An a.c generator makes use of electromagnetic induction to convert mechanical energy into useful electrical energy.
How does it work?
To have an overall picture on how an a.c generator work, you might want to look at the simulation here.
a.c generator works as a power supply. For position 1 and 5, by Fleming’s Right Hand Rule, induced current flows out from Slip Ring Q and back to coil from Ring P. Since current flows out from positive terminal to negative terminal, the Slip Ring Q must be positive. Likewise for position 3, Ring P must be positive.
Graph of magnitude of induced emf (or current)
Note that the graph is a sine graph or cosine graph depends on the starting position.
When the coil starts from horizontal position, it is a cosine graph;
When the coil starts from vertical position, it is a sine graph.
Factors affecting a.c Generator
d.c Motor v.s a.c Generator
Fixed Coil Generator
Instead of fixing the magnet and rotating the coil in a simple a.c generator, a fixed coil generator is made by fixing the coil and rotating the magnet. A bicycle dynamo is an example of this as shown below.