A positive charge moving up enters a magnetic field pointing out of the screen. What is the direction of the magnetic force on the charge?

The answer is not "left".

Right

Explanation:

When a moving charge enters a region with a magnetic field, the charge experiences a force of magnitude

where

q is the charge

v is the velocity of the particle

B is the strength of the magnetic field

And this force is perpendicular to the direction of motion of the particle. In particular, the direction of the force can be determined by using the right-hand rule:

- Direction of the index finger: direction of the velocity of the particle

- Direction of the middle finger: direction of the magnetic field

- Direction of the thumb: direction of the force

This is valid for a positive charge: for a negative charge, the direction of the thumb must be reversed.

In  this problem we have a positive charge, and we have:

- Index finger: upward (velocity is upward)

- Middle finger: out of the screen (magnetic field)

- Thumb: to the right

Therefore, the direction of the force is to the right.