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Equal and opposite forces with electricity

  1. May 5, 2014 #1
    I have a question concerning the generation of electricity and equal and opposite forces.
    I've been having a hard time describing it. Sorry if I don't get my point across.
    If I start off with using a generator to generate charge in capacitors, I have the equal and opposite forces being the work done by the generator and the charge in the capacitors. Then I take a system such as a rail gun where current is the direct cause of the forward momentum.
    Then I had the question, if I have saved up work to balance an equal and opposite force with my charge in the capacitor. Wouldn't losing charge be the balancing factor in the recoil of a rail gun? but then I found this source and it states.

    "Recoil forces in EM railguns appear wherever the
    breech of the railgun is closed electromagnetically.
    This means recoil forces may appear on power supply
    leads, switches, or power supply components them-
    selves. Careful attention is required on the part of
    the railgun designer to control the location of the
    recoil loading and provide means for sustaining the
    loads. Careless design can result in undesirable
    forces being applied to the projectile armature as
    well. On the other hand a thorough understanding of
    where and how recoil forces are generated can be used
    to good advantage in some EM gun systems. In closing
    we offer aspiring railgun designers one bit of advice
    originally offered to HPG machine designers bys by Hr. B.
    0. Lamme in 1906, 'You can't fool the flux.' "

    But now I am stuck with what is the balancing force of the discharge of the capacitors? Or if I removed the capacitors and had a powerful enough generator to fire a rail gun, what would be the balancing force of the generator? like I am putting force in and getting a projectile out. before it was the static force between the capacitors increasing.

    I guess I'm stuck thinking that the energy dissipated by the capacitors would equal the kinetic energy of the projectile plus the heat lost and the force on the rails. then I just don't see any magnetic fields crossing current to cause any recoil.

    Or does the projectile really just push back on the field and that pushing back is translated back into the system? does anyone know the equations for that if this is indeed what happens?
  2. jcsd
  3. May 5, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    You appear to be confusing energy and force descriptions.
    To understand the 3rd law force pairs in electrical systems, you need to describe the effects purely in terms of electrical and other forces.
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