6 min read Mathew Wahome

Electromagnetic Induction

The discovery

Michael Faraday found that a voltage (e.m.f.) is induced in a conductor when the magnetic field around it changes. This is electromagnetic induction, the basis of electricity generation in Kenya's hydro, geothermal and wind power stations.

Induction by experiment

Move a bar magnet into a coil connected to a galvanometer: the needle deflects.

Faster movement gives larger deflection.
More turns give greater e.m.f.
Stronger magnet gives greater e.m.f.
No movement gives no e.m.f.
Reversing movement reverses the current.

Laws

Faraday's law: induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage. E = -N (change in flux / time).

Lenz's law: the induced current flows in a direction that opposes the change causing it. This follows from conservation of energy. Pushing a north pole into a coil makes the near end a north pole, resisting the motion.

Fleming's right-hand rule gives direction of induced current: thumb = motion, first finger = field, second finger = current.

Generators

A coil rotating in a magnetic field produces an e.m.f.

A.c. generator: slip rings and brushes give alternating current.
D.c. generator (dynamo): split-ring commutator gives direct current.

Output increases with more turns, stronger magnets, larger coil area and faster rotation. In a power station, steam, falling water or wind turns the turbine connected to the generator.

Transformers

Two coils wound on a soft iron core. An alternating current in the primary creates a changing flux, inducing a voltage in the secondary.

Vs/Vp = Ns/Np
Step-up: Ns > Np; step-down: Ns < Np.
Ideal: input power = output power, VpIp = VsIs.
Efficiency = (output power / input power) x 100.

Worked example: A transformer reduces 240 V to 12 V. If the primary has 2000 turns, secondary turns = 2000 x 12/240 = 100 turns.

Energy losses

Resistance of coils (copper loss): use thick wire.
Eddy currents: laminate the core.
Flux leakage: wind coils on the same core.
Hysteresis: use soft iron.

Power transmission

Kenya Electricity Transmission Company (KETRACO) carries power at high voltage (132 kV, 220 kV, 400 kV). Power loss = I^2 R. Raising voltage lowers current, so losses fall. Step-down transformers near towns reduce voltage to 240 V for homes.

Example: 100 kW sent at 10,000 V gives current I = P/V = 10 A. Over a line of resistance 5 ohms, loss = 10^2 x 5 = 500 W.

Other applications

Induction cookers, microphones, tape readers, electric guitars, and mobile phone wireless chargers.

Safety

High voltage lines are dangerous; keep away from power lines and follow KPLC safety warnings.

Try it

Explain why a transformer will not work on a car battery.
A step-down transformer has 1,200 primary turns for 240 V. How many secondary turns give 6 V?
Describe how a bicycle dynamo lights a lamp.