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Made an electromagnet, curious about how this works exactly.

  1. Mar 30, 2012 #1
    Hi, I'm a sophmore in high school, and I haven't taken the Physics course yet. So I'm a bit green. Anyway, I was playing around with an electromagnet I made with a nail and some 1mm wire, and I've got a few questions about the magnetism it generates:

    -First, why does that iron core need to be in there? There's no current flowing through it, and from what I understand a coil already generates a magnetic field on it's own. But I can't do much with the coil itself, I've got to have that iron core.

    -If I have a fixed energy supply, what makes a stronger magnetic through the coil: high voltage, low current or low voltage, high current?

    -If I understand how transformers work correctly, they're simply two coils held together where one induces a current flow in another. So shouldn't a wire held up next to a powered coil show some sort of voltage? But if I do this, my multimeter can't detect any power in that second wire. So I don't understand this right. Do transformers require an iron core too?
  2. jcsd
  3. Mar 31, 2012 #2
    Replied inside the quote.

    I'll give you some tips.
    H - magnetic field intensity vector
    N - turns
    I - current

    If you have a closed core with average length C, then you will have the following relation, excluding flux dispersion and a uniform magnetic field within the core:
    |H|*C = N*I

    B = u*H, considering a linear model.
    u - core magnetic permeability
    B - magnetic flux density vector

    F = |B|*S, considering B uniform and perpendicular to the core cross section area.
    F - Flux
    S - core cross section area

    V = d(N*F)/dt
    V = voltage in a coil with N turns within a flux F.
    Last edited: Mar 31, 2012
  4. Mar 31, 2012 #3
    So transformers only work with AC current? what do you do if you have to voltage-change in a DC-only circuit?

    uhh... what's that? :redface:
  5. Mar 31, 2012 #4
    Yes, that's true. Increasing or decreasing voltage in a DC circuit is not so simple. You can use a voltage regulator which dissipates with resistors the additional voltage (very inefficient). On the other hand, if you want to increase the voltage, a more complex system such as a switching mode power supply should be used.

    About flux: http://en.wikipedia.org/wiki/Flux
  6. Mar 31, 2012 #5
    ok, thanks. So flux is flow through a surface area? Like water through a certain area filter?

    Wait, how does magnetism "flow"? It isn't a type of waves/radiation, is it?:confused:
  7. Mar 31, 2012 #6

    Magnetic fields and electric fields are related by the following Maxwell Equations:



    E - Electric field intensity vector
    H - Magnetic field intensity vector
    B - Magnetic flux density vector
    Jf - Free current density

    Considering a linear relation between H and B in a space free of charge, you obtain the following:


    That's an homogeneous wave equation. If you decompose the vector in three coordinates and solve the equations, you will obtain three wave functions.
    Therefore, you will have a wave only when E or H varies in time.
    Last edited: Mar 31, 2012
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