Here's the project: I'm building a capacitor by hand to essentially measure the permittivity constant. I am doing so by building a capacitor of known dimensions, setting up an RC circuit and plotting the cap. discharge. With the known resistance, the cap. should discharge according to the model
V(t) = V0e^-t/RC
where V0 = the battery voltage (i'm using a 1.5V Dcell), R = the resistance of the circuit (i'm using megaohm resistors - between 3-22MΩ - i have 10 of each), and C = the capacitance = KEA/d where K = dielectric constant (3.85 for paper), A = plate surface area, d = distance between the plates (im separating the plates with paper but may try other materials) and E = the permittivity of free space.
I'm measuring v(t) with a program called logger pro. It basically does exactly what i need; it plots voltage over time.
So here's my problem. I can get everything to work perfectly with a pre-built capacitor. The circuit works, the resistors work as predicted, etc.
But when I hook my one of my many attempts at a home-made resistor (one in which i can actually know the dimensions), I start seeing really weird things.
I hook up the cap. to the battery, and the voltage across the cap goes up to about 1.5V so the cap is storing charge. But as soon as I disconnect it from the battery, the voltage drops instantly. Then rises. Sometimes it rises to a voltage greater than 1.5V! It goes through these really random curves. Keep in mind, at this point, the circuit isnt even completed! its just a cap with a voltage meter and open wires!
The actual circuit design has resistors on a switch so that I can charge the cap, flip the switch, and once the switch is flipped, current flows through the resistors and i can track the discharge. Whenever i try that, again i see really random patterns. As soon as i flip the switch, the voltage across the cap drops into the negatives, then starts gaining voltage, and even gains positive voltage!
I've tried a few different cap designs, but i think I'm clearly missing something. My current model has two small strips of copper tape across a piece of paper. I made sure that the copper sheets are not in contact, and since there is a voltage across the cap, it must work.
I've also tried aluminum foil sheets over paper; similar results. but again, the curves and voltage fluctuations are different every time.
Can anyone think of solutions? Perhaps a better cap design, or a better circuit design?
thank you guys so much! again, i dont know if this really belongs on this forum but i appreciate any help u guys can offer!!