# AC Generator With Repelling Magnets

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1. Aug 2, 2015

### benofer90

Hi Everyone,

I have 4 scenarios in question . Will The EMF(Voltage) be the same in all of the scenarios?
- the difference between the two photos are the polls: one repel one attract .
- i understand that the current(amps) will change due to the wire length assuming same gauge

1) Photo 1: d1 = 1cm, d2=20cm, wire length=240cm, Wire Exposed to field=80cm, each magnet is 1 Tesla(at surface), coil moves at 6meter/second
2) Photo 1: d1 = 1cm, d2=10cm, wire length=200cm, Wire Exposed to field=80cm, each magnet is 1 Tesla(at surface), coil moves at 6meter/second
3) Photo 2: d1 = 1cm, d2=20cm, wire length=240cm, Wire Exposed to field=80cm, each magnet is 1 Tesla(at surface), coil moves at 6meter/second
4) Photo 2: d1 = 1cm, d2=10cm, wire length=200cm, Wire Exposed to field=80cm, each magnet is 1 Tesla(at surface), coil moves at 6meter/second

Photo1:

Photo2

Thank you

2. Aug 3, 2015

### BvU

Hello Ben, welcome to PF !

Do you have an expression for the EMF ? The F in EMF stands for force; forces are vectors: they have a magnitude and a direction, so you'll need to think about both of these.

3. Aug 3, 2015

### benofer90

hi Bvu, yea I'm aware of it . the EMF(V) will be generated in the oppose direction . but still I'm looking for an answer to the above question. and when i ask if the EMF(Voltage) be the same i mean in magnitude

4. Aug 3, 2015

### CWatters

Do you have an expression for the magnitude of the EMF ?

5. Aug 3, 2015

### stedwards

In your drawings #1, no current is generated in the loops.

6. Aug 3, 2015

### benofer90

what do you mean ?

7. Aug 3, 2015

### benofer90

Please explain why you think that ?

Last edited by a moderator: Aug 3, 2015
8. Aug 3, 2015

### stedwards

Use the right hand rule on the horizontal top section of a loop, and again, on the bottom section. The EMF of one section opposes the other. Symmetry says they are equal and opposite.

Last edited by a moderator: Aug 3, 2015
9. Aug 3, 2015

### benofer90

then how is it any difference from this ?

10. Aug 3, 2015

### BvU

Blue and red move in opposite directions

11. Aug 5, 2015

### benofer90

So essentially option 1,2 would not work as they are canceling each other and 3,4 would work?

12. Aug 5, 2015

### benofer90

So essentially option 1,2 would not work as they are canceling each other and 3,4 would work?

13. Aug 16, 2015

### benofer90

any thoughts ?

14. Aug 16, 2015

### benofer90

any thoughts ?

15. Aug 16, 2015

### BvU

Plenty of thoughts. One of them: the situation isn't all that clear to you yet. What can be done to improve the situation ?

Are you familiar with the Lorentz force on charge carriers ? $$\vec F = q\, \left (\vec E + \vec v \times \vec B \right )$$
There's a picture (bottom one: motion $\Rightarrow$ emf) in post #16 in this thread

16. Aug 16, 2015

### benofer90

all new to me. making sure I am getting it correctly .

17. Aug 16, 2015

### BvU

In you picture, e.g. at the lower end, you have M pointing to the left.
M is the magnetic field.
At the lower "horizontal" section of the wire frame, the magnetic field is pointing up.
The velocity vector is pointing to the left. So the Lorentz force on the charge carriers in the wire is pointing away from the viewer.

Your turn to try and find the direction of the emf for the upper "horizontal" section of the wire frame !

18. Aug 16, 2015

### benofer90

Hold on, when it say magnetic field "right hand rule" little picture I added on top of the drawing. it is referring to the permanent magnet field? and the force is referring to in my case the direction of the coil.

if that is the case then i believe this is the correct drawing .

Last edited: Aug 16, 2015
19. Aug 16, 2015

### BvU

One out of three. Still for the bottom "horizontal" section of the wire frame:

The magnetic field thumb is indeed the field from the magnet.
So yes, the middle finger is up (no pun intended ).

The current in this situation is the motion of the charge carriers in the frame. The $q\vec v$ in the Lorentz force expression.
So index finger points to the left.

That leaves the thumb pointing into the screen. Thus representing the direction of the force that these charge carriers "feel". Corresponding to an emf that wants to move these charges.

Your turn to try and find the direction of the emf for the upper "horizontal" section of the wire frame !

And photo 1, where the emf allegedly oppose each other.

20. Aug 16, 2015

### benofer90

But, if i look at "fleming's right hand rule" then:
1) the thumb is pointing to the movement(motion of the coil in my case and we can also call it force)
2) First finger (also called Index Finger) is the magnetic field (in my case the magnetic field due to the permeant magnet)
3) second finger (also called middle Finger) is the direction of the current (EMF,V)

no pun was taken

so where am I getting it wrong in photo #2?