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Homework Help: How to explain alternating emf?

  1. Oct 19, 2012 #1
    1. The problem statement, all variables and given/known data
    Explaining alternating current is simple as the direction of current will change accordingly. For example: a magnet is pushed through a solenoid. Explain why is there an induced alternating current.

    As it is pushed forward, the solenoid experiences an increase in magnetic flux linkage. Thus by faradays law there is an induced emf across the solenoid. In order to oppose the increase in magnetic field, by Lenz law there is an induced current flowing towards the right. In the middle of the solenoid, there is no change in magnetic flux linkage so there is no induced emf and hence current. When the magnet leaves the solenod, the solenoid experiences a decrease in magnetic field. Hence, in order to oppose the decrease in magnetic field by Lenz law there is an induced current flowing to the left. Thus there is an alternating current when the magnet is pushed through.

    However, in a question they showed a turbine with magnets on them which turn around. On the side is a solenoid. And they asked us why would there be an alternating emf.

    2. Relevant equations


    3. The attempt at a solution
    I'm not sure how to explain this. Do I start with Lenz law? Thanks for the help!
  2. jcsd
  3. Oct 19, 2012 #2
    What are the directions of the magnetic dipole moment, the axis of rotation of the magnet, and the a (cylindrical?) solenoid?
  4. Oct 19, 2012 #3
    Hi I'm unable to draw this picture now. But in a case where there is a magnetic pushed into a solenoid, and I am asked why is there an alternating emf? Then how would I phrase this?
  5. Oct 19, 2012 #4
    Your second paragraph did a pretty good job. I'm not sure what the problem is. As for the turbine, what I think is happening is there are multiple solenoids. The magnet's dipole moment is perpendicular to the axis of rotation, and the outer solenoids are oriented so that their axes are radial from the axis of the magnet's rotation.
  6. Oct 19, 2012 #5
    Hi but didn't I explain alternating current and not emf?
  7. Oct 19, 2012 #6
    I see now. Yes, you did explain AC, not EMF. Were you taught Faraday's Law of Induction? It's the equivalent to EMF that Lenz' Law is to current. Are you trying to prove Faraday's Law?
  8. Oct 20, 2012 #7
    I learned that when there is a change in magnetic flux linkage then there is an emf but I'm not sure how to explain an alternating emf since the emf doesn't flow like in current so there isn't a direction to it. Thanks for the help!
  9. Oct 20, 2012 #8
    EMF does have a direction. If you're going around the curve one way, it might have a positive EMF, but if you're going around the curve in the opposite way, it will have a negative EMF.
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