Learning outcomes
- Describe electromagnetic induction.
- State factors affecting induced e.m.f.
- Use a galvanometer to detect induced current.
- Explain direction using Lenz’s law.
- Apply induction to practical situations.
16.1 Producing an induced e.m.f.
An e.m.f. is induced when the magnetic flux linking a conductor changes. This can occur by moving a conductor through a magnetic field, moving a magnet relative to a coil, changing the current in a nearby coil, or changing the area or orientation of a coil in a field.
Motion itself is not enough. A magnet held stationary inside a coil produces no continuous induced e.m.f. because the flux linkage is not changing. The galvanometer deflects only while the magnet or field changes relative to the coil.
16.2 Relative motion
Pushing a north pole into a coil gives a deflection. Holding it still gives zero. Pulling it out gives a deflection in the opposite direction. Moving the coil while holding the magnet fixed produces the same effect because relative motion and flux change are what matter.
Faster movement produces a larger induced e.m.f. because flux linkage changes more rapidly. A stronger magnet, more turns or a soft-iron core can also increase flux linkage and therefore the induced signal.
16.3 Faraday’s principle qualitatively
The magnitude of induced e.m.f. depends on the rate of change of magnetic flux linkage. The detailed mathematical law is beyond the syllabus, but the qualitative phrase “faster change gives larger induced e.m.f.” is essential.
Increasing coil area, number of turns or field strength may increase total flux linkage. Rotating through a field creates a continuously changing linkage and is the basis of generators.
16.4 Lenz’s law
Lenz’s law states that the induced current acts in a direction that opposes the change producing it. If a north pole approaches a coil, the near face of the coil becomes north so it repels the approaching magnet. If the north pole moves away, the near face becomes south to attract it back.
The law is a consequence of conservation of energy. If the induced force helped the motion, the magnet could accelerate while generating electrical energy without an energy input. Instead, external work is needed to overcome the opposing magnetic force.
16.5 Determining current direction
First identify whether magnetic flux through the coil is increasing or decreasing and which pole is approaching or leaving. Next choose the induced pole that opposes that change. Finally use the solenoid right-hand rule to infer conventional current around the coil.
Questions often require only the direction of galvanometer deflection relative to a previous case. Reversing either the magnet polarity or the motion reverses induced current; reversing both restores the original direction.
16.6 Energy transfer and magnetic braking
When a magnet moves near a conducting loop, induced currents produce forces opposing motion and mechanical energy is transferred to internal energy in the conductor. This principle is used in eddy-current braking and damping.
A solid metal plate may show stronger eddy currents than a slotted plate because slots interrupt current loops. Transformer cores are laminated for a related reason: to reduce unwanted eddy-current heating.
Worked examples
Increasing deflection
A magnet is pushed into the same coil twice as fast. The flux linkage changes more rapidly, so induced e.m.f. and galvanometer deflection are larger.
Direction change
A north pole moving into a coil gives a right deflection. Pulling the same north pole out reverses the flux change, so deflection is left.
Lenz reasoning
An approaching north pole induces a north pole at the near face of the coil, producing repulsion and opposing the approach.
Practical focus
Investigation
Connect a coil to a centre-zero galvanometer. Move a bar magnet in and out at different speeds, reverse the pole, and vary the number of coil turns if possible. Record deflection size and direction. Keep movement distance similar and return the needle to zero between trials.
Examination guidance
- Induction requires changing flux linkage, not merely a magnetic field.
- Faster change produces greater induced e.m.f.
- Lenz’s law opposes the change, not necessarily the field itself.
- Stationary magnet and coil give no continuous induction.
Check your understanding
- Why is there no galvanometer deflection when the magnet is held still inside the coil?
- Give three ways to increase induced e.m.f.
- What energy principle explains Lenz’s law?
Answers
- Magnetic flux linkage is not changing.
- Move faster, use a stronger magnet, increase turns, or add an iron core.
- Conservation of energy.
