Electric AC Generator: Understand Magnetic Fields & Brushes

[genevievelily]

Hi! I have been studying AC generators. I am confused about why there is no current when the armature is perpendicular to the magnetic field (when it is vertical). Also, what is the purpose of the brushes? Probably a really obvious answer, but it is confusing me. Any help would be appreciated, thanks!

 

[Philip Wood]

Are you considering a single flat coil rotating in a uniform magnetic field? If so, when the coil is in the position you describe its area is perpendicular to the field at this instant, so its area projected on to a plane at right angles to the field is a maximum and, at this instant, not changing. So the magnetic flux linked with the coil is not changing. So there is no induced emf.

Can't think why there should be brushes, unless there is also a commutator to turn the ac generator into a dc generator. Alternatively we're dealing with a different sort of ac generator from the simple type I assumed in the first paragraph.

 

[jtbell]

Can't think why there should be brushes

If the magnets are stationary and the loop of wire rotates, you need some mechanism to keep the leads to the loop from twisting around themselves ad infinitum, no?

Alternatively, you could use permanent magnets and a fixed loop, and rotate the magnets instead.

 

[Philip Wood]

I was being stupid. Not for the first time.

 

[jtbell]

Don't worry. We all end up wearing the dunce cap sometimes, myself included.