Lesson 2 of 5
The direction opposes the change
I can deduce induction from changing magnetic fields, the opposing direction and factors affecting induced e.m.f. from experiments.Syllabus
Syllabus K323, 19(a). Deduce induction from changing magnetic fields, the opposing direction and factors affecting induced e.m.f. from experiments.
Make a guess. You are not marked.
The north pole of a bar magnet is pushed towards one face of a coil. The coil is part of a complete circuit. What pole does the near face of the coil become while the magnet approaches?
Show the answer
A north pole, which repels the approaching magnet.
The induced current always opposes the change that causes it. The change is a north pole coming closer, so the coil makes a north face to push it away.
The induced e.m.f. has a polarity that, in a closed conducting circuit, drives a current whose magnetic effect opposes the change producing it.
- Lenz's law
- The direction of the induced e.m.f., and of any induced current, is such that it opposes the change producing it.
Everything in this lesson is Lenz's law at work. Mark schemes often write "by Lenz's law". Say what the change is, such as "the N pole approaches", then say the induced current opposes it.
Like magnetic poles repel and unlike poles attract. Viewed from one face of a current-carrying coil, anticlockwise current makes that face N; clockwise current makes it S.
North approaches: oppose the approach
As a north pole approaches a closed coil, the field through the coil changes. The induced current makes the near face north, so it repels the approaching north pole. Viewed from the magnet end, that current is anticlockwise.
North withdraws: oppose the separation
When the same north pole withdraws, the change is reversed. The coil's near face becomes south, attracting the magnet and opposing its withdrawal. Viewed from the same magnet end, the current is clockwise.
The induced effect opposes the change
Here the conducting loop is closed, so an induced current can flow. In each lower view, look from the magnet towards the loop. Brown shows current, teal shows the induced field, blue shows magnet motion and purple shows magnetic force on the magnet.
North approaches: near face becomes north
Repulsion opposes the approach. The induced field points out of the near face.
North withdraws: near face becomes south
Attraction opposes the separation. The induced field points into the near face.
South approaches: near face becomes south
Repulsion opposes the approach. The induced field points into the near face.
South withdraws: near face becomes north
Attraction opposes the separation. The induced field points out of the near face.
| Magnet motion | Near coil face | Current in that face view |
|---|---|---|
| N approaches | N: repels | Anticlockwise |
| N withdraws | S: attracts | Clockwise |
| S approaches | S: repels | Clockwise |
| S withdraws | N: attracts | Anticlockwise |
Predict a new case
A south pole moves away
First identify the change: the magnet and coil are separating. To oppose that separation, the near coil face must attract the south pole, so it becomes north.
A north face requires anticlockwise current when viewed from the magnet end. Work from the change to the pole, then from the pole to the current direction.
The induced field does not always oppose the magnet's original field. During withdrawal, it acts to maintain a field that is decreasing. The rule concerns opposition to the change.
Where does the electrical energy come from?
When the induced current supplies a load, work is needed to keep moving the magnet or coil against the opposing magnetic effect. That external work provides the electrical output and any heating losses.
The opposing effect need not prevent the motion: an external force can maintain it while doing work. The magnet is not steadily consumed as fuel. In a generator, the mechanical energy source must keep turning the rotor.
Drill
Lenz's law: five in a row
Streak: 0 of 5
The north pole of a magnet moves towards a closed coil.
Check your understanding
A north pole is withdrawn from a closed conducting coil. Viewed from the magnet end, which induced pole and current direction oppose that withdrawal?
Show the answer
South face and clockwise current, which attracts the north pole as it is withdrawn.
Attraction opposes the increasing separation. The right-hand grip rule gives clockwise current when the face being viewed is south. The response opposes the change, rather than always opposing the magnet's original field.
Practise an exam question
Write your answer first, then tick the mark points it makes.
Exam-style question
State Lenz's law. The north pole of a bar magnet is pushed towards one end of a coil connected to a galvanometer. State the magnetic pole formed at that end of the coil and explain why. [3]
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