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Is a battery needed for electromagnetic induction?

  1. Jul 16, 2015 #1
    I am just scraping the surface of electromagnetic induction, and I have some questions. First, let me illustrate what I know. I know that in a simple circuit with a battery and a switch, when the switch is turned on, a current flows which produces a magnetic field of strength in proportion to the amps of the current. Now, the reverse scenario. I know that when you have a circuit and a magnet, if the magnet moves around near the circuit, a current is produced, and Faraday's law tells us how much voltage is produced. However, this is confusing to me. What is the point of Faraday's law in telling us how much voltage is produced when there is a battery connected to the circuit with a specified amount of voltage? Does Faraday's law imply that we don't need a battery since the magnet supplies the electromotive force necessary to start a current? Finally, if a battery IS needed for electromagnetic induction, then how does the magnet interact with the battery to elicit an emf that is stipulated by Faraday's law?
     
  2. jcsd
  3. Jul 16, 2015 #2
    You are confusing electromagnetic induction with the magnetic field created by currents. To create electromagnetic induction, you need a magnet that moves in front of a conductive loop (or many such loops, like a solenoid), or a loop that moves in front of a magnet. More generally, you need a magnetic field varying in front of a wire.
    Conversely, if a magnet is allowed to move in front of a conductive loop and a current is applied to the loop, then the magnet will move (and the same is true if the loop is allowed to move in front of a fixed magnet and the magnet is fixed). A battery connected to a circuit creates no electrical induction, but only a magnetic field (every current flowing through a conductive wire creates a magnetic field, and in a solenoid, the magnetic field of each loop add each to the other to give a strong field). Of course, if you replace the magnet by a solenoid connected to a battery (so it generates a magnetic field like a magnet), and if you move this solenoid in front of one or several loop (say another solenoid), then you create electrical induction in the second solenoid (and, to say the full truth, this current in the second solenoid generates back some electrical induction in the first solenoid, this is why "mutual induction" is considered in such cases, but this is more complicated).
     
    Last edited: Jul 16, 2015
  4. Jul 16, 2015 #3
    The induced electric field is entirely separate from that which is created by the battery. They are added together. The battery drives a current through the circuit. If you move a magnet near the circuit, then you will see an additional "EMF" in the circuit that can contribute to the total current (constructively or destructively, depending on the geometry of the magnet, N/S pole orientation, etc.) They are two completely separate sources of the electric field.
     
  5. Jul 16, 2015 #4
    Thanks, that was precisely what I was asking. So does this mean that there can be a circuit with no battery, solely powered by electromagnetic induction?
     
  6. Jul 16, 2015 #5

    Dale

    Staff: Mentor

    Yes, most mains power is of that type.
     
  7. Jul 16, 2015 #6
    Yes, the electromagnetic device which generates current for the circuit is known as a dynamo, (or generator).
    A common example of such a circuit would be a dynamo on a bicycle which powers the lights.
     
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