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

Rechargeable electromagnet?

  1. Jul 8, 2009 #1
    I was wondering if it is possible to make a rechargeable electromagnet like this:


    Please bear with me as I have poor drawing skills. :grin:

    It consists of a capacitor, 9V battery, and a electromagnet.

    Will it work?
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Jul 9, 2009 #2
    Hi there,

    I don't seem to really grab what you want to do??? An electromagnet is "rechargeable" by definition, meaning that you can turn it on and off as you wish.

    If this is what you mean, then your drawing could work, using the right material as a magnet.

  4. Jul 9, 2009 #3
    What you have drawn is the classical experimentation of electromagnetism(with the capacitor making no difference except a 90 degree phase change).
    It works with the capacitor if the souce is an ac source.
    However,"rechargable electromagnet" doesn't have a meaning as the electromagnet does not store charges.
    Last edited by a moderator: Apr 24, 2017
  5. Jul 9, 2009 #4
    Ah. I probably should have put more thought in to it. Thanks for the help though!
  6. Jul 9, 2009 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    As drawn, not only is this not a "recharger" it simply will not work. You have an electromagnet as long as current flows. In your circuit current flows until the capacitor is charged up. Depending on the size of the cap this will happen in a few milliseconds.

    You show it with a battery, batteries are DC sources, the magnet would work with an AC source.
  7. Jul 9, 2009 #6
    If you charge up an electromagnet with energy (volts and amps), and then short the two electromagnet current leads together, the electromagnet will discharge with a time constant called "L over R" or L/R, where L is the inductance of the electromagnet, and R is the coil resistance. The decay of the current is I = I0 exp[-R t/L], where t is time. In superconducting coils like in MRI "machines", the coil resistance is zero, so the L/R time constant is infinite. The real problem is keeping the coil cold, like at liquid helium temperatures.
    Last edited: Jul 9, 2009
  8. Jul 9, 2009 #7
    Is the circle with a "C" inside supposed to be a capacitor? With an "A" it's used to represent a current meter.
  9. Jul 9, 2009 #8


    User Avatar
    Science Advisor

    No, it is not. Despite the popular conception, superconductors do not have exactly zero resistance. It's merely very, very close to zero. MRI superconducting magnets run in persistent mode need to be refreshed every few months due to this and other losses within the system.
  10. Jul 9, 2009 #9

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Superconductors have exactly zero resistance. Exactly. Superconducting cable, however, does not. There are imperfections in any cable - splices, voids, etc. that can give it picoohms or even nanoohms of resistance. Just ask the LHC guys what happens when you get a few unexpected nanoohms of resistance.
  11. Jul 9, 2009 #10


    User Avatar
    Science Advisor

    It's never been conclusively proven to be exactly zero as far as I'm aware (for practical superconductors, of course--anything's possible in the theoretical realm); and I have no reason to believe that it is.
  12. Jul 9, 2009 #11
    My idea is that I complete the circuit until charge is stored onto the capacitor, then switch the wires are connect them to the charged capacitor. That's what I meant by "rechargeable,"
    although judging from the answers here, I don't think it will work. :frown:
  13. Jul 10, 2009 #12

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    So you make an assertion, and your only justification is "it's never been conclusively proven".

    That's not the way things work in science. If you make an assertion, it's your responsibility to prove it.

    A supercurrent is a superfluid. It flows without energy loss. Measurements of time constants of physical superconductors show that they will flow for many tens of thousands of years. Indeed, the limitations on this come from things like finite size and temperature effects.
  14. Jul 10, 2009 #13


    User Avatar
    Science Advisor

    I did not make the original assertion. I'm merely disputing it. If Bob would like to back up that assertion, he's more than welcome to.

    FYI: things are never "proven" in science. Someone who lambasts others for their alleged lack of scientific understanding should know this.
  15. Jul 10, 2009 #14
    Every time this thread is boldened there is nothing useful in it
  16. Jul 10, 2009 #15
    In electrical circuits (excluding mechanical systems, etc.), there are two known ways of storing energy.
    1. Storing energy in dielectrics (electrical charge in capacitors). The stored energy is (1/2)C V2. In theory, the energy can be stored indefinitely. This requires that the conduction band in the dielectric be thermally (exp[-eV/kT]) far away from the valence band.
    2. Storing energy in magnetic fields (electrical current). the stored energy is (1/2) L I2 . In theory, these are perfect storage devices (no energy leakage). In actually, only type II superconductors (probably elemental, not alloy) at low fields (under the lower critical current Hc1) approach being perfect superconductors. The contact resistance of the contacts for shorting the inductance when it is "charged" is one of the main ways the inductance loses energy.
    Last edited: Jul 10, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Rechargeable electromagnet?
  1. Electromagnetism ! (Replies: 45)

  2. Recharging at Moon (Replies: 10)