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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.

enter image description here

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!

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    \$\begingroup\$ You might also consider using two sheets of FR4 copper clad board (google it) as this would give you electrodes of a measurable area. \$\endgroup\$
    – MarkU
    Commented Dec 4, 2015 at 8:16
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    \$\begingroup\$ If the DMM has a built-in capacitance meter function, then you definitely should not apply an external DC source; this would interfere with the measurement. And if you plan to measure the decaying current as the capacitor charges to a battery voltage, the DMM needs to be in series rather than in parallel. Though because of the time dependency, an oscilloscope would be more appropriate. As drawn I'd be concerned that your proposed setup will destroy the DMM(or blow its internal fuse). There's nothing to limit the current through the DMM's shunt, except (hopefully) the internal fuse. \$\endgroup\$
    – MarkU
    Commented Dec 4, 2015 at 8:20
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    \$\begingroup\$ Another low-cost way to demonstrate building a capacitor, is use two strips of aluminum foil, covered by two strips of plastic wrap, then rolled up -- that's essentially the construction of an aluminum electrolytic capacitor. You can get a lot of area into a very compact space with this arrangement. \$\endgroup\$
    – MarkU
    Commented Dec 4, 2015 at 8:23
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    \$\begingroup\$ Battery, DMM, and breadboard is good enough to demonstrate conductor/insulator material properties, Ohm's law, and Kirchoff's current and voltage laws (i.e. DC circuits analysis), as well as some digital logic; but since capacitors and inductors have a time-dependent response, you really need a function generator (like an audio oscillator) and an oscilloscope to get a good look at what's happening. \$\endgroup\$
    – MarkU
    Commented Dec 4, 2015 at 8:29
  • \$\begingroup\$ I suggest you take a look at the top rated capacitor+measurement questions: electronics.stackexchange.com/questions/tagged/… \$\endgroup\$
    – MarkU
    Commented Dec 4, 2015 at 8:34

1 Answer 1

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  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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  • \$\begingroup\$ +1 - actually, during my uni Electricy & Magnetism classes, the "capacitance show & tell" was very similar: two metal (alu, as I recall) disks with known area (about 1/2 m radius each), set at a known distance from each other (in order of cms); lab PSU connected to terminals at each plate for a given time with A-meter, current measured over time, then PSU disconnected, some Rload connected, a V-meter connected to the terminals instead, voltage measured over time, plots were done, on plots is was clearly visible that plates had a capacitance with exp/log V/A behaviour, QED. \$\endgroup\$
    – user20088
    Commented Dec 4, 2015 at 13:55
  • \$\begingroup\$ A pointer - Connect the DMM to the foil before you charge up the cap. Then start the experiment by disconnecting the power supply, moving around as little as possible. Your body will tend to affect local electric fields, and with the high impedance of the DMM any movement may affect the reading. Depending on local circumstances this may not be appreciable, but keep it in mind. \$\endgroup\$
    – WhatRoughBeast
    Commented Dec 4, 2015 at 21:39

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