Hi Mario, In physics, they have another definition of capacitance which is related to our EE concept but not exactly the same. This physics concept is this: consider that I bring electrons in from infinite distance toward an object and deposit them on the surface of that object. If the amount of energy required per electron is E, then at that moment they consider that the voltage of the object with respect to any point at infinite distance is V=3DE/q where E is the energy and q is the charge of one electron (if E is = in Joules and q is in Coulombs then V is in Volts). As you add more and more negative charge to the object, it will repel electrons and more work will be required to deposit them onto the surface of the object, so V will go up. Using the familiar equation Q=3DCV, where Q is the total charge on the object and V is the Voltage right now, then C would be considered the capacitance of the object. In this way, even single conductor objects are considered to have a capacitance. The Earth is considered to have a continuous conductive surface for the purpose of this thought experiment. That's probably not too far off considering how much of its surface is salt water. Using this definition, the capacitance of a sphere is about 1 pF per centimeter of radius. So, since the radius of the Earth is 6.4x10^6 meters (6.4x10^8 cm), its capacitance is 6.4x10^(8-12)=3D6.4x10^-4 Farads or 640 microfarads, so you are very close. To make use of this capacitance, you would have to have current flow between the Earth and something not in electrical contact with it. If you have a metal plate held above the surface of the Earth with insulating poles, then the capacitance between this plate and the Earth would involve the Earth to infinity capacitance, but it gets difficult to calculate it out since charges brought from infinity now encounter the E fields from both the Earth and the plate. Experience suggests that we can usually just consider the local geometry between the plate and the Earth and compute the capacitance as a two-conductor capacitor :) Sean On Sun, Dec 14, 2014 at 9:37 AM, Electron wrote: > > At 09.40 2014.12.14, Sean Breheny wrote: > >I didn't read the entire stackexchange posting but I suspect that the > >threshold and subsequent decreasing voltage has something to do with str= ay > >capacitance in the measuring set-up and also possibly capacitance betwe= en > >the setup and earth. Your voltmeter/electrometer has some tiny internal > >capacitance plus capacitance between its leads. This is in parallel with > >the test capacitor. As the voltage increases, some of the charge in the > >test capacitor is transferred to the parasitic capacitance. When the tes= t > >capacitance begins to approach the value of the parasitic capacitance, > then > >the voltage will no longer be able to increase much because increasing > >voltage would require more charge to be transferred to the parasitic > >capacitance. > > By the way, I've read somewhere that the Earth has about 400 uF > capacitance. > > How could I use the Earth capacitance in an experiment? > > When I was a kid I tried to see if current flowed when I put a battery po= le > on Earth, but I could measure no current. > > Yes of course to see a current there must be flow back, but I thought tha= t > the Earth was so almighty and huge that my battery's electrons would try = to > travel towards that route, and before they came back to me saying (there'= s > no flow of current possible) they had to reach the other end of the world= .. > :) > > Have a nice sunday, > Mario > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .