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1/ A proton enters a magnetic field in an evacuated chamber.Its initial velocity is at right angles to the magnetic field. Explain why the kinetic enrgy of the proton stays constant.

2/ There are 2 point charges separated by a distance of 4d in a vacuum. Point Z is situated on a line between q1 and q2 at a distance 'd' from q1.

a/ Write an expression in terms of q1 and d for the electric field at point Z due to charge q1.

b/ the strength of the electric field is equal to zero.

What is the ratio of the point charges in terms of q1:q2

Any help/guidance is greatly appreciated- especially for the second question. Thanks a lot