A negatively charged particle moves across a page from left to right. A magnetic field is turned on, and the particle is initially deflected toward the bottom of the page. In what direction is the magnetic field pointing?

We can answer this question by using Fleming's left hand rule, which allows to determine the direction of the force exerted by a magnetic field on a charged particle.

In particular:

- The index finger corresponds to the direction of the magnetic field

- The middle finger corresponds to the direction of the motion of the particle

- The thumb gives the direction of the force (the direction must be reversed if the charge is negative)

In this problem, we have:

- The particle is moving from left to right (middle finger)

- The force is toward the bottom (thumb), because when the magnetic field is turned on, the particle is deflected towards the bottom. However, since the charge is negative, we have to reverse the direction: so, we put our thumb upward

- Therefore, the direction of the middle finger gives us the direction of the magnetic field: so, into the page.