Magnetic flux - magnitude of the change

1. Apr 24, 2017

superslow991

1. The problem statement, all variables and given/known data
A 200-loop coil of cross sectional area 8.5 cm2 lies in the plane of the page. An external magnetic field of 0.060 T is directed out of the plane of the page. The external field decreases to 0.020 T in 12 milliseconds.
(a) What is the magnitude of the change in the external magnetic flux enclosed by the coil?

2. Relevant equations

Φ=BAN
3. The attempt at a solution
I did 200*(8*10^-4)(0.06-0.02) and gotthe answer i got for this problem was 6.8*10^-3
Teacher said it was wrong. Did i mess up somewhere? i calculated it twice and got the same answer so im not sure how the answer would be wrong?

2. Apr 24, 2017

collinsmark

What is the rate of change of the magnetic field?

Above, as part of your work, you've calculated the total change of magnetic field as part of the answer, but what is the rate of change?

[Edit: see next post. I might have misinterpreted the question.]

Last edited: Apr 24, 2017
3. Apr 24, 2017

superslow991

Im not following, are you saying to divide the work i did by the time?

4. Apr 24, 2017

collinsmark

Actually on second thought, re-reading the specific question, there might be some misunderstanding for what is being asked.

The way I read the question verbatim [after re-reading it], it isn't asking about rate of flux change, but rather the total change in flux.

Also the use of the word "external" might play a part here. Does the "external" flux depend upon the number of loops in the coil?

Last edited: Apr 25, 2017
5. Apr 24, 2017

collinsmark

Eventually, perhaps in part (b), I'm guessing that the finding the rate of change of magnetic flux comes into play. And in that case you'll want to divide the total change of flux by the time of the change, to obtain the rate of change.

But given the specific wording of part (a), maybe that's not what your teacher is asking for here.

Instead, for part (a), you might want to focus on the word "external." Does the number of turns in the coil affect this "external" flux?

6. Apr 24, 2017

superslow991

I mean probably not since the flux would be outside the coil? not too sure.
Either that or i was suppose to subtract 0.02 from 0.06?

7. Apr 24, 2017

collinsmark

Between you and me, I don't like the way the question was worded.

I think by "external magnetic flux" your teacher was asking you to find the flux, defined by magnetic field strength and and area, of that place in space, whether the coil is actually there or not.

Then find the change from the starting time to the end, to find the change in flux.

I don't like the question because you need the coil to compute the area component, but you ignore the coil for the number of turns it has.

Anyway, that's how I interpret the question after re-reading it.

Subtracting 0.02 from 0.06 will give you the change in magnetic field strength. You'll still need to throw the area in there somehow to obtain the change in flux.

8. Apr 24, 2017

superslow991

yea idk either its asking for flux=BA or flux = BA/t not to sure bout this question

9. Apr 24, 2017

collinsmark

Flux is equal to $\Phi = \vec B \cdot \vec A$.

The rate of change of flux involves dividing the change of flux by the change in time, but I don't think that is what part (a) of the question is asking for.

10. Apr 24, 2017

superslow991

hmm ok. So what do you think part A is asking for?

11. Apr 25, 2017

collinsmark

You spell it out.

12. Apr 25, 2017

ehild

The area was 8.5 cm2. You calculated with 8 cm2.

13. Apr 25, 2017

superslow991

Typo sorry
idk I mean I thought my equation would solve for the flux but your saying I have to solve for the change of the flux? Went through different calculations just don't know what would be the right answer

14. Apr 25, 2017

collinsmark

No, your original attempt solved for the change in the flux, which I believe is correct (forget about post #2, that was my mistake).

But I don't think you need to include the number of turns in the coil for part (a), given the specific way it was worded.

15. Apr 25, 2017

superslow991

hmm ok thanks for the help ill try to see what happens today