[Catalist] Average emf induced in a coil formulae

[Elizabeth Kirby]

Thank you for this response, Peter. Most people have responded similarly. What I do find disconcerting is that the worked solutions for the new STAWA book for the Problem Set on Induction, I think it's set 11, questions 18 to 21, uses Faraday's Law and the time for a quarter turn and according to the solutions, it is the max emf that has been calculated. Subsequent parts of the question asking for the RMS value then uses that emf and divides it by square root of 2. It just seems contradictory. Instead of the new book using the new formula when the frequency of the coil is given, it still shows the 'old' method.
Kind regards
Elizabeth

Sent from my iPhone

> On 17 Sep 2016, at 13:35, P. de Lacey <p.delacey at physics.org> wrote:
> 
> 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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