If we suppose that the magnetic poles in the AC generator are regular rectangular bar magnets, then the field lines are supposed to be straight lines as in the photo below:

ac generator

And then the direction of velocity will be inclined with an angle to flux lines, so we will need to multiply by sinθ as in the following equation:


But the poles are actually concave so I suppose that the direction of velocity will always be perpendicular to the flux lines as in the following photo:

concave poles

So if the concave poles are the ones really used, why do we need sinθ in the equation?

  • $\begingroup$ Note that in a real-life generator, the gap between the loops and the pole piece faces is very small. $\endgroup$ – niels nielsen Dec 17 '18 at 0:28
  • $\begingroup$ @nielsnielsen what does this mean? $\endgroup$ – user3407319 Dec 17 '18 at 7:01
  • $\begingroup$ it makes the geometry of the motor and field magnets easier for you to understand. $\endgroup$ – niels nielsen Dec 17 '18 at 7:10
  • $\begingroup$ I still don’t get what does the gap distance have to do with the shape of the flux lines $\endgroup$ – user3407319 Dec 17 '18 at 13:24
  • $\begingroup$ I mean even if the gap is small, aren’t the flux lines still supposed to be perpendicular to the lateral sides making multiplying by sine the angle useless? $\endgroup$ – user3407319 Dec 17 '18 at 13:26

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