A.C. generator -> why does the current reverse?

[ellieee]

Homework Statement

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Relevant Equations

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why does the current reverse when the coil is at this position in A.C generator ?

 

[BvU]

Because the number of field lines (the magnetic flux) moves away from a maximum (or towards it, depending on which way it's rotating).
So the time derivative changed sign (or will change, depending on which way it's rotating) .

A better way to look at this is to use the Lorentz force: The velocity with which the pink and blue conductors move through the B field changes sign.

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[kuruman]

Here is an alternate explanation in terms of Lenz's law.

According to Lenz's law, the induced current will run in a direction that will oppose the proposed change in magnetic flux. Look at the picture. The direction of the current is such that it tends to reduce the flux lines through the loop. This means that the proposed change is to increase the field lines through the loop. For that to happen, the loop must be turning counterclockwise.

Having established the sense of rotation, note that the current in the top (pink) wire is into the screen. After the loop has rotated by 180^o, the current in the top wire must be into the screen for the same reasons as before. But the top wire now is the blue wire. If the current in the blue wire did not reverse direction somewhere in between (specifically when the field lines are in the plane of the loop), the current in the top wire would be out of the loop and that would violate Lenz's law.