Setup page for PHYS142 Coulomb's Law lab.
Day One
Apparatus
The figure below shows all of the apparatus needed for a single station for both days of the lab. Not all apparatus should be set out at the student stations however. The masses for the whole room should be should be in the general apparatus area over on the bench by the blackboard. This mass collection should contain a variety of masses generally found in the lab including things like mass clips, air cart masses, etc. The idea is to allow the students to figure out what they need in order to accomplish various tasks during the experiment.
- There should be calipers (the good ones with the dial indicator) and a couple micrometers in the middle cabinet..
- The apparatus and the HV supply should be wired up.
- Make sure that the three micrometer screws are set to provide a gap of anywhere from a few mm to a cm between the capacitor plates. They don't have to be precisely set, just make sure they are not screwed all the way in or out, and that they are in a reasonable starting position.
- Note that the acrylic cross bar with the micrometer insert is NOT used for the first day and should NOT be set out yet.
- The feet on the scales should be adjusted to be extended about 50% of their full range. Otherwise the micrometers will not be able to move the capacitor plates close enough together.
Additionally, there should be two micrometers, for measuring the diameter of the spacer ball, and two scales (needed more for day 2 than day 1, setup for the room.
Other than micrometers, all the apparatus should be in cabinet 1 (closest to the door).
Testing
The main thing to test is that the two plates can be brought close enough together that they produce a force of about 40g at 1500V before sparking begins. Typically they can be brought close enough to reach 50g of force. But 40g is enough to see the effect.
This requires that both the top and bottom plates be flat and free of nicks and dings where the electric field will concentrate. The locations where the screws tap into the bottom plate are a main source of premature arcing, as are the edges of the plates when they get nicked from drops. Often times a good buffing with a scotchbrite pad will improve a performance. Other times I have to take a LARGE flat file with very fine tooth from the shop to carefully smooth the bottom of the plates.
Check the condition of the grounding strap between the bottom plate and the BNC connector. These straps are frequent failure points as they get twisted and flexed in use. Typically if they fail it is at the connection to the plate. The design intent is to provide a reliable ground connection for the 2keV potential voltage WITHOUT applying downward force on the plate.
Note. I am not suggesting this is a good solution, it is how a predecessor designed the apparatus. In fact I recommend finding a new solution for this connection if this lab is to continue. The current design is not robust and the braided wire breaks too easily. The main concern is that whatever serves to connect the bottom plate to ground not put downward or upward pressure on the capacitor place as this will bias the measurement of the force between the plates.
Also check the stability of the output of the HV supply when it is turned up to its max. Arcing between the plates will cause the output voltage to drop of course. But sometimes the output of a supply will slowly drift up and down over the course of a minute, such supplies should be replaced.
Day Two
Apparatus
- Replace the acrylic top bar that holds the top plate of the capacitor with the Al cross bar with micrometer insert as shown in the above photo.
- The feet on the scales should be adjusted to be extended about 50% of their full range. Otherwise the micrometers will not be able to move the capacitor plates close enough together.
- The bottom plate with the grounding strap may or may not be needed, so keep it near the setup.
- Each station should have five 10g masses. Other 10g and 20g masses can be put in the parts area near the blackboard. If not out on the bench by the board, they should be located in Cabinet 1.
Testing
Replace the acrylic bar and top plate with the acrylic bar and micrometer insert.
Reset the three micrometer screws on the base of the apparatus so that there is enough space to attach the piece with the micrometer insert.
Note that the micrometer insert should be set so that it is not fully racked out in order to fit. Often times students will adjust this piece past the point where the threads run out. In these cases I have sometimes had to resort to using a pair of pliers to loosen them.