The a.c. generator
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
Structure of a Simple A.C. Generator An a.c. generator converts kinetic energy to electrical energy using electromagnetic induction. FIG 4.5.5: Simple a.c. generator Components shown: rectangular coil between poles of permanent magnet (N and S).
Coil mounted on axle that can rotate. Two slip rings attached to ends of coil. Carbon brushes press against slip rings to make electrical contact. External circuit connected to brushes. Key Components Coil: Rectangular wire coil that rotates between magnetic poles Permanent magnets: Provide the magnetic field Slip rings: Two separate rings attached to each end of the coil Brushes: Carbon contacts that maintain connection as coil rotates How the Generator Works The coil is rotated (by an external force, e.g., turbine) Coil wires cut through magnetic field lines E.m.f.
is induced in the coil (electromagnetic induction) Slip rings rotate with the coil Brushes transfer the current to the external circuit As coil rotates, direction of induced e.m.f. reverses every half turn Why A.C.?
The coil sides alternately move up and down through the field as it rotates, so the direction of the induced e.m.f. reverses each half rotation, producing alternating current . E.M.F. – Time Graph FIG 4.5.6: E.m.f.
vs time for a.c. generator Sinusoidal wave: E.m.f. on y-axis, time on x-axis. Wave oscillates between +peak and −peak values. Key positions marked: 0° (zero, coil horizontal), 90° (maximum positive, coil vertical), 180° (zero), 270° (maximum negative), 360° (zero, one complete cycle).
Coil Position and E.M.F. Coil Position Cutting Field Lines E.M.F. Horizontal (0°, 180°, 360°) Maximum rate Maximum (peak) Vertical (90°, 270°) Parallel to field (zero rate) Zero Increasing Generator Output Rotate coil faster (increases frequency and peak e.m.f.) Use more turns on coil (increases peak e.m.f.) Use stronger magnets (increases peak e.m.f.) Use larger coil area (more field lines cut) Alternative Design: Rotating Magnet Some generators have a stationary coil with a rotating magnet inside.
This avoids the need for slip rings on the output coil.
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