Smallest possible length of copper wire, where I can measure capacitance via multimeter

capacitancecapacitorelectromagnetismpower supply

I am a new highschool physics instructor and I want to show to my students the basic concept inside capacitors and how they work.

I plan on showing them 2 simple copper wires side by side , supplied with battery, constructed on a breadboard, and measuring via multimeter (which can measure mA), we can measure capacitance.

But I am afraid it would fail, the school does not have that much supply though, I do not have my own multimeter but planning to buy one if somebody here at Physics stackexchange can confirm a setup that I can measure capacitance and tell me the appropriate parameters.

I got this idea from the formula, which states that at some point, at the correct Area and distance to each other, 2 copper wires supplied with voltages can give me capacitance.

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So, if you guys have any spare time and device, can you please experiment on what length 2 copper wires, side by side, is enough to measure capacitance, I do not also have any idea if 9V battery is enough, maybe I can series battery to reach higher voltages.

Please help. Thanks!

Setup:
enter image description here

Hope somebody can try. Thanks!

Best Answer

  1. You can't easily measure capacitance with a multimeter unless it has a capacitance meter built in.
  2. Capacitance in parallel wires will be tiny because A (area) is so small and d (distance) is too great. This will make it very difficult to show.
  3. You won't be able to make any dynamic readings with a digital meter. The numbers will jump around for a fraction of a second but you will not be any wiser afterwards. With an analog (moving coil) meter you may see the needle flick in proportion to the current or voltage.

As MarkU has suggested, two large sheets of aluminium foil separated by a layer of cling-film / food wrap may be more helpful. I've never tried this but here are some experiments that come to mind.

  1. Measure the are of the alu-foil plate. Place it on the table.
  2. Cover with cling film. If you can measure the film thickness and look up the dielectric constant for PVC you may be able to calculate the capacitance value.
  3. Apply the second layer of foil and press the whole sandwich together by weighing down with books.
  4. Somehow, make an electrical connection to each plate.
  5. Charge up the plates using a battery or power supply.
  6. Connect a multimeter set to volts and measure the voltage across the plates.
  7. Disconnect the power supply and watch the voltage drop as the capacitor discharges through the multimeter. Try and estimate how long it takes to discharge by 63%. This is the RC time constant where R is the input resistance of your multimeter and C is your home-made capacitor. Typically digital meters have a 10 MΩ.

If all that works you can repeat the experiment with additional layers of food-wrap between the foil. This should reduce the capacitor value as the 'd' term is increasing.

Has anyone else tried this? Any other ideas?

Let us know how you get on.

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