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Name: David
Status: N/A
Age: 17
Location: N/A
Country: N/A
Date: 1999 


Question:
I'm really having a problem understanding magnetic flux and Lenz's Law of electromagnetic induction. I'm beginning to feel really stupid and the explanations I read are confusing me more and more. Can you help????? I'd be really grateful.


Replies:
Sometimes exactly the same words with an entirely different mental picture helps me get something I've been having trouble with: think of wind going through a hula hoop. Wind velocity is going to play the role of the magnetic induction B, and the hula hoop is going to represent a wire loop. Wind velocity is a vector field: it varies both in magnitude (breezy here, calm over there) and in direction (it can come from the north or from the south, and it can swirl around). A magnetic field can't be more complicated than the wind because they're both just vector fields.

The magnetic flux through a wire loop is exactly analogous to the amount of air going through the hula hoop. It's the dot product of the field strength and the cross-sectional area of the loop. Turn the hula hoop edge-on to the wind and the flux drops to zero because no air is getting through it. Keep turning and the flux goes negative because now the wind is blowing the other way through the hoop.

Now Faraday's law says the force on charges in the wire loop depends on the *change* in flux through the loop. The charges don't care how strong the wind is; they only care how rapidly the amount of air going through the hoop changes. If you rotate the hoop very quickly in a light breeze, they'll get excited. If it's really windy, a very slow rotation will do the same thing. If it's gusty, you can leave the hoop stationary and the change in wind speed will do the trick.

These days, most hula hoops have beads in them. Let's pretend that if the beads move inside the hoop they will generate wind through the hoop. Let's say if they go clockwise, they generate a wind going in the direction a screw would go if the beads were turning it. Let's say if you increase the amount of air going in that same direction through the hoop, the beads will feel forced to move (so far we have Faraday's law) counter-clockwise (now we've added Lenz's law).

Tim Mooney



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