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Force that holds two wires together when current flowing between them.

  1. Nov 10, 2008 #1
    I'm wondering what causes the force is that holds wires together when their is a current flowing through them, and how to calculate it for different currents. As an example, when I am hooking my car battery up and you touch the jumper cable to the battery to connect the terminals, it kinda "sticks" and takes a little extra force to pull off the electrode. Other examples I've heard but not experiences is that a high-voltage line shorted to ground through a person will not be easy to pull away to break the short - it will take a significant force or different (lower R) connection to ground before the person can be broken away from the wire.

    I would guess this force arises from either the magnetic field generated by the current through the wire, _or_ perhaps from a coulomb force/interaction that occurs when attempting to cause a short (the voltage immediately forming at the short is like a parallel plate capacitor where the plates are pulled towards each other because of the different voltage on each plate). What which one is it, and how could it be measured for a wire?

    I'm curious because, in a superconducting wire, I'm wondering if this force could be used to strengthen the wire if very high currents are applied through the wire.
  2. jcsd
  3. Nov 11, 2008 #2

    Please be very careful with car batteries. Their voltage is safe, but their short circuit current is above 1000 A. This explodes about any wire or metal sheet. A friend of mines almost lost his finger as the current flew through his wedding ring: no ring more, no skin more, no muscles more... Said friend is an electrical engineer, among the very best I know.

    Contacts can stick after a high current because they're welded over a small surface. To check that this is the correct explanation, stop the current by another means and observe whether the contact still sticks.

    Currents contract muscles, so victims sometimes grip conductors. No magnetic effect here.

    To avoid this, always touch life conductors first with the back of you hand instead of the palm, and choose your arm's position so that folding brutally the elbow removes you from the conductor without knocking the surroundings. Also wear insulating shoes, and work with one hand - keep the other in you pocket. Thankful thoughts to my old professor, it helped several times.

    If a victim shorts a high-voltage line (like 200kV) first-aid workers are told to keep away from the victim (in other words: let the victim die) and prevent people from getting close to the victim, as potential drops within the soil would kill persons simply getting close. So yes, you need to short-circuit the line before removing the victim's ashes.
  4. Nov 11, 2008 #3
    Always a good reminder to never short-circuit a car battery. That was not what I had done, but was in fact referring to when I complete the charging circuit when jump starting a different car. The current in the case I've experienced is much smaller, not enough to "weld" the wires in my opinion, although I wouldn't know for sure without testing - perhaps the next time I jump start a car. I'm pretty sure once the current stops, the holding/sticking force also stops though, since I can only remember the force when breaking the circuit, not when removing the other jumpers.

    You don't sound to convinced that this effect is even due to electric or magnetic forces?.... but I'm pretty certain I've seen it elsewhere too and the welding explanation doesn't seem plausible. Even when working in the lab with wires at relatively low to medium currents, that they "stick" together a little and breaking the connection takes a bit more force. Perhaps I should check again though, would be quite easy with a battery and small wires...

    I searched the web and couldn't find mention of it (put two wires and force into a search and you just get tons of homework problems about two parallel wires, which is not at all what I'm interested in here).

    Thanks for your thoughts though Enthalpy,
    Last edited: Nov 11, 2008
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