Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Back EMF (electromagnetic induction)

  1. Apr 3, 2008 #1
    1)When the current through the coil of a motor flows it cases the coil to spin.
    2)The spinning motion causes the wires in the coil to cut through the magnetic field.
    3)The current induced (back EMF) in the wires by this spinning motion opposes the change that causes it. (Lenz's Law)
    4)Current supplied against this EMF requires electrical energy.
    5)It is this energy that is converted to the kinetic energy of the motor.

    I don't understand 4) and 5). Where does this electrical energy come from? I thought the motor's kinetic energy comes the the initial electrical current supplied to it...

  2. jcsd
  3. Apr 3, 2008 #2
    I am a bit confused too but I think that:
    Whenever a current carrying coil is put in a magnetic field, a torque acts on it. This is responsible for its rotational motion and forms the working principle of galvanometers.
    But this rotational motion causes the induction of opposing emf and this tends to resist the rotation. So to keep up the same rotation rate more energy needs to be supplied. So it is this energy which gives the motor its kinetic energy.
  4. Apr 3, 2008 #3
    The energy comes from the Battery or the power supply supplying the motor. The back emf generated is very small compared to the source emf.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook