understand and explain experiments that demonstrate: • that a changing magnetic flux can induce an e.m.f. in a circuit • that the induced e.m.f. is in such a direction as to oppose the change producing it • the factors affecting the magnitude of the
Electromagnetic induction is the process by which a changing magnetic flux through a closed circuit induces an electromotive force (e.m.f.) in that circuit. The phenomenon underpins the operation of generators, transformers and many modern devices.
Key Concepts
Magnetic flux, \$\Phi_B = BA\cos\theta\$, where \$B\$ is magnetic field strength, \$A\$ the area of the loop and \$\theta\$ the angle between \$B\$ and the normal to the loop.
Mount the coil on the motor shaft so it can rotate within the gap of the magnet.
Connect the coil leads to the voltmeter.
Start the motor at low speed; record the peak voltage.
Increase the motor speed and record the new peak voltage.
Repeat with different numbers of turns (N) and different coil areas (A).
Peak e.m.f. \$\mathcal{E}_{\text{max}} = NAB\omega\$, where \$\omega\$ is angular speed.
Voltage is proportional to speed, number of turns, coil area and magnetic field strength.
Suggested diagram: Rotating rectangular coil between the poles of a horseshoe magnet, with connections to a voltmeter.
Experiment 3 – Lenz’s Law Demonstration (Opposing the Change)
Two common demonstrations illustrate the direction of the induced e.m.f. and its opposition to the cause.
Falling Magnet Through a Conducting Ring
Drop a strong magnet through a vertical copper ring attached to a spring balance.
Observe that the magnet falls more slowly than in free fall.
Explanation: The falling magnet changes the magnetic flux through the ring, inducing a current whose magnetic field opposes the magnet’s motion (Lenz’s law).
Current‑Carrying Loop Near a Magnet
Place a rectangular coil on a frictionless air track near a fixed magnet.
Pass a current through the coil and release it.
The coil moves away from the magnet, showing that the induced magnetic force opposes the motion that would increase flux.
Suggested diagram: Magnet falling through a conducting ring with arrows indicating induced current direction and magnetic field opposing motion.
Factors Affecting the Magnitude of the Induced e.m.f.
From Faraday’s law, the magnitude of the induced e.m.f. depends on how rapidly the magnetic flux changes. The main experimental variables are summarised below.
Factor
How it Influences \$\mathcal{E}\$
Experimental Control
Rate of change of magnetic field (\$\frac{dB}{dt}\$)
Higher \$dB/dt\$ → larger \$\mathcal{E}\$
Move magnet faster; use stronger magnets.
Area of the loop (A)
Flux \$\Phi_B = BA\cos\theta\$; larger area gives larger change in flux.
Use larger coils or increase coil dimensions.
Number of turns (N)
Induced e.m.f. is proportional to N (multiple loops add their e.m.f.).
Wind more turns onto the same core.
Angle between field and loop normal (\$\theta\$)
Flux varies as \$\cos\theta\$; rotating the coil changes flux sinusoidally.
Rotate coil at different angular speeds.
Electrical resistance of the circuit (R)
Higher R reduces the induced current \$I = \mathcal{E}/R\$, but does not change \$\mathcal{E}\$ itself.
Insert resistors to observe current change while \$\mathcal{E}\$ remains constant.
Summary of Key Points
A changing magnetic flux through a closed circuit induces an e.m.f. (Faraday’s law).
The induced e.m.f. always acts to oppose the change that produced it (Lenz’s law).
The magnitude of the induced e.m.f. increases with:
Faster change of flux (higher speed of magnet or coil rotation).
Greater magnetic field strength.
Larger loop area.
More turns in the coil.
Experimental setups such as a moving magnet through a coil, a rotating coil in a uniform field, and falling‑magnet demonstrations provide clear, observable evidence of these principles.