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Primary coil is used to overcome the back e.m.f

  1. Nov 1, 2004 #1
    I was told that P.d. supplied to the primary coil is used to overcome the back e.m.f. so that the the flux will keep changing, why is it so?
    What happened if the P.d. supplied is different to the back e.m.f.?

  2. jcsd
  3. Nov 2, 2004 #2
    Help is greatly appreciated, any opinoin is fine.

    Nb. : P.d. is potential difference, e.m.f. is electromotive force.
  4. Nov 2, 2004 #3
    You must provide an A.C. supply to a transformer to get a changing flux. Otherwise you only get an output when you turn it on or off!
  5. Nov 2, 2004 #4
    Yes, the source is from A.C. supply, and the question is why must the value be equal to the back e.m.f.?

    and what does this sentence means? Can u explain it more clearly?

  6. Nov 3, 2004 #5
    The p.d. of the supply doesn't 'have to be' equal to the back emf - the supply pd creates the back emf. If they were equal at all times, no current would flow!

    The sentence above means that unless the supply p.d. is varying, then you won't get a fluctuating flux in the core. You can not generate a pd in the secondary coil without a fluctuating flux.

    If you turn on or off a DC supply, the sudden change in current generates a flux in the core. When the DC supply remains connected, current flow is constant, so you get no flux change and no output pd in the secondary coil.
  7. Nov 3, 2004 #6
    Here is the paragraph given by my lecturer,
    " P.d. supplied to the primary coil is used to overcome the back e.m.f. (equivalent to action and reaction). Hence, the p.d. sipplied to the primary, V = back e.m.f. "

    Here is another paragraph adapted from college physics,
    "If the resistance of the primary coil is neglected, this e.m.f. must be equal in magnitude to the external voltage applied to it."

    and from understanding physics,
    "the changing flux through the primary coil induces a back e.m.f. in the coil, the back e.m.f. opposses the applied p.d. So the applied p.d. must equal the back e.m.f. to keep the flux changing."

    So, it seems like the 2nd one is more accurate whereby the 1st and third one will give u misconception, since it would imply no current will actually flow. And the 2nd one will only have current passsing through when there is resistance.
    And, the p.d. supplied is not exactly the same as the back e.m.f., it is only approximately.

    Am i right?
  8. Nov 3, 2004 #7
    Hmm... It seems that some things are unclear here. As you apply a varying pd to the PRIMARY coil, this generates a changing flux in the Iron core. This changing flux generates a back EMF in the PRIMARY Coil. This emf does oppose the inducing pd as described by Lenz's law. In a perfect world, this back emf would be equal to the applied pd, but as there is no such thing as a perfect (ie 100% efficient) transformer, it is not quite the case.

    I don't quite see where the statement "So the applied p.d. must equal the back e.m.f. to keep the flux changing" comes from. If you have a varying supply pd you MUST have a varying flux! Maybe I have missed something here though??
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