Magnetism

[SKaminski]

I've been struggling a bit with magnetism, and while I understand how/when to use the equations and the (right hand) rules, I struggle any time a passage discusses coils moving in a field.

Is there a good resource for learning about pulling a loop out of a field? About a rotating coil in a field?

A good illustration of my problem is here:

https://www.youtube.com/watch?v=wchiNm1CgC4

The I vs t graph (the bottom one) looks identical to a 1001 EK Physics question (878) that is Force vs degrees. EK states that when the coil's angle with the magnetic field is 0, force is the greatest! This makes no sense to me! At all times the "long arms" of the coil are perpendicular to the field, so to find the greatest force generated, wouldn't you want that to be when "small arm" is perpendicular as well? This doesn't appear true in either case...

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[Endure]

I've been struggling a bit with magnetism, and while I understand how/when to use the equations and the (right hand) rules, I struggle any time a passage discusses coils moving in a field.

Is there a good resource for learning about pulling a loop out of a field? About a rotating coil in a field?

A good illustration of my problem is here:

https://www.youtube.com/watch?v=wchiNm1CgC4

The I vs t graph (the bottom one) looks identical to a 1001 EK Physics question (878) that is Force vs degrees. EK states that when the coil's angle with the magnetic field is 0, force is the greatest! This makes no sense to me! At all times the "long arms" of the coil are perpendicular to the field, so to find the greatest force generated, wouldn't you want that to be when "small arm" is perpendicular as well? This doesn't appear true in either case...

It's funny that you're using that video - I used the same one when taking physics last summer. Since you're discussing force, I'll try to clarify how a motor works.

A motor takes advantage of the fact that moving charge experiences a magnetic force when passed through a magnetic field. If you use a rectangular wheel and connect it to a battery, such as the one in the video, and place it within a uniform magnetic field, you'll create torque around its axis of rotation. Imagine a system where the plane of the rectangular wheel is parallel to the direction of the magnetic field. If you apply the right hand rule, the vector of the magnetic force is perpendicular to the direction of the magnetic field and will cause the wheel to spin. If you do the same for the wheel when its placed perpendicular to the magnetic field, the magnetic force points parallel to the plane of the wheel resulting in 0 torque.

Thus, t(torque) = Fr = (BIA)sinθ, where θ is the angle between the magnetic field and the line normal to the plane of the rectangular wheel.

 

[Gauss44]

Is there a good resource for learning about pulling a loop out of a field? About a rotating coil in a field?

I think I watched lecture 17 for that: http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/

If you are still looking for a good resource after that, one of these classes/online notes might work: http://www.fatwallet.com/forums/free-stuff/1247592/

 

[SKaminski]

Hey!
@Gauss44
Thanks for the link, I found this video at khan academy (via your second link): https://www.khanacademy.org/science/physics/electricity-and-magnetism/v/magnetism-9--electric-motors

and it helped me a lot. It visually illustrated Endure's point. I feel pretty confident with rotating coils. The only thing I'm a little confused with is pulling a loop out of a magnetic field. I don't think I need to know the formula's specific to pulling a loop out of a field, or even ε=NBAωsing(ωt) and
ε= -N {ΔΦb/Δt}. I think I just need to understand the idea conceptually.

Is it essentially: The more of the loop thats in the field, the greater the force on the electrons, the greater the current induced?

 

[Gauss44]

Hey!
@Gauss44
Thanks for the link, I found this video at khan academy (via your second link): https://www.khanacademy.org/science/physics/electricity-and-magnetism/v/magnetism-9--electric-motors

and it helped me a lot. It visually illustrated Endure's point. I feel pretty confident with rotating coils. The only thing I'm a little confused with is pulling a loop out of a magnetic field. I don't think I need to know the formula's specific to pulling a loop out of a field, or even ε=NBAωsing(ωt) and
ε= -N {ΔΦb/Δt}. I think I just need to understand the idea conceptually.

Is it essentially: The more of the loop thats in the field, the greater the force on the electrons, the greater the current induced?

The induced current opposes the change in magnetic flux in both direction and magnitude. Here's some examples (#25, #26, #27, #102 and #103). http://www.rhs.rcs.k12.tn.us/teachers/yorkda/documents/Walker4_ISM_Ch23.pdf

If anyone else can explain this well, feel free to jump in.

 

[Gauss44]

accidentally posted twice. please delete.