Principles Of Electromagnetic Induction & Lenz's Law

1. Definition

Electromagnetic induction is the production of an e.m.f. in a conductor when the magnetic field (magnetic flux) linking it changes.

2. Key Ideas

• A changing magnetic field/flux induces an e.m.f..
• Induced current flows only if the circuit is closed.
• Lenz’s law: in a closed circuit, the induced current produces an effect that opposes the change causing it.
• A faster change in flux → a larger induced e.m.f.
• Factors that increase induced e.m.f. (qualitative):
  • move magnet/coil faster
  • use a stronger magnet
  • increase number of turns in the coil
  • increase coil area

3. Detailed Explanations

A. When is an e.m.f. induced?

An e.m.f. is induced whenever the magnetic field linking a conductor/coil is changing, for example:

• moving a magnet into/out of a coil
• moving a coil in a magnetic field
• changing the current in a nearby coil (changing magnetic field)

Investigation: magnet and coil

1. Connect a coil to a sensitive centre-zero galvanometer. Check that it reads zero when nothing moves.
2. Push one pole of a bar magnet into the coil and record the direction and size of the deflection.
3. Hold the magnet still inside the coil. The reading returns to zero because the flux linkage is no longer changing.
4. Pull the same pole out. The deflection reverses because the change in flux linkage has reversed.
5. Repeat by moving the magnet faster, using a stronger magnet or increasing the coil turns. Change only one factor at a time and compare peak deflections.

The experiment separates three ideas clearly: motion that changes flux linkage produces an e.m.f.; reversing the change reverses its direction; increasing the rate of change increases its magnitude. If the circuit is open, use a voltmeter to observe the induced e.m.f.—do not claim a continuous current.

Typical induction graph: induced e.m.f. when a magnet moves in and out

A schematic induced e.m.f. vs time graph: one pulse when the magnet enters the coil and an opposite pulse when it leaves.

Scroll across the graph to read all labels.

Open full-size graph

View figure data

Values for Typical induction graph: induced e.m.f. when a magnet moves in and out

Time (s)	e.m.f.
0	0
2	0
3	1
4	0
6	0
7	-1
8	0
10	0

Values for Typical induction graph: induced e.m.f. when a magnet moves in and out

Time (s)	e.m.f.
0	0
2	0
3	1
4	0
6	0
7	-1
8	0
10	0

B. Lenz’s law (direction)

Lenz’s law says the induced current produces a magnetic effect that opposes the change that produced it. It does not oppose the magnetic field itself.

Example with a coil and a magnet:

• If a north pole approaches the coil, the coil produces a north pole on the near side to repel it (opposes approach).
• If a north pole moves away, the coil produces a south pole on the near side to attract it (opposes separation).

C. Lenz’s law and Fleming’s right-hand rule

Use Fleming’s right-hand rule when a conductor moves in a magnetic field (generator rule):

• first finger: magnetic field (N → S)
• thumb: motion of the conductor
• second finger: induced current

4. Common Mistakes

• Saying a constant magnetic field induces an e.m.f. (you need a changing field/flux).
• Forgetting: induced current only flows in a closed circuit.
• Using “induced e.m.f.” and “induced current” as if they mean the same thing. A changing flux can induce an e.m.f. in an open circuit, but continuous current needs a closed path.
• Writing “the induced current helps the change” (it opposes the change).

5. Exam Tips

1. Look for the trigger: “moving magnet/coil” or “changing magnetic field”.
2. For direction, write “opposes the change” (Lenz’s law) and explain with attract/repel.
3. For magnitude: “bigger/faster change → bigger induced e.m.f.” (faster motion, stronger magnet, more turns, larger area).
4. If the question gives an open circuit or voltmeter, discuss e.m.f. first; only claim a current when there is a complete conducting path.

6. Worked Examples

7. Mind Stretchers

8. Practice and next step

Predict the sign and size of each induced-e.m.f. pulse in the Magnetism & Induction Lab. Then use the Electromagnetic Induction check before continuing to the A.C. Generator.