Measurement of e/m
Prelab Exercises
Phy 123 L
Cal Poly Pomona - Physics Department
Electrons all have the same charge (
), and the same
mass (
). Today we will measure the ratio of these two values, (
) by observing electrons as
they travel through
a gas that illuminates their trajectory. Lorentz Force
We're going to shoot electrons with a controlled velocity (v)
into a region with a uniform magnetic
field (B)as shown
below. 
The X's indicates the vector quantity is going away from you.
The dots indicate that the vector quantity is going towards you.
When a charged object with charge (q) moves through a magnetic field (B), at a velocity (v), it experiences a magnetic force (F) given by the vector equation,
.(The boldfaces type of the F, v and B
indicate that these quantities have a vector character, that is..they
have a specific direction, sometimes, textbooks write vectors with
boldface type, so the same vector equation can be written as....F=q v X B.)
This equation governs the way that charged objects behave in a magnetic field.
- the faster the charge moves (v), the greater
the force
(F).
- the greater the magnitude (B), the greater the force (F).
- q can be either positive or negative. For electrons, q is negative.
- The funny-looking cross means that the force is.......perpendicular to both the velocity
AND the magnetic field.
and this.
In this lab, we will shoot electrons into a uniform magnetic field and observe this circle.
Helmholtz Coils
Making the uniform magnetic field requires a pair coiled wires called
Helholtz coils. It isn't hard to make a magnetic field, all you need is to run a current down a wire. The field goes around the wire as seen in this demonstration.
By placing two coils parallel to eachother, we can create a regtion of magnetic field between them, where the field is uniform.
Try to do this with this little demonstration.