[Catalist] Average emf induced in a coil formulae

[P. de Lacey]

Hi Elizabeth,
If students are using the formula Emf(av) = 4NABf they need to 
understand clearly what they are calculating and how it differs from 
Emf(rms) or Emf(max)
The formula Emf(av) = 4NABf gives the average e.m.f. of a 
fully-rectified sine wave. (This is not the same as Emf(rms).)
Its relationship to Emf(max) is Emf(max)= (pi/2)Emf(av), which reduces 
to the formula Emf(max) = NAB2(pi)f.
  Average voltage is not a useful value when dealing with (sinusoidal) 
a.c. because the average value over a full cycle is zero. Peak and 
r.m.s. values are more useful, so students should, preferably, use these.
Best regards,
Peter de Lacey


On 14/09/16 06:28, Elizabeth Kirby wrote:
> Esteemed Catalistians
> Could someone in the realm of Yr 12 Physics clarify for me whether or
> not students can still use the formula Emf (av.) = 4NABf, when they are
> given the frequency of rotation of a coil – provided they explain that
> there are 4 flux changes per rotation? This formula gives the same
> result as Faraday’s Law if one calculates the time for a quarter turn
> and one uses that quarter time as the time for a flux change. The new
> formula Emf (max) = NAB2 pi f, does not give the same answer even when
> one adjusts with the V peak and V RMS formula.
> Urgent help required with this please.
> Kind regards
> Elizabeth
>
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