Physics 7B Lab Days are really interesting

1 minute read

In this lab we explored the motion of a charged particle in a uniform magnetic field. First we heated up a cathode held at low voltage and set up an anode held at high voltage a few centimeters away. This causes electrons to accelerate from the cathode to the anode. most electrons are stopped by the anode but a small hole allows a stream to escape.

Since Human eyes cannot see electrons, we encase the whole apparatus in a bulb filled with helium. When the gas molecules are struck by the electrons, they emit a radiant blue color, shown in the picture. The blue is not the electrons themselves but an illuminated path of where they were.

To produce a uniform magnetic field, we place 2 large circular coils of wire (Heimholtz coils) around the bulb. Without going into too much detail, this creates a uniform field through the middle of the bulb. When turned on, the magnetic field created affects the path of the electrons by the "left hand rule", that is, if you point your fingers of your left hand in the direction of the initial path of the electrons, and align the top of your hand with the magnetic field, your perpendicular thumb will be pointing in the direction of the force. Since the electrons are shooting out from the bottom right of the pic, and the magnetic field is pointed somewhat towards the camera, the force felt by the electrons is up.

The electron path then follows typical Newtonian physics (F=ma=m*v^2/r) and forms a circle. What is the point? We were able to see how magnetic fields and current running through solenoids can affect trajectories of charged particles. Also... it just looks cool.

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