Two charged particles are projected into a magnetic field that is perpendicular to their initial velocities. If the charges are deflected in opposite directions, what does this tell you about them? (Ignore the interaction between these two particles.)

The two charged particles are opposite charge (one positive, the other negative)

Explanation:

This can be explained through Fleming's left hand rule. As we know in Fleming's LH rule

Thumb: represent motion of the charge

Fore finger: direction of magnetic field

Centre finger: direction of current

In this case, the direction of magnetic field is in one way (perpendicular to direction of the particles motion)

If the charge is positive, equivalently it's direction is the same as the direction of current. If the charge is negative, it's direction is opposite the direction of current.

Using the fleming left hand rule, we'll see that as we change the direction of current without changing the direction of field, the direction of motion will change towards the opposite way.

For example, let's say the particles is moving away from you and the magnetic field is from left to right:

If the charge is positive (hence direction of current is away from you), the particles will move downward.

If the charge is negative (hence direction of current is toward you), the particles will move upward.

With these evidence, no doubt that the two particles have different charges.