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EMF Induction in Solenoid

  1. Oct 17, 2013 #1
    1. Assuming that both the windings of a transformer is wrapped around an iron core, how is an EMF induced in the secondary winding - isn't it necessary for flux to cut through the winding in order to induce an EMF? The flux cutting through the winding would apparently be very low..as most of it is contained inside the iron core.

    2. If a magnet approaches a solenoid along its horizontal axis, the flux cutting through the solenoid will increase. However, the motion is parallel to the direction of magnetic field. Will an emf be induced across the solenoid?
  2. jcsd
  3. Oct 17, 2013 #2
    The flux of a solenoid must complete a loop. Yes the flux inside the solenoid is largely contained in the iron core. However the flux also leaves the iron core and completes its loop outside the solenoid. When the field is reversed, it must pass through the windings inducing a current.

    Do you mean if a magnet approaches a solenoid in line with the coil and perhaps even enters the solenoid? If so, then consider the magnetic field a torus and you will see that the field must cross the windings. If the magnet approaches the solenoid perpendicular to the center of the coil, the flux lines will cross the windings but the field intersecting the coil on one side of the coil will cancel the field on the other side and very little current will be induced.
    Last edited: Oct 17, 2013
  4. Oct 17, 2013 #3
    Thanks, skeptic2.

    I didn't get the first point. As most of the flux from the primary remains inside the iron core while completing its loop and very little of it cuts the secondary winding (in an ideal transformer all the flux should remain inside the core), very little power should be transferred to the secondary winding - but in a transformer nearly all the power from the primary is transferred...?

    Magnet approaching a solenoid in line with the coil: i think i've understood. The magnetic field is torroidal and hence even when the magnet is far away, the field lines cutting through the upper portion of the solenoid coil would be tilted slightly upwards and those passing through the lower portion would have a slight downward direction. Is that what u meant to say?
  5. Oct 18, 2013 #4
    Not only must the flux complete a loop, the loop must contain the windings with the current that creates the flux. In other words the loop must go around the windings. The loop of the flux cannot be completely contained by a core that only passes through the center of the solenoid.
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