IGCSE Physics 4.5.2 A.C. Generator Notes | Electricity and magnetism 2025

Type	Description	Examples
Direct Current (D.C.)	Flows in one direction only.	Batteries, cells.
Alternating Current (A.C.)	Changes direction continuously; magnitude also varies with time.	Mains electricity (e.g. 230 V A.C. in India).

Consists of an armature coil, slip rings, carbon brushes, and a magnet.
When the coil rotates between magnet poles, it cuts magnetic field lines, inducing an e.m.f. according to Faraday’s Law.
As the coil rotates, the direction of induced e.m.f. reverses every half turn, producing an alternating current.
Slip rings maintain continuous connection between the coil and the external circuit as it spins.

When the coil is perpendicular to magnetic field lines → no cutting → induced e.m.f. = 0.
As coil rotates → sides of coil cut magnetic field lines → e.m.f. increases, reaching maximum when coil is parallel to field lines.
Continuing rotation → e.m.f. decreases to 0 again after half a turn (coil upside down).
Next half-turn → e.m.f. reverses direction, producing the negative half of the a.c. cycle.
After one complete revolution, one full cycle of a.c. is produced.

Component	Function
Armature coil	Cuts magnetic field lines and induces e.m.f.
Slip rings	Maintain electrical contact between rotating coil and external circuit.
Brushes	Transfer current from rotating slip rings to external circuit.
Magnet	Provides the magnetic field needed for induction.

The induced e.m.f. reverses direction every half-turn → output current is alternating.
The time for one full revolution of the coil corresponds to one complete a.c. cycle.
Increasing speed of rotation → increases frequency and amplitude of the induced e.m.f.

Basis	D.C. Motor	A.C. Generator
Energy Conversion	Electrical → Mechanical	Mechanical → Electrical
Electricity	Uses electricity to produce motion	Generates electricity from motion
Principle	Current-carrying conductor in magnetic field experiences a force	Induced e.m.f. produced by changing magnetic flux
Fleming’s Rule	Left-hand rule	Right-hand rule
Commutator Type	Split ring	Slip rings
Current	Supplied to armature windings	Induced in armature windings

Induced e.m.f. arises due to flux change.
Direction of current reverses every half-turn because coil sides interchange positions in the field.
Slip rings ensure smooth a.c. output; split rings would instead produce pulsating d.c.
Speed increase causes both higher frequency and larger amplitude of induced e.m.f.

4.5.2 A.C. Generator