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Thread: Faraday's law

  1. #1
    Join Date
    Jun 2011

    Faraday's law

    I just wanted to confirm: for the same voltage, if I reduce the number of turns then the magnetic field strength increases and vice-versa?
    Last edited by philippeb8; 2021-May-21 at 04:38 AM.

  2. #2
    Join Date
    Aug 2002
    No, it does not work that way, you cannot change this equation around, the voltage induced is created by the time change of the magnetic field in the coil, the Delta B / Delta t in the equation is actually a derivative dB/dt.
    Just putting a voltage V to a coil with N windings will drive a current I through it, with I = V/R with R the resistance.
    The magnetic field of a coil carrying a current I is given by:

    B = mu0 * I * N / L

    where L is the length of the coil (along the axis)
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  3. #3
    Join Date
    Apr 2011
    Norfolk UK and some of me is in Northern France
    Quote Originally Posted by philippeb8 View Post
    I just wanted to confirm: for the same voltage, if I reduce the number of turns then the magnetic field strength increases and vice-versa?
    You have to think of the magnetic field as an energy store that takes time to establish. When you put a fixed voltage onto a coil, the current is not instant, it creates a magnet. If you measure resistance it takes time to get to the value, although often that is too small to notice.

    Once the field is established a direct current flows. Now if you break the circuit, the energy will cause a so called back emf which will spark at the switch. The spark shows a bigger voltage is generated. The more turns you add, the more time it will take to establish the field and the more energy is stored.

    In alternating voltages and currents you deliberately reverse the field and that can transfer to a second coil, which is how transformers work. If the second coil has twice the number of turns, you get twice the voltage ac out.

    The ingoing ac volts then “sees” the back emf opposing the current flow. That is the impedance which usually is many times greater than the resistance in practical coils.

    In one special case you do get the effect you mention, in magnetising coils to creat permanent magnets. The impedance can limit the field strength and a small number, like one turn , is used. Then a very high voltage pulse causes a high current and field to magnetise material. If you try to use more turns you need more volts and that causes insulation breakdown. Probably more than you wanted to know!
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  4. #4
    Join Date
    Jun 2011
    Awesome, thank you very much!

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