Magnetic Field of a Moving Charge
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Phys 4, Section 2                                                         Feb 20, 2002



Magnetic Field of a Moving Charge

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

μo ≡ 4π 10-7 Ns2/C2 is called the permeability of free space. The constant εo that is used in electric field calculations is called the permittivity of free space. Note that εoμo = 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?

FE = kq2/r2

To get FB 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 FB to FE:



Last modified on February 27, 2002