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## Physically Grounded

This analogy needs to be taken with a grain of salt but may give an idea of what a ground is in a circuit.

Check it out further here: http://hyperphysics.phy-astr.gsu.edu...ic/watcir.html
 Recognitions: Gold Member Science Advisor Pressure too is a differential measurement. Mechanical engineers generally use local atmospheric pressure as their reference. Pressure measurements that are referred to local atmosphere are "gage pressure" , psig, but usually the g suffix is omitted as in that picture. Pressure measurements that are referred to absolute zero are called "absolute pressure", psia. Absolute pressure differs from gage pressure by local atmospheric presure, of course. An absolute pressure gage has an evacuated chamber for reference. In a barometer it's the empty space in the glass tube above the mercury. Observe that in Dlgoff's example from http://hyperphysics.phy-astr.gsu.edu...ic/watcir.html :: Provided the reservoir in the picture is open to atmosphere it has an absolute pressure of ~14.7 psia. Well, that is if it's someplace like Miami which is at sea level. The gage showing 50 psi is relative to atmosphere. Were it an absolute gage it'd show 64.7. If instead it's in the mountains west of Denver whewre atmospheric pressure is more like 12 psia, the same absolute pressure gage would show 62 psia.. We poor electicals don't know earth's absolute potential so we assign it value zero. That may be obvious to most folks but i struggled with the concept. Sorry if i bored you, and dont think i was in any way "talking down"... old jim
 Thanks a lot Jim, your very helpful information is greatly appreciated. I will have to struggle myself with the concept too.