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**1. The problem statement, all variables and given/known data**

In a cathodic ray tube, the canon launches a circular electron beam on the screen. The beam has a 0.22 mm diameter, the electrons have a kinetic energy of 25 keV and 5.6*10^14 electrons reach the screen each second. Find the magnetic field produced by the beam on a point located at a 1.5mm distance of the beam axis.

The answer is 12 nT

**2. Relevant equations**

I know that the magnetic field of a circular arc is

B = (u*i*angle)/(4*Pi*distance) but in this case I don't have an angle so im pretty sure this is not the correct formula

Also we could integrate using the biot savart rule but we don't have the limits of integration.

**3. The attempt at a solution**

I have found the current by using the fact that 5.6*10^14 electrons reach the screen each second. By converting electrons to coulombs, I get 8.97 * 10^-5 Amperes. Then I am stuck, I have no idea how to continue

thank you