Answer:
a) DOWN direction, b) directed INTO THE SCREEN, c) F = 0
Explanation:
The direction of the force is
for electric force
F = q E
where we assume a positive test charge, for which the force has the direction of the electric field.
For a magnetic field
in this case the direction of the force is given by the right hand rule.
For a positive test charge, the thumb points in the direction of velocity, the other fingers extended in the direction of the magnetic field, and the palm gives the direction of force for a positive charge.
F = q v x B
Let us apply these considerations to our case.
a) negative charge moving to the left
in a magnetic field points away from the screen
In this case the thumb goes to the left, the fingers extended outwards and the palm points upwards, but since the charge is negative the force has a DOWN direction.
b) negative charge moves to the left
in electric field it points off the screen.
The outside is in the direction of the electric field and since the charge is negative, the force is directed INTO THE SCREEN
c) positive charge moves down
in magnetic field points up
in this case the velocity and the field have the same direction so the vector product of them is zero
F = q v B sin 0
F = 0
