Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Q: A/C Electromagnet

  1. Sep 15, 2012 #1
    I am looking at electromagnets and discovered an article using an A/C version.
    What I am surprised by is that there is no mention of how the A/C is being used.
    Wouldn't someone creating this version of an electromagnet kill themselves?
    There's no mention whether the author uses any kind of lamp or anything to control the power running into this thing.
    If this thing was wired into an outlet, wouldn't a fuse at the main's blow?
  2. jcsd
  3. Sep 15, 2012 #2
    You forgot the part on that page which says "Connect up a variac to the coil so you can control the voltage and current going to it."

    The variac is also called auto-transformer. They usually have a fuse which would blow if you try to short circuit it.
  4. Sep 15, 2012 #3
    I did read that wrong. I don't understand what that is yet, but I'm gonna check wikipedia and see.

    Why can a single coil like the one primary coil of a transformer have voltage applied to it without being a short circuit? Does the winding of wire create resistance? A transformer doesn't have any resistance attached to its primary coil does it?
  5. Sep 15, 2012 #4
    Coils are inductors whether in transformers, electromagnets or something else.

    When you apply DC voltage to an inductor, it tries to create a magnetic field which creates a force to resist the rise of current. The higher the frequency of breaking on and off the circuit with DC voltage, the longer it takes for the current to rise. if the frequency is high enough, the current in inductor shall never be able to rise to become a short circuit.

    For a standard inductor with L Henry inductance, connected across AC of frequency f Hz, the effective resistance is 2*PI*f*L ohms. This is also called reactance.
  6. Sep 16, 2012 #5


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    the primary winding will have a DC resistance maybe a few 10's of ohms or 100's of ohms

    But in an AC circuit we dont so much look at that resistance value ... rather because its a coil ( an inductor) it has an impedance to the AC current flowing through it

    DC circuit we call it resistance
    AC circuit we call it impedance

    its that impedance that stops a relatively low wire resistance from becoming a short circuit to the AC current

    Do a google search on Impedance ( AC impedance) and have some fun learning :)

  7. Sep 16, 2012 #6
    That is absolutely amazing. Thanks for the replies.

    How many wraps/turns of a wire constitutes a coil that can resist(DC)/impede(AC) a short circuit?
    A 12 gauge wire can be connected to a 120v outlet and as long as there are enough turns, we have a transformer primary without short? And if I create another coil of wiring that's brought closer to it, with more turns, a higher voltage will be induced across it, becomes the secondary?
  8. Oct 31, 2012 #7
    Yea, I guess so. But the coils of the wire should be insulated from each other, thats important.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook