# Need people with good info on electromagnetic induction 18 replies

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random_soldier1337

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17th June 2008

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#1 12 years ago

So I'm in a bit of a pinch. There's this self made science project we were supposed to do. My group did it on electromagnetic induction and try to find a relation between the velocity at which a magnet was passing through copper coils, i.e. the velocity at which the field lines were changing, being cut or otherwise (correct me if I'm wrong) and the amount of electric potential energy produced within the circuit to which the copper coils were connected. The graph showing the relationship should be linear but for some reason it is coming out as a parabola. Any suggestions as to what is wrong?

Mr. Pedantic

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#2 12 years ago

Are you plotting electric potential over velocity of magnet? And how are you doing the experiment?

Flodgy

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#3 12 years ago

A parabola? Wow.

First off, is the coil perfect or as close to a perfectly designed coil you can get? Because if there are major inconsistencies in it this will warp the results you get.

Just assuming here as I haven't looked at this kind of stuff in a while, but wouldn't the velocity be proportional to the amount of a current induced? If so, then your answer of a linear graph would be correct. If you could give us a bit more information on how you physically carried out the experiment and what your aim was, it would be a great help.

random_soldier1337

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#4 12 years ago

The experiment I am doing by dropping a bar magnet through copper coils of 240 turns from different heights so that velocity changes each time it passes. The coils are connected to a voltage sensor that is connected to a Vernier CBL unit connected to the computer which finds the voltage changes at different instances in time. I should note that the opening through the coils was small so we had to make a funnel type apparatus that we made out of material similar to magazine covers, so that the magnet would slip through. The friction it faced could have affected results but I doubt that because the magnet fell with the same consistency as that of a regular weight in free fall. I don't know the exact strength of the magnet but it was able to attract another magnet and pieces of metal over a distance of 5 to 6 cm. Yes, I was plotting Electric potential versus velocity. What do you mean by a perfect coil? Do you mean perfectly circular from one view? If that's what you mean then, no. Although it was inside of a box so I don't know if it was hugely inconsistent but I don't think there should have been such large inconsistencies. Possibly an oval, not egg shaped but rectangular with curved corners. I don't know how well that sounds but I don't think it was shaped irregularly or a shape without symmetry. According to the equation EMF = B*l*v, where EMF stands for electromotive force, B stands for strength of the magnetic field, l stands for length of something that I'm not sure about as I have yet to study the topic in detail myself in my physics classes and v is the velocity of the changing magnetic field, the graphs should have been linear. My aim was to study the effect of the velocity of the moving magnetic field on the electromotive force produced. I'm putting up the word document with the data and the graphs that came out incase you need to have a look at them. It may seems like the graphs have outliers but I think otherwise and so do the physics teachers I have asked. Even they are a bit baffled.

Mr. Pedantic

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#5 12 years ago

Can you assume that the current through the coils is constant? Because if you can't, then that may be why your results are off, you're measuring voltage, not energy transferred.

And l stands for the length of your wire.

And I think there's something wrong with your method, but I can't put my finger on it, I just have a feeling there's something with the way you're doing this that's not right.

Shizzle my nizzle

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#6 12 years ago

If the magnet is accelerating past the coil then of course the results will be parabolic-like (I would guess they were more exponential, but it's a similar shape). The voltage is being induced by the changing magnetic field, which is itself accelerating, so the voltage induced will appear to accelerate too.

Try moving the magnet through the coil at constant speed with your fingers if it's possible and see the resulting (nearly) straight line you'll get. Induction by magnetic flux depends on velocity, so if there is a changing velocity (i.e. acceleration) then your results will appear exponential.

random_soldier1337

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#7 12 years ago

I thought that it would be the acceleration. How would you suggest noting the movement of my hand? Use a motion sensor? It's gonna be hard knowing what speed my hand is moving at or supposed to move at over such a short distance. I don't think current through the coils is constant. I don't know but I think my teacher mentioned something about AC current being produced in the coils when moving the magnet which could make my results vary. Not sure about this but the power supply to the CBL was also AC, so it could have made the results vary. I don't know what could be wrong with my method. As far as I know, the only thing is the friction of the funnel thing we made, the acceleration and the AC current. Doesn't matter though. I can't do it again now. I'll have to justify it somehow in my analysis and conclusion. I thought my calculations would be wrong but the design itself is wrong so I'll just have to put up something for that. Any other ideas on what else could have gone wrong other than non-constant current and the changing velocity of the magnet?

Shizzle my nizzle

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#8 12 years ago

When I was talking about using your hand I was just explaining a way you could prove what I said that current is produced with constant speed and not just acceleration of a magnet through a coil. I wasn't suggesting a change to your actual experiment.

Something else which will effect your results is the fact that you are measuring the time when the magnet is only partially in the coil (when it is entering and exiting), and not just the time where it is entirely within the coil. This will effect your results because it's a weaker field inside the coil at those given times.

random_soldier1337

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#9 12 years ago

Actually, I was recording the changes in potential difference at all times with the help of the voltage sensor and took all the peak/maximum potential difference changes I got when the magnet passed through the coils. In a way it was being recorded while the magnet was entering, in the middle and leaving the coils, so I am not sure whether what you say might hold. Do you think that the magnet weakened due to usage? I only used it for about like 1-3 hours each time I did the experiment. It was an ALNICO bar magnet, incase that helps somehow.

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