Investigating Electromagnetic Induction

Electromagnetic induction is when a voltage (e.m.f.) is made in a conductor because the magnetic field linking it changes. If the conductor is part of a closed circuit, this e.m.f. drives a current. In short: move a wire across a magnetic field, or change the magnetic field through a coil, and a voltage appears.

Simple investigation (magnet and coil)

Apparatus: a coil of wire, a sensitive meter (galvanometer), and a bar magnet.

• Connect the coil to the meter. Hold the magnet still: the needle stays at zero.
• Push the magnet into the coil: the needle deflects one way (a current is induced).
• Pull the magnet out: the needle deflects the opposite way (current reverses).
• Move faster: the deflection is larger (bigger e.m.f.).

What changes the size of the induced e.m.f.?

• Speed of change: faster motion or faster field change gives larger e.m.f.
• Magnetic field strength: stronger magnets give larger e.m.f.
• Number of turns: more turns in the coil give larger e.m.f.
• Area and orientation: a larger area and cutting field lines at right angles increase e.m.f.

In symbols (qualitative): $\text{e.m.f.} \propto \text{rate of change of magnetic flux}$.

Direction of the induced current

Lenz’s law: the induced e.m.f. always opposes the change that causes it. The coil acts like a “magnetic seatbelt” that resists changes in magnetic field through it.

Fleming’s right-hand rule (generator rule): use your right hand with three axes at right angles. First finger = magnetic field (N to S), Thumb = motion of the conductor, Second finger = induced current (conventional current). Set any two, the third gives the direction.

From induction to generators

In an a.c. generator, a coil rotates in a magnetic field and continually cuts field lines, producing an alternating e.m.f. that changes sign as the coil turns. Slip rings and brushes maintain contact with the moving coil.

Common misconceptions

• A strong magnet held still induces no e.m.f.; there must be a change.
• Only the component of motion cutting field lines counts; moving along field lines gives little or no e.m.f.
• Induction gives a current only while the change is happening; it is not a steady battery unless the change continues.