Why do eddy currents flow the opposite direction to the right-hand-screw-rule?

[toneboy1]

I've noticed in demonstrations that when you've got a magnetic field going through an iron core, say coming out of the page at you, that the eddy currents circulating perpendicular to them will flow clock-wise. But if you had a wire loop and put a current through it clockwise that it would produce a magnetic field going in the page, the opposite direction.

What is it that makes the electrons or whatever in the core go that way instead of adhereing to the rule?

Thanks very much.

 

[sophiecentaur]

You will, surely, only get eddy currents when the magnetic field changes?

 

[toneboy1]

Yes, but for the short time when the magnetic field is pointing out of the page.

I think this is Lenz's law, but I don't get why it doesn't follow the Right hand curl or screw rule.

 

[sophiecentaur]

OK. What causes eddy currents? Answer that first.

 

[toneboy1]

The magnetic field line passing through the material...thats the phenomenon.

 

[I_am_learning]

Nope. Eddy currents are produced by changing magnetic field, and they follow lenz law.

 

[toneboy1]

yeah, so changing magnetic field through material, but you're saying lenz's law dictates the direction of the eddy currents will be the opposite to right hand rule?

Cheers

 

[sophiecentaur]

Lenz's law is the electrical equivalent to Newton's third law. It has to work that way round or any current would just get bigger and bigger - like the impossible situation where the reaction force was in the direction of the original push . . . . . .. zooooooooom!
Inductance is very analogous to Mass - just as Capacitance is analogous to a spring constant.

 

[goodname]

Lenz's law is simply the outcome of law of conservation of energy...Change in current induce magnetic field and change in magnetic filed produce current and this cannot go like chain violating law of conservation of energy...

 

[I_am_learning]

yeah, so changing magnetic field through material, but you're saying lenz's law dictates the direction of the eddy currents will be the opposite to right hand rule?

Cheers

What is right hand rule used for? To find the direction of magnetic field setup by a wire loop.
Since the magnetic field you are talking about isn't created by the eddy current loop, why do you think it need to follow right hand rule. Here, instead, the eddy current loop is setup by the magnetic field (changing). You need to understand the difference.
You are missing one thing here, the eddy current lopp also creates magnetic field. And this Eddy-Magnetic-field will be created in accordance to right hand rule. And the lenz-law now comes into play. It says that, in response to change in applied-magnetic field, the eddy current magnetic field will be setup so as to oppose the change. That tells you the direction of eddy current.

Remember the clasic experiment where you point a North pole towards a coil and observe
1. No motion no current
2. Moving towards the coil, some current
3. Moving away from coil, some current but opposite direction

In each case, there is magnetic field pointing toward the coil. The current in coil is equivalent to eddy current.

 

[toneboy1]

I understand that Lenz's law it due to concervation of energy and those analogies.

Thanks for making that distinction clear, that the right hand rule is for magnetic fields due to current, not current due to changing fields (if I understand you correctly). But what I want to know is, is there an actual physics explanation for the contrary direction, or is this just part of the phenomina of the universe, part of the laws like gravity which we can't explain but we can observe keeping everything in order?

Thanks

 

[I_am_learning]

But what I want to know is, is there an actual physics explanation for the contrary direction

Can you Kindly tell me where is there a contradiction?

 

[sophiecentaur]

"Contradiction" doesn't mean "contra direction".

The "actual Physics reason" is energy conservation. You can't get more Physicsy than that.

You want a hand-waving, mechanical explanation? No point; the basic Energy consideration says it all.

 

[toneboy1]

Can you Kindly tell me where is there a contradiction?

no 'contradiction' in concept, in opposite direction

 

[toneboy1]

"Contradiction" doesn't mean "contra direction".

The "actual Physics reason" is energy conservation. You can't get more Physicsy than that.

You want a hand-waving, mechanical explanation? No point; the basic Energy consideration says it all.

I do appreciate your help. But energy conservation, makes sense, but there is cause and effect, like there aren't energy faries that come down and make the eddy currents when you move a magnet :P

Kind Regards,

Rob

 

[pfmember]

I realize this thread is old but here is a figure from a textbook:



In this figure you can see the direction the author claims the eddy currents travel in. Notice that for the leading edge eddy current (the on on the right hand side of the picture in the direction of the F_pull vector) that the external magnetic field of the dipole is decreasing. Lenz's law tells us that the direction of the induced magnetic field will oppose the change in the external magnetic field. Since the external field is decreasing, in order to oppose this decrease the induced magnetic field will point in the same direction as the external field, which in this case is from North to South. From here by the right hand rule it is clear that the eddy current for the leading edge will be in the direction shown by the author.

Likewise the same logic works for the trailing edge where the external field is increasing, by Lenze's Law the induced field will oppose the change by being in the opposite direction, and the right hand rule gives us the direction shown in the image.

I added this post because the discussion was unclear without a picture representing what was being talked about.