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I studied that when alternating current is passed through the coil ,the coil produce magnetic field (primary) as shown in figure,when any conducting material placed near to that field, eddy current is induced on that material and that eddy current produce secondary magnetic field on that conducting material,so

i build a coil ,which wounded over a plastic bobbin,which inductance value is 88mh, to produce secondary magnetic field on some metal when i place near to it .

does this coil produce eddy current only in AC,what happen if i supply DC? what should i do TO make strong secondary magnetic field ?

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Eddy (electrical) currents (or Foucault currents) are produced only when there are changes of Magnetic flux in time. So if you will supply the coil with DC, you will have only primary magnetic field.

It comes from Maxwell-Faraday equation: $\oint_{\delta\Sigma} {\mathbf{E}~d\mathbf{l}} = - \frac{d}{dt}~ \int\int_\Sigma \mathbf{B}~d\mathbf{S} $

Magnetic flux $ \Phi = \int\int_\Sigma \mathbf{B}~d\mathbf{S}=L~I$.

The stronger changes of magnetic flux are, the stronger resulting eddy (and therefore secondary magnetic fields) will be. Shortly, in order to make secondary magnetic field stronger you should increase either AC frequency, amplitude or inductance of the coil.

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  • $\begingroup$ what if i give pulsed DC like a square ware (0v and 5v),it will produce eddy current? @headcrabby $\endgroup$
    – masternone
    Commented Oct 7, 2015 at 8:56
  • $\begingroup$ if piece of wood is immersed in water and it is moisturized,will eddy current induce on that wood and produce secondary magnetic field? $\endgroup$
    – masternone
    Commented Oct 7, 2015 at 8:59
  • $\begingroup$ 1. Any changes from constant current will produce eddy current (the question is how strong it will be). For the pulse DC changes of voltage from 0V to 5V will also result in changes of current and so on. So yes, it will produce eddy current. $\endgroup$
    – headcrabby
    Commented Oct 7, 2015 at 10:00
  • $\begingroup$ 2. Well, theoretically, if you keep a piece of wood (it also depends on the type of wood) in the water for long time, it wont be insulator anymore, so you can have some eddy currents inside which will produce secondary magnetic fields. Can't say how strong will it be, to answer that you will need to check the resistivity of the given "wet" piece of wood. $\endgroup$
    – headcrabby
    Commented Oct 7, 2015 at 10:14

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