4.5.1 Electromagnetic Induction
Definition
Electromagnetic induction occurs when a magnetic field is cut by a conductor such as a wire, coil, or solenoid, inducing an e.m.f. or current in it.
Conditions for Induction
- The conductor must move across magnetic field lines or the magnetic field around it must change.
- No current or e.m.f. is induced if the conductor is stationary or moves parallel to field lines.
- Induction occurs only in conductors, not in insulators such as nylon or plastic.
Demonstrating Induction
- Moving wire in magnetic field: move a wire up and down between magnet poles → galvanometer shows deflection in opposite directions for opposite motions.
- Moving magnet in coil: push magnet into solenoid → current flows one way; pull it out → current reverses; stationary magnet → no current.
Factors Affecting Induced e.m.f.
| Factor | Effect |
|---|---|
| Speed of motion | Faster motion → greater rate of change of flux → larger e.m.f. |
| Magnetic field strength | Stronger field → greater flux change → larger e.m.f. |
| Number of turns | More turns → greater total induced voltage. |
| Length of conductor in field | Longer conductor → larger e.m.f. generated. |
Ways to Increase or Change Induced Current
- Move the conductor or magnet faster.
- Use a stronger magnet or increase coil turns.
- Reverse the motion or flip the magnet’s poles to reverse current direction.
Finding the Direction of Induced Current
Use Fleming’s Right-Hand Rule:
- First finger → direction of magnetic field (N → S)
- Thumb → motion of conductor
- Second finger → direction of induced current
Lenz’s Law
The direction of an induced current is always such that it opposes the change that caused it. For example, when a magnet approaches a coil, the coil becomes an electromagnet whose near face develops the same pole as the approaching magnet, thus repelling it.
Summary Table — Lenz’s Law Outcomes
| Bar Magnet Motion | Coil Reaction | Polarity Induced | Current Direction |
|---|---|---|---|
| North pole enters coil | Repels magnet (stops entry) | North | Anticlockwise |
| South pole enters coil | Repels magnet (stops entry) | South | Clockwise |
| North pole leaves coil | Attracts magnet (prevents leaving) | South | Clockwise |
| South pole leaves coil | Attracts magnet (prevents leaving) | North | Anticlockwise |
Key Laws & Rules Recap
- Faraday’s Law: The magnitude of induced e.m.f. is proportional to the rate of change of magnetic flux linkage.
- Lenz’s Law: Direction of induced current opposes the flux change producing it.
- Fleming’s Right-Hand Rule: Predicts direction of motion, field, and induced current.
Exam Practice Summary
- State that no e.m.f. is induced if there is no motion or flux change.
- Explain reversal of current when direction of motion or poles is reversed.
- Describe how to increase induced e.m.f. using speed, magnet strength, turns, or length.
- Remember: Induction only occurs in conductors, not in insulators.
- Relate observations to Lenz’s Law and Fleming’s Right-Hand Rule accurately.
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