# Earthing a conductor

This may be a basic question, but I have never understood it completely: why is an earthed conductor always at zero potential? I would say it is because theoretically one can suck up charge from the earth without doing work, hence it is at zero potential (all earth charge is at infinity), not sure if this makes sense.

The electrical potential has a gauge freedom because we can arbitrarily set the zero anywhere we want. This is because we can only ever measure differences in the potential and not the absolute value of the potential.

Conductors that are earthed are all at the same potential (because they are earthed to the same planet) so it is usually convenient to choose this as our zero. Any potentials that we measure are then the difference from the potential of the earth.

• To be fair "earthed are all at the same potential" is not really true. Lightening strikes often cause huge transient voltages on one earth ground and not another. This can effect things like serial connections that don't run the "ground" line. – Sam Sep 26 '16 at 17:22
• Agreed with @Sam. It's in fact a serious mistake, for example, to run a mains line to an out-building with its own grounding rod/plate and neglect to disconnect the neutral from the sub-panel earth bus. Lightning strikes can cause such large potential differences between the earth at the separate buildings that it can induce very large currents in the neutral conductor servicing the secondary panel. And ground loop noise...even within a building sharing a common earth that can be nightmare for copper communication systems, especially in an electrically noisy environment. – J... Sep 26 '16 at 21:05
• There does not even have to be a lightning. Some years ago we ran some data network between separate houses, maybe 30 m apart, and IIRC there was something like 90V on the shielding, even in normal weather. – Jan Hudec Sep 27 '16 at 12:11

Note also that "Earth" or "Ground" in a circuit may not even be phsyically connected to the real Earth (the planet), but instead refers to the common point from which zero volts is defined

"why is an earthed conductor always at zero potential"

Because that's the definition. But we could have chosen any other number too. Voltage is a relative measure between two values, not an absolute value.

Think about length. When I say "12 feet", what does that mean? It means 12 feet from one point to another. We simply pick one of those points to be zero. Maybe we use Mean Sea Level as one end, or maybe the floor of your house, or maybe a completely arbitrary point we call "the datum". We just pick something to be zero because for that particular example it's convenient.

And nothing says convenient in voltages like "stick wire in dirt". So we call that zero.

• I see, but in a physics problem, I was trying to use the method of images to solve for a charge outside an earthed spherical conductor. I managed to get the right imaginary charge by setting the potential of the sphere to zero, but if it was nonzero I would not get the same answer. Is the zero here justified because the potential of a field of a point charge is defined to be zero at infinity? – user209347 Sep 26 '16 at 16:11
• @user209347 Comments are for clarification. Your question in your comment is so different that you are probably better off creating a new question. – Patrick M Sep 26 '16 at 17:30