In a certain velocity selector consisting of perpendicular electric and magnetic fields, the charged particles move toward the east, and the magnetic field is directed to the north. what direction should the electric field point?

Down.

Explanation:

In the given question, the charged particle is moving towards the east direction. Suppose east direction towards the positive x-axis.

And the magnetic field directed towards to the north direction. Suppose north direction towards the positive y-axis.

Therefore force of magnetic field can be calculate by the formula,

Therefore force of the magnetic field is in positive z-axis means in an upward direction.

Now the force on the electric field must be in the opposite direction means it is in a downward direction.

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