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.