1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Shouldn't a wire (coil) just store energy and not give it away?

  1. Aug 8, 2013 #1
    Hello, I was thinking about induced currents in a wire(coil). For example when a current from an external source passes through a wire - the changing magnetic field induces a current opposing the current applied (back emf) like a Newton's 3 law and nothing would ever pass through a wire, but the good part is that the magnetic field produces back emf only when the field is changing, which means that sooner or later there will be no change and no back emf. No back emf - current can flow. Which brings me to the things happening to the wire when the current is stopped: current is stopped, magnetic field collapses and produces current in the opposite direction, but then i think - shouldn't the current produced sort of oppose the collapse of the magnetic field? Or if it can't oppose the collapse of the magnetic field, maybe it can by being created by magnetic field create it's own magnetic field, which would create current, which would oppose the creation of the current creating it and so on, on and on creating back emf, which creates back emf and then a back emf creates again back emf and so on and nothing happens.... The first part has some logic, at least for me, because when a current is applied by external source sooner or later the will be no change in magnetic field- hence no back emf and a flow of current, but in the case of a wire, which has energy induced in it in the form of the magnetic field, shouldn't the magnetic field just fight it self and nothing would happen, because of no external current source- any ideas?
     
  2. jcsd
  3. Aug 8, 2013 #2
    You need to get your logic sorted out.
    A changing magnetic field produces an induced emf, (not a current), the direction of the induced emf opposes the change in the magnetic field (not the current applied).
    Newtons 3rd law has nothing to do with induced emfs.
     
  4. Aug 8, 2013 #3

    Drakkith

    User Avatar

    Staff: Mentor

    This is incorrect. When the current begins to stop, the changing magnetic field generates an EMF that tries to keep the current flowing. Thus energy is stored in the magnetic field and delivered to the wire as EMF that causes current to flow after the external power source is removed.
     
  5. Aug 8, 2013 #4
    OK, back to text book:D
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Shouldn't a wire (coil) just store energy and not give it away?
Loading...