the coil.
- Recall. An iron core makes the electromagnet stronger.stronger
- Write the answer. stronger
Check. ✓
4
Which device converts motion into an alternating voltage? Write the name.
generator
Method. Recall the device.
- Recall. A generator converts motion into electricity.generator
- Write the answer. generator
Check. ✓
5
A transformer has 10 primary turns and 50 secondary turns, with 4 V input. Find the output voltage. Write the number of volts.
20
Method. Vs = Vp x Ns / Np.
- Calculate. Vs = 4 x 50 / 10 = 20 V.4 x 50 / 10 = 20
- Write the answer. 20 V
Check. ✓
6
What is the name of the part that reverses the current in a motor every half turn? Write the name.
split-ring commutator
Method. Recall the part.
- Recall. The split-ring commutator reverses the current every half turn.split-ring commutator
- Write the answer. split-ring commutator
Check. ✓
7
Name the effect in which a moving magnet induces a voltage in a coil. Write the name.
electromagnetic induction
Method. Recall the effect.
- Recall. Moving a magnet near a coil induces a voltage: electromagnetic induction.electromagnetic induction
- Write the answer. electromagnetic induction
Check. ✓
8
Is the force on a wire greatest when it is parallel or at right angles to the field? Write the word.
right angles
Method. The force is largest at right angles.
- Recall. The force is greatest when the wire is at right angles to the field.right angles
- Write the answer. right angles
Check. ✓
9
Explain why a magnetic compass points north, in one sentence. Accept any sentence that says the needle lines up with the Earth’s magnetic field, which acts like a large bar magnet.
Accept any sentence that says the needle lines up with the Earth’s magnetic field, which acts like a large bar magnet.
Method. Think about the Earth.
- Field. The Earth has a magnetic field, like a bar magnet.field
- Needle. The compass needle lines up with that field.lines up
- Write the answer. Accept any sentence that says the needle lines up with the Earth’s magnetic field, which acts like a large bar magnet.
Check. ✓
10
Which device uses a switchable electromagnet to pick up scrap iron? Write the name.
electromagnet
Method. Recall the device.
- Recall. A scrap yard crane uses an electromagnet, which can be switched off to drop the load.electromagnet
- Write the answer. electromagnet
Check. ✓
Total 30 marks. Allow about 30 minutes.
Exercise 20X Exam-style questions
1
RDescribe how an electromagnet can be made stronger, giving two methods. Accept any two of: increase the current, increase the number of turns, add an iron core.[2 marks]
Accept any two of: increase the current, increase the number of turns, add an iron core.
Method. Name two methods.
- First. Increase the current through the coil.current
- Second. Increase the number of turns on the coil, or add an iron core.turns or core
- Write the answer. Accept any two of: increase the current, increase the number of turns, add an iron core.
Check. ✓
2
PSA transformer has 300 primary turns and 60 secondary turns, with an input of 230 V. Calculate the secondary voltage. Write the number of volts.[2 marks]
46
Method. Vs = Vp x Ns / Np.
- Calculate. Vs = 230 x 60 / 300 = 46 V.230 x 60 / 300 = 46
- Write the answer. 46 V
Check. ✓
3
RDescribe how the force on a wire is related to the current and the field, and use Fleming’s left-hand rule to describe the direction. Accept any answer that says the force increases with current and field strength, and the direction is given by the thumb in the left-hand rule.[4 marks]
Accept any answer that says the force increases with current and field strength, and the direction is given by the thumb in the left-hand rule.
Method. Describe the force and its direction.
- Size. The force increases with the current and with the strength of the field.increases
- Direction. Fleming’s left-hand rule gives the direction: first finger field, second finger current, thumb force.left-hand rule
- Maximum. The force is greatest when the wire is at right angles to the field.right angles
- Write the answer. Accept any answer that says the force increases with current and field strength, and the direction is given by the thumb in the left-hand rule.
Check. ✓
4
RExplain how an electric motor turns, using the forces on the coil and the split-ring commutator. Accept any answer that says the forces on opposite sides act in opposite directions, creating a turning effect, and the commutator reverses the current every half turn so the coil keeps turning.[4 marks]
Accept any answer that says the forces on opposite sides act in opposite directions, creating a turning effect, and the commutator reverses the current every half turn so the coil keeps turning.
Method. Describe the motor.
