How to calculate resonant frequency of capacitor? / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

How to calculate resonant frequency of capacitor?

1 2 next

Move Thread

LAN_403

AuroraFlame

Fri Jul 06 2012, 05:18PM

Registered Member #5566 Joined: Tue Jul 03 2012, 10:55AM
Location:
Posts: 12

Firstly new member and first post here

I remember when I was a kid at school being told by one of the science teachers that the voltage difference between my head and feet whilst standing was over 100 volts. This has been known for hundreds of years and the voltage is about 100 volts per meter rising from the earthâ€™s surface.

This means if you got a helium balloon, coated it in aluminum foil, attached it to a fine copper wire and let it rise 100 meters into the air you would have ten thousand volts between the end of that wire and ground!
Obviously the amperage is miniscule but in the early days of electricity it was shown that electrostatic motors could be run from this atmospheric electricity.

Ok so here is what I want to do and I am looking for some help from you guys if you would be so kind. I want to experiment with charging a large capacitor using the Earth as the power supply. Its just for fun really but here comes the difficult part (difficult for me anyway) I want to tune that capacitor to match the resonant frequency of the earths energy field which is about 7.86 hertz.

Obviously if the capacitor is tuned to resonate with the earth it will capture the maximum amount of energy. I know that tesla tried this although he used a coil instead of a capacitor. I just want to make a simple two plate disc capacitor.

I donâ€™t know how to calculate the size of the capacitor that would have a resonant frequency of 7.86 hertz. I know that at such a low resonant frequency it must be large, perhaps over a meter diameter?

I am looking to use 2mm thick acetal sheet which has a dielectric strength of >50kv per mm therefore breakdown voltage is over 100kv. I will simply cut a single disc from this sheet and sandwich it between two aluminum discs. I will probably design the capacitor so it can be tuned once I get an idea of size range.

The experiment is obviously inexpensive and low-tech, the only difficulty being working out the size of the capacitor needed.
Technical data for acetal sheet can be found here

Thank you for any help!

Sulaiman

Fri Jul 06 2012, 10:24PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The 100 V/m is a 'static' electric field,
consider it as direct current

the Schumann resonances are alternating currents,
with very little power

on its own an 'ideal' capacitor has no resonant frequency
resonance requires inductance and capacitance,
the tether/line/wire from the balloon would have inductance (about 1 uH/m)
but to get resonance at around 8 Hz quite large values of capacitance and/or inductance are required ( F = 1/2.pi.sqrt(L.C))

your wire would probably leak the charge back into the air due to corona

flying a tethered balloon 100m up requires permission (CAA etc.)

you would require a very large balloon to collect a useful amount of power

What do you actually want to do?

AuroraFlame

Sat Jul 07 2012, 12:13PM

Registered Member #5566 Joined: Tue Jul 03 2012, 10:55AM
Location:
Posts: 12

That a parallel two plate capacitors donâ€™t have a resonant frequency goes against what I was taught whilst at collage. I actually remember the lecturer setting us the task of finding the resonant frequency of a capacitor as a test. We simply used a scope and signal generator connected to the capacitor, adjusted the frequency until max amplitude wave was observed and noted it down as the resonant frequency of the capacitor.

As for the Schuman frequency I always understood them to be pulsed dc. The ionosphere is positively charged and the earth is negatively charged like two plates of a capacitor. The sun charges the earth capacitor up with charged particles until a potential is reached when a lightning strike occurs. This process occurs rhythmically and thus the field around the earth builds and collapses rhythmically.

Antonio

Sat Jul 07 2012, 01:18PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

These concepts are quite confuse. A capacitor really resonates with its own inductance, this is a known problem in high-frequency circuits, and is measurable. But the resonance occurs much above the normal frequencies where the capacitor would be normally used.
The Schumann resonance occurs simply due to the time that an electrical signal takes to propagate around the Earth. Any lightning strike excites it, but it is a low-Q resonance, difficult to observe. Lightning does not occur due to charges received from the Sun, that would not affect anything on the ground due to the Faraday shielding effect of the atmosphere. Lightning occurs due to triboelectric effects in storm clouds.

Pinky's Brain

Sat Jul 07 2012, 03:18PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Antonio wrote ...
Lightning occurs due to triboelectric effects in storm clouds.

Presumably rain figures into it as well at some point, I've always imagined ground strike lightning like a Kelvin water dropper writ large.

Carl Pugh

Sat Jul 07 2012, 05:17PM

Registered Member #1064 Joined: Tue Oct 16 2007, 05:04PM
Location:
Posts: 42

Maybe I'm getting old and cautious, but:
They say quite a few people have been killed while flying kites using wires. (Benjamin Franklin's kite experiment)

I worked with an engineer who told the story that they had an antenna on the roof that was connected to a cable in the ceiling. He came in one morning and the cable was moving around. He grabbed the cable and was knocked across the room. The put a meter on the cable and measured 10,000 volt.

WHAT YOU ARE TRYING TO DO IS DANGEROUS. DON'T DO IT WITHOUT A LOT MORE RESEARCH.

Steve Conner

Sat Jul 07 2012, 06:59PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Carl Pugh wrote ...

WHAT YOU ARE TRYING TO DO IS DANGEROUS. DON'T DO IT WITHOUT A LOT MORE RESEARCH.

That would make an excellent stock reply to 90% of posts on the forum.

However it is true that since Ben Franklin first flew a kite in a thunderstorm, many people have been killed trying to replicate the experiment.

Static charges on radio antennas are a well known phenomenon. Every ham radio rig I've poked around in has had some sort of device for dissipating them to ground.

The energy in the Schumann modes comes from lightning strikes, of which there are about 120 per second averaged over the whole world. I believe the general consensus is that it's too small to be worth "harnessing".

AuroraFlame

Sun Jul 08 2012, 05:13PM

Registered Member #5566 Joined: Tue Jul 03 2012, 10:55AM
Location:
Posts: 12

I am aware that such experiments could be dangerous but only during bad weather, during fine sunny weather that would not be so.

Here are some electrostatic motors which are running off electricity collected from weather balloons.

"You can measure this voltage with an earth-field antennaâ€”a wire with a sharp point at the top to start a corona, or with a bit of radioactive material that ionizes the air in its immediate vicinity. Near the earth, voltage is proportional to altitude; on an average day you might measure 1,200 volts with a 10-meter antenna"

I'm not really interested in making motors however, I just want to see how fast a large capacitor would charge using this setup and how big the sparks can be got from this.

Carl Pugh

Mon Jul 09 2012, 04:58PM

Registered Member #1064 Joined: Tue Oct 16 2007, 05:04PM
Location:
Posts: 42

I am aware that such experiments could be dangerous but only during bad weather, during fine sunny weather that would not be so.

Would you bet your life that the preceding statement is correct? You are:

radiotech

Fri Jul 13 2012, 06:13AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

want to experiment with charging a large capacitor using the Earth as the power supply

Circuits have two ends. Could you sketch your earth-balloon circuit showing the flow paths
and indicate the EMF source.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob