# AC generator and DC motor problem

1. Sep 6, 2012

### sgstudent

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
none.
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.