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Is the term loop loosely defined in Faraday's law?

  1. Apr 23, 2014 #1
    Is the term "loop" loosely defined in Faraday's law?

    A voltage will be induced in a loop of wire when the magnetic field flux through the loop changes

    If you're reading this, you probably know what happens when you hook up the leads from an oscilloscope to a microphone. Nothing, lol. There needs to be relative motion between the voicecoil and the permanent magnet for you to view any voltage fluctuations.

    And here is the origin of my confusion,

    Disconnect the leads. The voicecoil is now open ended. In other words, it is NO LONGER a true loop. Will a voltage still be induced?

    Also, voltage is a measure between two points or ends in the case of a voice coil. How am I supposed to find the "ends" in a circle?
  2. jcsd
  3. Apr 24, 2014 #2

    Simon Bridge

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    You mean - does the loop have to be closed?
  4. Apr 24, 2014 #3
    Without the loop closed there will be a voltage and current induced in the loop. By the way, the scope is a very high input impedance and having it connected is very close to having the loop open. With the loop closed, the voltage around the loop gets smaller and the current gets bigger. The voltage around the loop is defined for a specific path. If we start at point A, go around the loop once and end on the same point, there is a definite voltage along that path.

  5. Apr 24, 2014 #4


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    Using a moving coil "dynamic" microphone, you should get an output if you talk into it and this may be visible on the oscilloscope trace if you have enough gain in the oscilloscope.

    These microphones are now fairly rare and you might be using an "electret" microphone which requires a source of DC voltage to make it work.
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