Magnetic Flux vs. Time (1 Viewer)

[taeyang]

I got the answer as A, but apparently the answer is D.

Here is my reasoning; the emf produced is faraday's equation thing right? tge = -n X (change in magnetic flux)/(change in time)

Hence, it's basically the derivative of the function of the magnetic flux, given in the question. Therefore if you take the gradient of the tangent at any point across the curve you will be given the appropriate value for potential difference of the emf, and hence you will be able to derive the curve for the rate of change of magnetic flux. now take for example the gradient when Time = 0, the gradient is a maximum positive value, which means that the EMF produced is equal too the negative of this value, so HOW IN THE WORLD does it make sense that the emf produced is 0???????????


I need help bros.

 

[hscishard]

Shouldn't it be B

 

[_deloso]

emf = -Wb/t therefore at a certain time, the emf would be - times Wb. for eg when t=6, Wb=-2 so emf=-x-2 =2 when t=6. hence the answer is d. I don't think it's the derivative since it is the induced emf not the rate of change of the magnetic flux

 

[K4M1N3]

Umm isnt induced EMF defined as the rate of change of magnetic flux?

 

[umz93]

emf = -Wb/t therefore at a certain time, the emf would be - times Wb. for eg when t=6, Wb=-2 so emf=-x-2 =2 when t=6. hence the answer is d. I don't think it's the derivative since it is the induced emf not the rate of change of the magnetic flux

Agreed. Just sub the values into the formula -Wb/T, it will give you the graph!

 

[jamesfirst]

Wait, I thought if magnetic flux was maximum, the induced emf is 0

 

[Fizzy_Cyst]

It is all to do with the wording of the question.

It is a graph of mag flux vs time in a solenoid.

It wants the graph of EMF vs time in the SAME solenoid, (i.e., a graph of EMF vs time which would cause that flux vs time graph), not the graph of induced EMF in a different solenoid.

 

[weirdguy99]

http://www.boardofstudies.nsw.edu.au/hsc_exams/exam-papers-2007/pdf_doc/physics-07.pdf

Check out question 8. It's pretty much the same thing, except the answers are in a slightly different order.
The answer to question 8 is D (from the HSC notes, cross checked with Excel Success One).

I'm not sure how OP's question is different, as they both ask for 'induced emf' or 'potential difference generated', which is the same thing right? So therefore, if both questions are the same and the actual HSC question has D, then the OP's answer of A must be correct

If you put your hand and make a tangent to the curve at 0ms, you'll see it is a maximum, but it is negative due to Lenz's Law (I think?). If you make a tangent at 2ms, there is no induced emf as there is no change in flux.

So the answer is A and OP should be right.

What question is this from anyways?

 

[Haemolymph]

I got D.
Because it's in a solenoid shouldn't the way just be the opposite?

 

[weirdguy99]

Lets say this was a short answer question, how would you answer it?

I doubt you could just say "because it's in a solenoid shouldn't the way just be the opposite?"

I'm not trying to be rude or anything, but could you give a more qualitative answer?

 

[Haemolymph]

Lets say this was a short answer question, how would you answer it?

I doubt you could just say "because it's in a solenoid shouldn't the way just be the opposite?"

I'm not trying to be rude or anything, but could you give a more qualitative answer?

I don't do physics so I don't know the theory behind it but honestly it's pretty simple imo.
May as well write the magnetic flux wave produced by the solenoid is converse to the voltage wave produced and this is because of some year 11 physics thing I forget.

 

[stampede]

Lets say this was a short answer question, how would you answer it?

I doubt you could just say "because it's in a solenoid shouldn't the way just be the opposite?"

I'm not trying to be rude or anything, but could you give a more qualitative answer?

theyll never ask a q like that in short answers, it would be a waste, they only ask these types of qs in mc

now, im taking a guess and sayin most if not all of you guys doing physics are also doing 2unit (if not, just know wat a sine curve and a cos curve looks like and that the differential of a sine curve is a cos curve, while the differential of a cos curve is a negative sine curve)

this will be the easiest way to answer these qs:

remember emf is defined as the negative rate of change of magnetic flux

re: the first q the OP posted, the original magnetic flux graph is in the shape of a sine curve, the differential of that is a cos curve, and the negative of that is a negative cos curve, which is the shape of the graph shown in A.

re: the hsc q that weirdguy99 posted, the original graph is not a sine curve exactly since it is linear, but the general shape of the curve is that like a sine curve. the differential of that is a cos curve, and the negative of that is a negative cos curve, which is the general shape of the answer, D. (Note how the original graph had linear segments and how when u differentiate this you get straight line segments; eg y=2x, dy/dx=2 etc)

let me know if you guys are still confused about finding the right answers

 

[weirdguy99]

re: the first q the OP posted, the original magnetic flux graph is in the shape of a sine curve, the differential of that is a cos curve, and the negative of that is a negative cos curve, which is the shape of the graph shown in D.

D is another sine curve. A negative cosine curve would be A, as the OP had answered.

In regards to the HSC question that I posted earlier, there are other questions that use non-linear curves that give the same answer. The Jacaranda textbook also agrees with me (page 137 of the 2nd Edition) where it says "the flux threading the coil is at a maximum value. The emf is zero, as the gradient of the flux versus time graph is zero, which means that there is no change in flux through the coil at this instant".

Now, lets look at 2ms, where the flux is threading at a maximum. D shows that there is minimum emf being produced, which should be zero.

http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2002exams/pdf_doc/physics_02.pdf

Check out question 10. Answer is B (from BOS notes), showing that OP's answer is indeed A.

 

[clementc]

re: the first q the OP posted, the original magnetic flux graph is in the shape of a sine curve, the differential of that is a cos curve, and the negative of that is a negative cos curve, which is the shape of the graph shown in D.

They are all negative cos curves; just all with a phase difference. But if you think about it that way, if anything it should be A. D would be a negative sin curve.

Anyway yeah LOL I was just posting to show my support for option A. As demonstrated by , you simply find the negative of the gradient, so A should be the right answer I think =)

 

[stampede]

yes guys my bad, answer for the first q should be A

the method still holds true, try it out and see for yourselves

 

[hscishard]

Shit, -n...
Then yea it would be A