How much farad/kg does a metal act? - Page 1
Forum section: electronics / projects

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 12, 2022, 05:55:18 am
content: If you have sprang out of books you will find this question bizarre, it is normal that current should loop, a mass alone hanging on a circuit represents nothing even it is a military tank.When child, I watched my father repair tube TV sets testing the high voltage horizontal sweep by a screw driver, mid size large flat, approaching the plate to see the length of continues arc to the edge of the screw driver. Not slightest doubt, there is ac open loop current is passing from the metal and the plate charging and discharging the static potential. Bigger is the screw driver, longer is the arc, this why he used always the same.If you have a CRT TV, touch with a small screw driver any RGB pins of the tube and you will see that color in question gets delayed as if you added a capacitor.This phenomena can be seen on the damper high voltage capacitor, always the positive lead carrying more current than the negative, as it melts the solder and ends destoying itself by arcing.I looked on the net if I can find how much Coulomb does a kg of metal needs to be raised 1 static volts, Nothing. May be high voltage EEs know somthing about it.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 12, 2022, 06:03:59 am
content: I doesn't depend on the mass, it depends on the physical size, shape, and proximity to other objects.The self capacitance of a metal sphere in vacuum/air and far away (several diameters) from any other metal or dielectric has a capacitance of C = 4 * pi * r * eps0.For a soccer ball sized sphere, that comes out to be about 15 pF, so you need to add 15 pC to raise the potential 1 V relative to a distant ground plane.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 12, 2022, 06:22:46 am
content: Thank you for such precise reply, you should add this in Wikipedia.If I try to measure by low voltage, the output of a BF generator, I link the scope probe the output with 1M resistor and add a big mass and see the low pass cut frequency. Can such experiment give a reliable F/kg measure?

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 12, 2022, 07:48:57 am
content: I did the experiment, with 1Mohm and 625g of hammer head I got approximately 1.6khz -3db.This makes 100pf. Or 160pf/kg.Is this the right value?

author: strawberry
author_rank: Super Contributor
timestamp: October 12, 2022, 08:03:34 am
content: lead uranium better pF/kg

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2022, 08:29:29 am
content: The capacitance is identical whether it's a foil shell or solid metal. Mass of a bulk shell capacitance is a meaningless measurement.Tim

author: magic
author_rank: Super Contributor
timestamp: October 12, 2022, 09:04:47 am
content: You are not working in the vacuum and infinitely far from ground.Any conductor is also a capacitor to Earth and other things like mains cables. In fact, it's a capacitive voltage divide with respect to mains: there is some capacitance from live wires to the object, and from the object to ground. With a high input impedance amplifier it's possible to see how there is a small mains frequency AC voltage on any unshielded conductor. A big PITA, because usually this is not the thing you want to observe.Sorry for a mostly off-topic answerBTW, there is a "rule of thumb" that human body has about 100pF capacitance to ground and I checked with a capacitance meter (black terminal to earth, red terminal to me) that this number is about right. That's what enables those neon light voltage detectors to work.I guess a screwdriver is less than 100pF, then.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2022, 02:10:56 pm
content: Physics curiosity: what is theminimumF/kg of a single bound object?Something spherical to be sure, and something big to be sure: at given density, mass goes as r^3, while capacitance goes as r.The interesting part is that, as objects get heavier, eventually they get smaller, as gravity crushes against the compressibility of the material. Degenerate matter goes as r ~ m^(-1/3). Presumably, you'd want to find the densest, heaviest degenerate matter, just before it collapses into the next state (unless that next state is even denser). Well, that describes either a neutron star or a black hole, so I guess that's that.Incidentally, a black hole possesses charge, so it's still meaningful to speak of its capacitance.Black holes (non-rotating) have radius ~ mass, so it would seem the F/kg becomes fixed in this limiting case. Heh, neat. So it doesn't matter (at least to a first order) whether it's a microscopic or supermassive one.Tim

author: strawberry
author_rank: Super Contributor
timestamp: October 13, 2022, 02:41:23 am
content: some capacitance meters are simple AC resistance meterleakage will give increased reading

author: Terry Bites
author_rank: Super Contributor
timestamp: October 13, 2022, 10:19:15 am
content: Planet Earth has a capacitance of 710 μF. Go figure.