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Homework Help: AC generator and DC motor problem

  1. Sep 6, 2012 #1
    1. The problem statement, all variables and given/known data
    For an AC generator, my teacher told us that when turning it, you will slowly manage to turn it at a constant speed only because of the induced current causing a force in the other direction to the turning motion. Why is this so?

    For a DC motor, my teacher told us that after it turns for a while, it will slow down because of the induced current flowing in the opposite direction. As a result the net current (can I use this term or what should I use instead?) is smaller resulting in a smaller force. Why is this so?
    2. Relevant equations
    3. The attempt at a solution
    For the AC generator: when you first turn it, by fleming's right hand rule you experience a net current. But by Flemings left hand rule you also experience a force in the opposite direction. But as you first turn it, it has a net moment so the coil has a net acceleration which means the speed increases at a increasing rate. But by fleming's left hand rule, since the motion keeps on increasing so the opposing force also keeps increasing. this causes the net moment to drop as the acceleration drops. The acceleration will drop to 0 as the opposing force will keep increasing until there is no net moment and it rotates with a constant speed. (this is in a perfect case where there is no friction) when there is friction, using the same principles initially the net moment is the person turning it and the friction it experiences. So by Flemings left hand rule, it experiences a opposing force which increases as you rotate with a increasing speed. However, this time due to the friction you start to turn with constant speed earlier as there is that pre existing friction to hinder the motion.

    I'm not very sure about this so I hope someone can give me a better explanation or a link for it.

    For the DC motor:If you apply a fixed current, the net moment is also fixed as the magnetic field is also kept constant. So by fleming's right hand rule, it will experience a opposing current. But at the first instance, the opposing current is smaller than the current by the battery. And as there is a net moment, so it will speed up which also causes the induced current to increase as well. However, the initial current does not increase (only the speed of rotation) so the net current will slowly drop which causes the net moment to drop. The net moment will drop until there is 0 net moment at which the induced current will be the same as the current supplied. So at this point of time, there is no more moment causing it turn with constant speed but it has no current. (this is when there is no friction) but if there is friction then using the same principles, initially the moment is the force minus the friction and it has a net moment and as the induced current increases, it will eventually reach no net moment. But this time since there is friction this time, the induced current does not need to be equal to the battery's current for a no net moment to occur. So because of this when there is 0 net moment, there is still a current flowing through the coil as it rotates with constant speed.

    Then again this is really confusing to put together so I'm not sure how to explain this. Hopefully some of you geniuses can explain these to me or provide a link for a explanation. Thanks for the help guys! :£
    Last edited: Sep 6, 2012
  2. jcsd
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