4.4.1Electricity and Magnetism
Electromagnetic Induction — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions
5 min read 11 parts Lesson 25 of 31
Part 1 of 9Induced e.m.f.
1Electromagnetic induction
Electromagnetic induction an e.m.f. is induced in a conductor whenever there is relative movement between the conductor and a magnetic field
- The conductor can move in a stationary magnetic field.
- The conductor can be stationary in a changing magnetic field, for example when a magnet moves through a coil.
2Cutting field lines
- Cutting field lines the induced e.m.f. appears while the conductor cuts magnetic field lines
3Complete circuit
- Complete circuit a current is induced and can be detected by an ammeter or galvanometer
- A sensitive voltmeter measures the size of the induced e.m.f.
4No relative movement
- No relative movement a stationary magnet and coil, or movement parallel to the field lines, induces no e.m.f.
Part 2 of 9Demonstration with a magnet and a coil
1
Connect a coil to a sensitive voltmeter and hold the bar magnet still inside or outside the coil.
- The reading is zero because no field lines are cut.
2
Move the bar magnet into the coil.
- The coil cuts the field lines and there is a reading on the voltmeter.
3
Move the magnet back out of the coil.
- The reading has the opposite sign because the direction of the e.m.f. has reversed.
4
Conclude that an e.m.f. is induced only while the magnet is moving through the coil.
Part 3 of 9Demonstration with a wire and a magnet
1
Connect a wire to a voltmeter; no current flows through the wire to start with.
2
Hold the wire still between the magnetic poles; the reading is zero because no field lines are cut.
3
Move the wire between the poles across the field; the wire cuts field lines and an e.m.f. is induced.
4
Move the wire back out; the reading has the opposite sign.
5
Increase the length of the wire in the field or move it faster to get a larger e.m.f.
Part 4 of 9Factors affecting the size of the e.m.f.
Factors affecting the size of the e.m.f.
Speed of movement
- Speed of movement a faster movement cuts field lines at a greater rate
Number of turns
- Number of turns each turn of the coil cuts the field lines, so the total e.m.f. increases with every extra turn
Area of the coil
- Area of the coil a larger coil has more wire cutting the field lines
Strength of the magnetic field
- Strength of the magnetic field a stronger field has more field lines in a given area
Wording
- Wording say a stronger magnet, not a bigger magnet, because larger magnets are not always stronger
Part 5 of 9Factors affecting the direction of the e.m.f.
Direction of movement
- Direction of movement reversing the movement of the wire, coil or magnet reverses the e.m.f.
Orientation of the poles
- Orientation of the poles switching the poles of the magnet reverses the e.m.f.
Reversing both
- Reversing both doing both together gives the original direction
Part 6 of 9Lenz's law
1Lenz's law
Lenz's law the direction of an induced e.m.f. always opposes the change that causes it
2Magnet pushed into a coil
- Magnet pushed into a coil the end of the coil nearest the magnet becomes a north pole
- The induced current makes a magnetic field that repels the approaching magnet.
3Magnet pulled out of a coil
- Magnet pulled out of a coil the end of the coil nearest the magnet becomes a south pole
- The induced field attracts the magnet and so opposes its removal.
4Energy
- Energy work must be done to move the magnet against this opposing force, so energy is conserved
Part 7 of 9Right-hand dynamo rule
Right-hand dynamo rule
- Right-hand dynamo rule gives the direction of the induced e.m.f.
- (current) in a moving wire
- First finger points in the direction of the field.
- Thumb points in the direction of the motion of the wire.
- Second finger then points in the direction of the current, or strictly the e.m.f.
Direction of current
- Direction of current conventional current is from positive to negative
Part 8 of 9Uses of electromagnetic induction
Electrical generators
- Electrical generators convert mechanical energy to electrical energy
Transformers
- Transformers used in electrical power transmission
Part 9 of 9More from the specification
1
Transformer mutual induction between coils
- Transformer mutual induction between coils alternating magnetic flux through a laminated iron core links primary and secondary coils, stepping voltage levels.
2
Electromagnetic induction
- Electromagnetic induction a voltage is induced when a conductor cuts magnetic field lines or the field linking a coil changes.
3
Factors
- Factors the induced voltage is larger with a faster movement, a stronger magnet or more turns on the coil.
4
Alternating current generator
- Alternating current generator a rotating coil in a field produces an alternating voltage, the reverse of the motor effect.
5
Transformer
- Transformer a primary coil's changing field induces voltage in the secondary, with Vp/Vs = Np/Ns.
6
Solenoid
- Solenoid moving a magnet in or out of the solenoid induces a current, with the direction opposing the change.
7
Electromagnetic induction
- Electromagnetic induction a voltage is induced when a conductor cuts magnetic field lines or the field linking a coil changes.
8
Factors
- Factors the induced voltage is larger with a faster movement, a stronger magnet or more turns on the coil.
9
Alternating current generator
- Alternating current generator a rotating coil in a field produces an alternating voltage, the reverse of the motor effect.
10
Transformer
- Transformer a primary coil's changing field induces voltage in the secondary, with Vp/Vs = Np/Ns.
11
Solenoid
- Solenoid moving a magnet in or out of the solenoid induces a current, with the direction opposing the change.
12
Solenoid
- A coil of wire that produces a magnetic field like a bar magnet when a current flows.
13
Voltage
- A measure of the electrical energy supplied or changed in a circuit.
14
Alternating current
- A current that continuously changes direction.
15
Potential difference
- The work done by a unit charge passing through a component.
16
Step-up transformer
- A transformer with more turns on the secondary coil than the primary, raising the voltage.
17
Step-down transformer
- A transformer with fewer turns on the secondary coil, lowering the voltage.
18
Permanent magnet
- A magnet whose field is always present.
19
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
Key terms
| Term | Meaning |
|---|---|
| Electromagnetic induction | The production of an e.m.f. in a conductor when there is relative movement between the conductor and a magnetic field. |
| Induced e.m.f. | The e.m.f. produced in a conductor when it cuts magnetic field lines. |
| Lenz's law | The direction of an induced e.m.f. always opposes the change causing it. |
| Right-hand dynamo rule | First finger = field, thumb = motion, second finger = induced current. |
| Solenoid | A coil of wire that produces a magnetic field like a bar magnet when a current flows. |
| Generator | A device that converts movement into an electrical output by electromagnetic induction. |
Avoid these mistakes
1
Thinking a stationary magnet inside a coil induces a current — the field must be changing or moving relative to the coil
2
Saying a bigger magnet induces a larger e.m.f. — a stronger magnet does, because size does not decide strength
3
Thinking the induced e.m.f. has the same direction when the magnet is removed — it reverses
4
Using the left hand for induction — the right-hand dynamo rule gives induced current and the left-hand rule gives the force on a current
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