An electron traveling horizontally (in the +x direction) enters a region where a uniform magnetic field, B, is directed upward (in the +y direction). What is the direction of the force exerted on the electron once it has entered the field?

The direction of the electric force on the electron must be down.

Explanation:

The electric force is:

Where

q = charge

E = electric field

Like the charge q is positive (+x direction), the direction of the electric force is the same that the electric field.

Since the charge of the electron is negative, the direction of the electric force must be opposite to the direction of the field, so the direction of the electric force on the electron must be down.

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