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# Edexcel Jan 13 - magnet and coil watch

1. I have uploaded the Q and A.
Why is the region of 0 EMF so small. Surely it should be longer because the field is uniform for a longer section in the middle?
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2. (Original post by jsmith6131)
I have uploaded the Q and A.
Why is the region of 0 EMF so small. Surely it should be longer because the field is uniform for a longer section in the middle?
It's only a "sketch".
This means the graph shows the general principle involved, but doesn't need to be exactly right in every detail.
The principle is
-induced emf increases then decreases as the coil enters the field
-induced emf is zero when the coil is totally in the uniform field
-induced emf again as coil leaves the field with the curve being the reverse of the initial one.

I agree the central zero seems a bit short, but having said that, the coil is only completely in the uniform field for a short time. The uniform field is the section with straight parallel lines (yellow) and the coil must be completely in it. (the blue part of its path)

If you wanted to be really picky you could argue that, if the diagram shows the extent of the field, there is no field at the two regions on the left and right where I have placed the coil. These points are between A and the field and B and the field. You could argue that there should be no induced emf as the coil passes A or B until it actually enters the field.
In reality, of course, the field would reach beyond the limits shown and stretch as far as A and B.

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