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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: nos488 on November 10, 2011, 10:23:41 am

Title: confused...
Post by: nos488 on November 10, 2011, 10:23:41 am: Okay in the 2007 physics VCAA exam on question 7 i'm a bit confused in this whole area I understand that this coil has a thousand turns i understand that the delta t is 0.01 of a second but i don't understand where they got their change of flux to me it looks like they've used the flux through the coil as there change in the flux but this is kinda confusing since it only did a quarter of a rotation shouldn't the flux be only a quarter of that. Anyway thanks to anyone who will have a look at this for me
Title: Re: confused...
Post by: Lasercookie on November 10, 2011, 03:50:36 pm: VCAA 2007 Q7:

$EMF = -n(\frac{d \Phi}{d t})$

So the coil rotates a quarter of a revolution in 0.01s, so:
$dt = 0.01$
and we know that $n = 1000$

So at the original position, flux is: 3.0 * 10–^-4 Wb
After a quarter turn, (e.g. 90 degrees), the coil will be horizontally flat - so flux will be 0 Wb (no flux threading through the coil).

So $d \Phi = final - initial = 0 - 3.0 * 10–^{-4} Wb = - 3.0 * 10–^{-4}$

I know you can ignore the negatives, but I like to leave them in. But since you can ignore them (which solutions probably did), then the flux is pretty much the same.

$EMF = -1000(\frac{- 3.0 * 10–^{-4}}{0.01}) = 30 V$
Title: Re: confused...
Post by: nos488 on November 10, 2011, 05:04:54 pm: Thank you for that but if it was something similar (I know the vcaa exam had a diagram) does it matter what position it started off in as long as it tells you the turns the revolutions and the time?
Title: Re: confused...
Post by: Lasercookie on November 10, 2011, 05:43:42 pm: Quote from: nos488 on November 10, 2011, 05:04:54 pm
Thank you for that but if it was something similar (I know the vcaa exam had a diagram) does it matter what position it started off in as long as it tells you the turns the revolutions and the time?
Yes it does matter.

Change in flux is the change from initial position to the final position.

$d \Phi = final - initial = (BAcos \theta) - (BAcos \theta)$

If it started off where flux was 0, and then changed to where flux was about halfway from it's maximum - then the value would change

Or if it when 0 to maximum, then the value would be different, but since we only consider the magnitude, it'd effectively be the same thing as if it was going max to 0.

But yeah the initial position does matter and you need it to be able to figure it out (unless you're given dflux or even dflux/dt straight out, then you don't need to calculate it yourself). If you're not given a diagram, then you might be given how much of an angle it rotated or something like that.