It is critical that the ions arriving at step 3 have the same velocity. To ensure this the ions pass through a velocity selector. This a region with a uniform electric and magnetic field.

The electric field comes from a set of parallel plates. The field is perpendicular to the velocity of the ions and exerts a force of qE on the ions.

The magnetic field is perpendicular to both the ion velocity and the electric field. The magnetic force has a magnitude qvB, and is opposite in direction to the electric force. The forces balance when:

qE = qvB

E = vB

v

=

E B

For ions with a larger speed the magnetic force exceeds the electric force and those ions are deflected out of the beam. Ions with a smaller speed are deflected out of the beam in the other direction, because for those ions the electric force is larger than the magnetic force.

In which direction is the magnetic field in the simulation above?

1. Into the screen
2. Out of the screen

What would we need to change if the charges were negative instead of positive? To have negative charges of the right speed pass through the velocity selector without being deflected we would have to ______________

1. reverse the direction of the electric field
2. reverse the direction of the magnetic field
3. reverse the direction of one field or the other
4. reverse the directions of both fields
5. none of the above, it would work fine just the way it is
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   Flipping the sign of the charge flips the direction of both forces, but what matters is that they will still balance if the charges are going at just the right speed. So, why change anything?