Phys 4, Section 2                                                         Feb 20, 2002

Experiments show that the magnetic field of of moving charge can be expressed as:

μ_o ≡ 4π × 10^-7 N·s²/C² is called the permeability of free space. The constant ε_o that is used in electric field calculations is called the permittivity of free space. Note that ε_oμ_o = 1/c².

Two protons with a vertical displacement of r between them move in the x-y plane parallel to the x-axis at the same speed v (small compared to c). When they are both at x = 0, what is the ratio of the electric/magnetic forces between them?

F_E = kq²/r²

To get F_B acting on top charge, first find B caused by bottom charge:

So, the force this field exerts on the top charge is:

Comparing the ratio of F_B to F_E:

Last modified on February 27, 2002