Generators & Motors — A/L Physics

What must I remember about Generators & Motors?

• AC Generator (Alternator): Converts mechanical work into alternating electrical energy; armature coil rotates in magnetic field; slip rings and carbon brushes extract continuous sinusoidal AC: e = E0 sin(ωt).
• DC Generator: Employs split-ring commutator (instead of slip rings) to mechanically reverse external electrical connections every half-cycle, producing unidirectional pulsating DC.
• DC Motor Principle: Converts direct electrical energy into mechanical rotational kinetic energy; current-carrying loop in magnetic field experiences magnetic torque τ = B I A sin θ (Fleming's Left-Hand Rule).
• Back EMF (Eb) in Motors: Rotating motor armature coil cuts magnetic field lines, acting simultaneously as a generator that induces a Back EMF opposing the supply voltage (Lenz's Law): Net armature current Ia = (V - Eb) / Ra, where Ra is armature resistance.

Test yourself (1 minute)

1. Motor Startup Surge & Starters

   At startup (motor stationary, ω = 0), Back EMF Eb = 0; initial starting current Ia_start = V / Ra is dangerously high (can burn windings); starting rheostats add temporary series resistance until motor accelerates and builds protective back emf.

2. Motor Mechanical Power Output

   Total electrical power supplied V * Ia = Ia² Ra (Joule heat loss) + Eb * Ia (converted to mechanical power P_mech); Mechanical Power is maximized when Eb = V / 2.

3. Generator vs Motor Rule Distinction

   Generator uses Fleming's RIGHT-Hand Rule (Mechanical + Field => Induced Current); Motor uses Fleming's LEFT-Hand Rule (Current + Field => Mechanical Force).

Practise Generators & Motors questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 8: AC & Electromagnetism

What should I study first?

Next in the syllabus

Where does Generators & Motors fit in the syllabus?

Unit 8.7 · Year 2 · Term 1