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# Electromagnetic induction Q watch

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1. A flat circular coil of 120 turns, each of area 0.07m^2 is placed with its axis parallel to a uniform magnetic field. The flux density of the field is changed steadily from 80mT to 20mT over a period of 4seconds. What is the EMF induced in the coil during this time? A) 0, B)130mV, c)170mV, D)500mV

(answer in back of book: B)

So, what I did was: flux = BxA = (20-80)x10^-3 x 0.07m = -4.2x10^-3
EMF = change in flux/ change in time = -4.2x10^-3/4 = -1.05x10^-3 (which is obviously incorrect)

Help much appreciated
2. (Original post by lekha2611)
So, what I did was: flux = BxA = (20-80)x10^-3 x 0.07m = -4.2x10^-3
You have miscalculated the value in this step. Btw, as the qn is asking for the emf, a scalar quantity, you should use the bigger value to deduct the smaller value to yield a positive value.

The final value will not be exactly 130 mV, but close to that.
3. (Original post by lekha2611)
A flat circular coil of 120 turns, each of area 0.07m^2 is placed with its axis parallel to a uniform magnetic field. The flux density of the field is changed steadily from 80mT to 20mT over a period of 4seconds. What is the EMF induced in the coil during this time? A) 0, B)130mV, c)170mV, D)500mV

(answer in back of book: B)

So, what I did was: flux = BxA = (20-80)x10^-3 x 0.07m = -4.2x10^-3
EMF = change in flux/ change in time = -4.2x10^-3/4 = -1.05x10^-3 (which is obviously incorrect)

Help much appreciated
You have forgotten to take into account the number of turns.
4. Where do I take into account the number of turns - when calculating flux?
5. (Original post by lekha2611)
Where do I take into account the number of turns - when calculating flux?
Either when the question requires you to calculate the magnetic flux linkage or the emf, also the rate of change of magnetic flux linkage.
6. (Original post by lekha2611)
Where do I take into account the number of turns - when calculating flux?
Your calculation using the area of the coil and the flux density of the field passing through it is for ONE turn of wire. You have 120 turns, each with the same flux passing through it, and each getting the same induced emf.

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Updated: October 1, 2011
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