Spark image

Electron in electric and magnetic fields

Question:

An electron travels through a region of space with no acceleration
Which one of the statements is the best conclusion?
(a) Both E & B must be zero
(b) E must be zero, but B might be non-zero in the region
(c) E & B might BOTH be Non-Zero, but they must be mutually perpendicular
(d) B Must be Zero, but E might be Non-Zero
(e) E & B might Both be Non-Zero , but they must point in opposite directions

Answer

For the magnetic field to exert no force on the electron it must act in the same direction as that in which the electron is moving. The electric field will always exert a force on the electron no matter which direction it point in.

Let's consider the possible answers one by one:

(a) they don't both have to be. They could be arranged so as to cancel each other
(b) once again they don't have to be. They could cancel.
(c) this would be possible as E and B could cancel each other. The two fields would be perpendicular as the magnetic force will act at right angles to the path of the moving electron
(d) this would not be possible as E is non zero and B is zero. B could not cancel E.
(e) this would not be possible as E is non zero and B is not at right angles to E.

So the best answer is (c)
 

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