4.5.1 Electromagnetic induction
- Syllabus
- 0625–2026–2027
- Topic
- 4.5.1
- Level
- —
Learning objectives
4.5.1.1A conductor moving across a magnetic• Know: a conductor moving across a magnetic field or a changing magnetic field linking with a conductor can induce an e.m.f. in the conductor4.5.1.2An experiment to demonstrate EM• Describe an experiment to demonstrate electromagnetic induction4.5.1.3Factors affecting the magnitude of an• State the factors affecting the magnitude of an induced e.m.f.4.5.1.4Direction of an induced e.m.f. opposes• Know: the direction of an induced e.m.f. opposes the change causing it4.5.1.5And use the relative directions of• State and use the relative directions of force, field and induced current
When electromagnetic induction occurs
An e.m.f. is induced when a conductor moves across a magnetic field or when the magnetic field linking a conductor changes. Both descriptions mean that the conductor experiences a changing magnetic flux linkage.
| Situation | Induced e.m.f.? | Reason |
|---|---|---|
| wire moves across field lines | yes | the wire cuts magnetic field lines |
| magnet moves into or out of a coil | yes | the field linking the coil changes |
| magnet and coil remain stationary together | no | the field linkage is unchanged |
| magnet and coil move together at the same speed | no | there is no relative change in linkage |
An induced e.m.f. can exist across an open circuit. An induced current flows only when the conducting path is complete.
Motion alone is not enough: a conductor moving parallel to the field does not cut field lines. What matters is a change in magnetic flux linkage, not simply the presence of a magnet.
Demonstrate electromagnetic induction
Connect a coil to a sensitive centre-zero galvanometer and place a bar magnet on the coil's axis. Keep the same coil, magnet and meter while changing only the magnet's motion.
| Action | Observation | Conclusion |
|---|---|---|
| push the magnet into the coil | pointer deflects | changing field linkage induces an e.m.f. and current |
| hold the magnet still inside the coil | pointer returns to zero | unchanged linkage gives no induced e.m.f. |
| withdraw the magnet | pointer deflects the opposite way | reversing the change reverses the induced current |
| move the magnet faster | larger deflection | a faster change produces a larger induced e.m.f. |
The effect can also be demonstrated by moving a straight wire across the field between magnet poles while it is connected to a sensitive meter. Reversing the motion reverses the deflection.
Repeat each movement from the same starting position and compare peak deflections. A deflection only while the linkage changes is the essential evidence.
What makes induced e.m.f. larger
The magnitude of an induced e.m.f. increases when magnetic flux linkage changes more rapidly.
| Change | Why the induced e.m.f. is larger |
|---|---|
| move the magnet, wire or coil faster | the linkage changes in less time |
| use a stronger magnetic field | more magnetic flux is linked or cut during the change |
| use more turns on the coil | more conductors experience the changing linkage |
| use a longer conductor cutting the field, or orient motion more nearly perpendicular to the field | more field lines are cut per second |
For a fair comparison, change one factor at a time and compare the size of the peak meter deflection. Reversing motion or field direction reverses polarity; it does not by itself make the e.m.f. larger.
A magnet being closer to a coil is not sufficient on its own. If it is held still, the linkage is constant and the induced e.m.f. is zero.
Why induced effects oppose the change
Lenz's law states that the direction of an induced current is such that its magnetic effect opposes the change that produced it.
| Change near one end of a coil | Pole induced at that end | Effect |
|---|---|---|
| north pole approaches | north | repels the approaching magnet |
| north pole withdraws | south | attracts the receding magnet |
| south pole approaches | south | repels the approaching magnet |
| south pole withdraws | north | attracts the receding magnet |
Because the induced force opposes the motion, work must be done to keep the magnet, wire or coil moving. That mechanical energy is transferred to electrical energy in the circuit.
It is the magnetic effect or force that opposes the change—not necessarily the current direction itself. First identify the change, then choose the induced pole or force that resists it.
Use Fleming's right-hand rule
For generator action, hold the right-hand thumb, first finger and second finger mutually perpendicular. First finger points from N to S (field), thumb points in the conductor's motion, and second finger gives the conventional induced-current direction.
| Digit | Represents |
|---|---|
| thumb | motion of the conductor across the field |
| first finger | magnetic field direction, N to S |
| second finger | conventional induced current |
Reversing either the motion or the field reverses the induced current. Reversing both leaves the current direction unchanged because the two reversals cancel.
Use the right hand for induction or generator questions. Fleming's left-hand rule describes the motor effect, where an existing current experiences a force.