## Visionary01 2 years ago Examples where C-EMF can be generated? Hi, Do you know systems that generate C-EMF? Set ups like an Electric Motor? What other system do you have in mind? Another thing. I know that CEMF is caused by a changing electromagnetic field. How can an electromagnetic field be changed? Please elaborate!

1. Mashy

you just need a changing magnetic field.. faraday laws says that a changing magnetic field (more precisely magnetic flux) induces an EMF .. so that can be done in a numerous ways the magnetic flux that is linked with any coil is always given by the formula $\phi b = nABcos \theta$ where theta is angle between the normal to a coil of n turns and A cross section area and the applied field so you can induce the emf by a) changing area (sliding a coil into and out of the field) this is called as motional emf b) changing b itself ( sliding in and out a magnet into the coil if you may) c) by using an AC current d) by rotating the coil (OUR ENTIRE FREAKEN TECHNOLOGY RUNS ON ELECTRICITY WHICH IS ACTUALLY PRODUCED BY THIS METHOD.. we don't have ANY OTHER METHOD of generating electricity :O ).. ofcourse other alternatives are only taking baby steps like solar cells.. !! so any coil can generate a counter emf

2. gleem

Counter emfs occur when the magnetic flux generated by a current through a coil is changed rapidly as when the current flow is interrupted say by a switch. The emf generated depending on the inductance and the time interval of the interruption can be great enough to cause arc overs at the switch or arching through the insulation of the wire in the coil.

3. Vincent-Lyon.Fr

The loudspeaker is another example.

4. Visionary01

|dw:1358544557642:dw| In this case, where a movable magnet is getting attracted to the fixed electromagnet, will C-EMF be generated? And since it not going "into" the coil will it be a huge amount? I doubt it would... What do you think? @Mashy @Gleem @Vincent-Lyon.Fr Ow thanks for your replies really interesting and helpful points!

5. gleem

Only if the flux in the coil is decreasing will there be a counter emf and if it increases it will reinforce the current emf. Off course the rate of change in this case will probably be low compared to interrupting the current in the electromagnet. And also your orientation of the permanent magnet is not optimal for rapid flux change.

6. Visionary01

@gleem Why is the rate of change low? And I'd like to add that the magnet is movable. So its going to be attracted and moved from its origin to be near the electromagnet. And! The magnet's poles strength is very high compared to the electromagnet. What do you think will still happen?!

7. gleem

According to your diagram the magnetic flux of the bar magnet seems nearly perpendicular to the coil thereby making the component of its flux through the coil small and any change in flux "linking the coil" as it approaches the coil small. Unless the bar magnet is rotated 90 deg so that the its lines of flux are parallel and "literally shot into the coil the C-emf generated will not be large compared to just turning the current off in the coil.

8. Visionary01

@gleem I haven't drawn the flux lines jut the direction, and the line around the coil represents the windings. Now I'm staring to think would the magnet be perpendicular to the electromagnet's flux... Even so! If the flux lines were perpendicular, would the electromagnet sill be able to attract the magnet? + For C-EMF to be generated highly, the flux lines have to be parallel? I'm guessing its a yes.

9. gleem

From your diagram I can preitty well guess the orientation of the bar magnets flux line. They are in fact perp. to the coil's flux line. Yes the coil would attract it an if allowed rotate it so the N/S pole of the coil would attract the magnets opposite pole and pull it toward the coil and maybe into it if properly aligned. When parallel there is more flux passing into the coil. Only the component of the bar magnets lines of flux perpendicular to the cross sectional areal of the coil contribute to the emf.

10. Visionary01

@gleem thus! C-EMF will not be highly generated at all... It will be very. But do you think in an electric motor... The flux lines are parallel?

11. gleem

I believe they are designed that way

12. Visionary01

@gleem thank you! I drew up my set up all again with the inclusion of the flux lines you we're right. They are parallel. Now I don't have to worry about back EMF being generated highly... Actually I DOUBT it will be generated AT ALL.

13. Visionary01

@Mashy @Vincent-Lyon.Fr What do you think?

14. Visionary01

@gleem I was wondering... What kind of source are you relying here? That both the flux lines have to be parallel in order for the C-EMF to be generated?

15. gleem

Are we getting induced emf mixed up with with c-emf?

16. Visionary01

@gleem well to be honest... I feel that in order for C-EMF to be generated it has no relevance being in parallel or perpendicular. Could you please give me source to prove me wrong. Main reason is that, many people I asked would refute this idea. However, I'd like to use a source to refute them and make them agree. @Carl_Pham what do you generally think about this topic?

17. Visionary01

And yea @gleem I might be mixing up induced emf and c-emf here...

18. gleem

I kink you are too.

19. Visionary01

Ok, @gleem since there is a difference could you explain more? Generally why is it that the flux lines have to be parallel and what supports this?

20. Visionary01

@gleem Because I'm trying to study your point... But... I cant find a source.

21. gleem

Its not that the lines need to be parallel but the field applied to the solenoid must be perp.. to the crossectional area of the solenoid to have maximal effect.. So that if there is a flux in the solenoid to start with the applied field will then be parallel to that field.

22. Visionary01

So... Its not the flux lines that are important but rather the magnetic field lines? Could you draw an illustration please @gleem? I'd like to go in depth!

23. Visionary01

@gleem? Please describe what you ment with more detail...

24. gleem

The flux lines are the field lines.

25. Visionary01

So in order for a significant C-EMF to be generated, the flux lines have to be perp. of the electromagnet? I'd want to agree, but what do you have to prove this to be true? What resource can I review?

26. gleem

perp to the crossectional area of the electromagnet. You can refer to and general physics text or for a quick review http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html#c1 try this.

27. Visionary01

So perpendicular the cross-sectional area would be the inside of the coil or what ever area that the magnet would be close to the electromagnet. And I'm studying that reference. Thanks you @gleem so far!

28. Visionary01

While reading your reference what part does this specifically show? I'm looking for some relevance to C-EMF... But only see references to induced emf.

29. gleem

c-emf is the induced emf. it occurs when the current increases and again when it decreases but reversed. try http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indcur.html#c1

30. Visionary01

Thanks, so generally @gleem when a magnet comes near a cross sectional area of a electromagnet, in order for C-EMF to be significantly high, the field lines HAVE TO be perpendicular. I'll study your sources and get back to you. But my summary up is correct right?

31. gleem

Yes for maximal effect they should be perp to the crossectional area.

32. Visionary01

Thank you @gleem!