Endercraft_O t1_izglur6 wrote on December 8, 2022 at 11:22 PM

The 710uF is the capacitance with a conductor at infinite distance, for any circuit you build on Earth the actual capacitance will be much much larger.

Much of electrical engineering is about simplifying circuit calculations. Earth ground could be any source/sink of charge sufficiently large as to be effectively infinite in the context of your circuit. And Earth is effectively infinitely large compared to most electronics at human scales.

Imagine every circuit connected to earth ground as one terminal of a capacitor and the Earth as the other terminal. Each circuit could have a different amount of current flowing to earth ground with a non zero net current. But because the capacitance is so big, V=Q/C, any amount of human scale current we provide will have negligible effects on the voltage of earth ground.

So you are right, Earth is not a infinite source/sink but we treat it as one because it is large enough to not impact our calculations most of the times.

MiffedMouse t1_izhfpb3 wrote on December 9, 2022 at 3:13 AM

To add to this, the earth is also frequently not a large enough capacitor, especially for higher power applications. Large radio stations and radar stations often have a metal mesh placed on or under the ground nearby to make the “earth” a better capacitor. Lightning discharge spikes often extend deep into the foundations of buildings, or sometimes have a spike that extends below the foundations of the building, to get a better ground connection.

So the earth is often assumed to be infinite during the initial design stage, and if it turns out not to be “infinite” enough we can supplement the “earth” so that our design still works.