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Question
An electron beam projected along the positive x-axis deflects along the positive y-axis. If this deflection is caused by a magnetic field, what is the direction of the field? Can we conclude that the field is parallel to the z-axis?
Solution
As the particle gets deflected towards the positive y-axis, we can conclude that force is acting on the particle along the positive y-axis. Now, as the electron is moving along the positive x-axis, the current can be assumed to be flowing along the negative x-axis. Applying Fleming's left-hand rule, we find that the thumb points in the direction of force, i.e. the positive y-axis and the middle finger points in the direction of current, i.e. negative x-axis. Consequently, the forefinger gives us the direction of magnetic field, i.e. out of the plane of the paper or in the positive z-direction. So, we can conclude that the magnetic field is pointing along the positive z-axis.
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