- Forces. The current in the coil feels forces on the two sides, in opposite directions.opposite forces
- Turning. This makes a turning effect on the coil.turning effect
- Commutator. The split-ring commutator reverses the current every half turn.commutator
- Continue. So the coil keeps turning in the same direction.same direction
- Write the answer. Accept any answer that says the forces on opposite sides act in opposite directions, creating a turning effect, and the commutator reverses the current every half turn so the coil keeps turning.
Check. ✓
5
RExplain why a transformer is used to step up the voltage before the electricity is sent along the cables of the national grid. Accept any answer that says a higher voltage gives a smaller current for the same power, so less energy is lost as heat in the cables.[3 marks]
Accept any answer that says a higher voltage gives a smaller current for the same power, so less energy is lost as heat in the cables.
Method. Think about power and current.
- Power. For the same power, a higher voltage gives a smaller current.smaller current
- Loss. Less current means less energy lost as heat in the cables.less heat
- Write the answer. Accept any answer that says a higher voltage gives a smaller current for the same power, so less energy is lost as heat in the cables.
Check. ✓
6
PS[H] A step-up transformer has an input of 12 V at 5 A, and an output of 60 V. Assuming power in equals power out, find the output current. Write the number of amperes.[3 marks]
1
Method. Power in = power out.
- Power. Power in = 12 x 5 = 60 W.60 W
- Current. Is = 60 / 60 = 1 A.60 / 60 = 1
- Write the answer. 1 A
Check. ✓
7
RDescribe how a generator produces an alternating voltage, and say what happens to the output when the coil turns faster. Accept any answer that says the coil turns in a magnetic field, the flux changes, inducing a voltage that reverses each half turn, and a faster coil gives a bigger and more frequent voltage.[4 marks]
Accept any answer that says the coil turns in a magnetic field, the flux changes, inducing a voltage that reverses each half turn, and a faster coil gives a bigger and more frequent voltage.
Method. Describe the generator and the effect of speed.
- Coil. A coil is turned in a magnetic field, so the field through it changes.changes
- Voltage. This induces a voltage that reverses every half turn.reverses
- Speed. A faster turning coil gives a bigger voltage and a higher frequency.bigger, higher frequency
- Output. So the output is alternating current.AC
- Write the answer. Accept any answer that says the coil turns in a magnetic field, the flux changes, inducing a voltage that reverses each half turn, and a faster coil gives a bigger and more frequent voltage.
Check. ✓
8
RDescribe the magnetic field around a bar magnet, including the direction of the field lines outside the magnet, and say what the spacing of the lines shows. Accept any answer that says lines go from north to south outside the magnet, and closer lines show a stronger field.[3 marks]
Accept any answer that says lines go from north to south outside the magnet, and closer lines show a stronger field.
Method. Describe the field.
- Direction. Field lines go from the north pole to the south pole outside the magnet.north to south
- Spacing. Closer lines show a stronger field.stronger
- Write the answer. Accept any answer that says lines go from north to south outside the magnet, and closer lines show a stronger field.
Check. ✓
9
RExplain why a steady current in the primary coil of a transformer produces no output, in one sentence. Accept any sentence that says a steady current gives a steady field, so no voltage is induced in the secondary.[2 marks]
Accept any sentence that says a steady current gives a steady field, so no voltage is induced in the secondary.
Method. Think about the field.
- Field. A steady current gives a steady magnetic field.steady
- Induce. No change means no induced voltage in the secondary.no change
- Write the answer. Accept any sentence that says a steady current gives a steady field, so no voltage is induced in the secondary.
Check. ✓
10
RExplain why magnetism is induced in a paper clip next to a bar magnet, and why the clip is always attracted, in two sentences. Accept any answer that says the magnet magnetises the iron of the clip, and the induced pole is always opposite to the nearest pole, so it is attracted.[3 marks]
Accept any answer that says the magnet magnetises the iron of the clip, and the induced pole is always opposite to the nearest pole, so it is attracted.
Method. Describe the induced magnetism.
- Induce. The magnet magnetises the iron of the clip.magnetises
- Attract. The induced pole nearest the magnet is opposite, so the clip is attracted.attracted
- Write the answer. Accept any answer that says the magnet magnetises the iron of the clip, and the induced pole is always opposite to the nearest pole, so it is attracted.
Check. ✓