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Homework Help: Describe the EMF induced by a magnetic field

  1. Jul 16, 2015 #1
    1. The problem statement, all variables and given/known data
    A coil of insulated wire is wound many times around an iron bar. The wire is connected to a battery and a contact switch. The contact switch is alternately flipped closed and left closed, so the wire carries a current through it, and then flipped open and left open, so that the current stops. Describe what magnetic fields are created, if those magnetic fields in turn induce an emf, and whether the induced emf is in the direction of the original current or opposite to it. Justify your answer using Lenz's law and the conditions which can cause a magnetic field to
    induce an emf. You may draw a diagram if it helps you to visualize the apparatus.

    2. Relevant equations

    3. The attempt at a solution

    This problem is really confusing. I understand that the apparatus is a simple circuit with a solenoid, and when turned on, an electromagnet is produced from the solenoid. However, I don't understand what the question is asking. It asks if the magnetic fields induce an emf. But why would the magnetic field created by a circuit with a battery induce an emf on itself? I thought that only external magnets not associated with the circuit could induce an emf on it. Also, I am not sure how to determine the direction of the field in relation to the current. In addition, I am not sure how Lenz's law applies because since I thought that that had to do with an external magnetic field affecting the current and the subsequent field created by the current. If I could get some pointers I would really appreciate it.
  2. jcsd
  3. Jul 16, 2015 #2


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    I'm a little rusty on this stuff, but isn't the question hinting at inductance, which creates an 'inertia' in the flow of electric current. So when the switch is first flicked on, the inductance from the coils gives a magnetic field that resists the increase in current (ie the emf from the mag field is in the opposite direction of the current), and when the switch is flicked off, the inductance gives an emf that resists the decrease (ie the mag-induced emf is in the same direction as the current).

    In any case, a quick glance at Maxwell's laws should answer it.

    Edit to Add:
    That's kind of true, but needs rewording to something like
    "only external magnets not associated with an infinitesimally small part of the circuit can induce an emf on that part"
    When you have a straight wire, this means there's no emf against the increasing current. But when you coil the wire, you are bringing the mag fields from other parts of the circuit close to this part of the circuit, so they can produce an emf on it.
  4. Jul 19, 2015 #3

    rude man

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    The Faraday relation emf = - N dφ/dt applies to ANY magnetic flux φ cutting the coil, whether internally generated as in the case of your current-interrupted solenoid, or externally applied from an external electromagnet, or both.

    You need to read up on Lenz's law, what it states and how it applies in this case.

    BTW every potential drop voltage across a (pure) inductor is an emf. Reason is the voltage is generated from a differing source of energy (the magnetic field). In contrast, the voltage across a capacitor is not an emf, just a voltage..
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