
#1
Apr1712, 10:33 PM

P: 3

Hello everyone,
How are you doing? I have a doubt about electromagnetic induction, in three particular cases. I need to confirm that I have the right concepts, so I ask for your help. The main problem: Imagine that you have a permanent magnet, axially polarized and rotating on its axis with a constant angular speed. Surrounding this magnet, a coil (constant area section pointing in the same direction of magnet polarization). The main question is: will there be induced voltage? This is what I think: 1) We know that, for a constant Area, flux linkage ψ = B*A*cos θ. In this case θ = 0°, so ψ = B*A. And the induced voltage is ε = N*dψ/dt = N*A*dB/dt. In this main case, I think that there will be no variation in B, because the rotation does not change it at all. So dB/dt = 0, thus ε = 0. 2) Let's suppose the magnet is now radially polarized, but keeping the surrounding coil. In this case, can I affirm that rotation still doesn't change B at all (actually it does change B, but if we consider the whole thing it does not)? And not only because of this ε is zero, but θ = 90°, which implies ψ = 0. 3) Now suppose the coil doesn't fully surround the magnet. Let's say it covers only 270° of it (a little abstraction is needed, I know ). In this case of nonsymmetry, there will be a variation in B, but ε is still zero because θ = 90°. Am I correct? Did I miss something? Thank you, Marcus 



#2
Apr1712, 11:23 PM

Sci Advisor
PF Gold
P: 2,020





#3
Apr1712, 11:53 PM

P: 3

Ah, first, I forgot to tell that these permanent magnets are round magnets.
But the third case is a bit more complicated... well, you have just described a "common" generator, right? And thank you for your reply! If you let me, I want to ask you other geometry. This is going to help me understand a bit more. I drew it to make it easier to see the problem. The red/blue part of magnet is only north/south pole division (ie, in the picture, it is polarized along axis X). If this magnet rotates around axis X with a constant speed ω, as the coil remain still, should I expect induced voltage? I guess not, because, again, θ = 90°. But what I can't see is: what if the magnet is polarized in Z axis? Notice that this is very similar to case (2) I described before, but the coil is in front of the magnet, not surrounding it. Thanks again, Marcus 



#4
Apr1812, 06:29 PM

Sci Advisor
PF Gold
P: 2,020

Electromagnetic Induction
First partyou are right. Second partwhat do you think? Draw your magnet as a dipole, for example, and draw a few field lines around it to see what happens.




#5
Apr1812, 06:44 PM

P: 3

I think I see... dB/dt will not be zero.
Thanks for your help, marcusl. 



#6
Apr1812, 07:11 PM

Sci Advisor
PF Gold
P: 2,020

You are welcome.



Register to reply 
Related Discussions  
electromagnetic induction  Introductory Physics Homework  2  
Electromagnetic Induction  Classical Physics  6  
Electromagnetic Induction vs. Electromagnetic Radiation  Classical Physics  17  
ElectroMagnetic Induction  HELP!  Introductory Physics Homework  1  
electromagnetic induction  General Physics  22 