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## Homework Statement

Two narrow beams of electrons with velocities

**v**and

**-v**are injected into an evacuated chamber along the length

*l*. Each beam moves with constant speed and carries a current

*I*. The tendency for the beams to deflect each other through their mutual interaction is compensated by a magnetic flux density

*B*perpendicular to the plane containing the two electron beams so that they travel in parallel straight lines a distance

*d*apart (

*d << l*). Show that the ratio

*f*of the magnetic to the electric force on each of the beams due to their mutual interaction is vε0μ0

## Homework Equations

For two parallel currents I1 and I2 the force on a length

*l*of I2 is:

F= (μ0*I1*I2*

*l*)/2πd

## The Attempt at a Solution

Using the above equation and substituting I1 = I and I2 = -I:

F= -(μ0*I^2*

*l*)/2πd

I don't know what to do now, what is the electric force?

All I know is Coloumb's law but I thought that only applied to static charges?