Generation Question.

[SDPHD]

Is there a simple way of answering this question?

The answer in the back of the book does unholy things to my mind when I try to make sense of it...

[Lasercookie]

AC alternates between positive and negative voltage. DC current sticks to positive voltage.

The alternator works by rotating the coil in a magnet. This induces an EMF. The current switches direction every half turn. The EMF is then made usable by using a slip ring.

For DC you need to use a split ring commutator. The commutator ensures that the EMF outputted will remain positive, through the use of the ring/brushes. When the coil rotates to the 'split' bits, it cannot complete the circuit, so that would-be negative voltage is not transferred to the circuit.

If you want to refer to the 'basic principles', go into detail about Faraday's and Lenz's law. Be a bit more pedantic than I was with your wording as well. The difference between an alternator and a generator is really the split ring and the slip ring. You could use a graph of AC and DC to show how they look.

Take a look at the animations here if it's still not clear to you: http://www.animations.physics.unsw.edu.au/jw/electricmotors.html#mandg