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

Help with understanding equation for force Between two current carrying coils

  1. Sep 19, 2005 #1


    User Avatar


    I am currently involved in building a linear motor which is based around the coil gun principle. I am having trouble understanding the equation for force used to desribe the coilgun's action.

    Fx = I1*I2*dM/dX

    Fy = I1*I2*dM/dX

    Here we are considering two coils of wire, with an rapidly
    changing current allowed to flow through one of the coils (The active coil)which in turn induces a current in the nearby 'passive' coil (via Lenz Law). So I1 is the active current and I2 is the passive current.

    So bascially u end up with two magnetic poles with same polarity and there is a repulsion action. If the vertical movement is constrained, you are left with horizontal thrust which is what the linear motor makes use of.

    I can relate this force equation to physics textbooks' description of force betweentwo parallel current carrying conductors which uses the Lorentz Force equation (F = I*L*B) and Magnetic Field near straight wire (B = mu*I/(2*PI*R) to arrive at F = (mu*I1*I2)/(2*PI*D).

    I don't how to explain the dM/dX part.

    Can someone help me get from the F = I*L*B description to the
    F = I*dM/dX description. Thanks

  2. jcsd
  3. Sep 19, 2005 #2


    User Avatar

    oh..the M in dM/dX is the mutual inductance..(a coefficient to describe the linking flux between the two coils..coupling). I can see how 'good coupling' can help induce large currents however, i don't understand how its rate of change contributes to force..
  4. Sep 21, 2005 #3


    User Avatar

    *Fy = I1*I2*dM/dY
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook