Question

Use the equation for magnetic force on a moving charge to derive the equation for magnetic force on a current carrying wire. Show your work.

Answer 1

Answer:

The formula comes from Lorentz force law which includes both the electric and magnetic field. If the electric field is zero, the force law for just the magnetic field is F=q(ν×B) . Here, F  is force and is a vector because the force acts in a direction.  q  is the charge of the particle.  v  is velocity and is a vector because the particle is moving in some direction.  B is the magnetic flux density.

We can derive an expression for the magnetic force on a current by taking a sum of the magnetic forces on individual charges. (The forces add because they are in the same direction.) The force on an individual charge moving at the drift velocity vd.  Since the magnitude of B is constant at every line element of the loop (circle) and it dot product with the line element is B dl everywhere, therefore

                                                  ∮B dl=μ0 I

                                                  B ∮dl=μ0 I

                                                  B 2πr=μ0 I

                                                   B=μ02πr Id=μ0/4π I dl×rr3

Since, r can be written as r=(rcosθ,rsinθ,z) and dl as dl=(dl,0,0) And now, if we take the cross product we would get

                                               dl×r=−z dlj^+rsinθk^

and therefore the magnitude of dB is equal to

dB=μ0/4π I |dl×r|/r3=μ0/4π I z2+r2sin2θ−−−−−−−−−−√dl/r3

Thus, magnetic field is depending on r,θ,z.

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