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Simplifying Lenz's Law

  1. Dec 11, 2009 #1
    I am having great difficulty grasping the concept of Lenz's law. As far as I am unware it is derived as:

    "any current driven by an induced e.m.f opposes the change causing it"

    I do not really understand what this means - help would be greatly appreciated :)
  2. jcsd
  3. Dec 11, 2009 #2
    The law of Lenz can be described as follows:

    Induced quantities, such as current (mmf), voltage (emf), flux, & torque, are always oriented in a direction opposite to the original quantities.

    A motor is started. It's current draw is large when at a standstill. Once the rotor spins, mmf/emf are induced into the stator. This induced current opposes the original current. Hence the net current draw decreases.

    If I pedal a bicycle type generator under no load, and I control the speed to 300 rpm, for example. I then load the generator with 50 watts. The added load current has a magnetic field which produces a an additional torque. This torque counters my pedaling torque.

    Otherwise, a motor or generator would create its own supply of energy. Induced quantities, electrical as well as mechanical, always act in the *opposite direction* to the corresponding original quantities.

    Is this helpful?

  4. Dec 11, 2009 #3
    This Law is often demonstrated by moving a magnet towards the end of a long coil of wire. (Solenoid)
    The result is that an emf is generated/induced in the coil.
    If a current is able to flow in the coil, it flows in such a direction that the magnetic field it generates opposes the one that produced it- the one due to the magnet.
    In practical terms, if you moved a N pole of a magnet towards the end of the coil, the current generated at the end of the coil would produce a N pole there - the N pole would be opposing the one producing it. (Like poles repel)
    If you Google Lenz's Law demo you will find a better explanation of this than is possible in words, using some animations. There's also some stuff on YouTube.
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