Earthing needs a conducting path

1. A charged object has an imbalance of electrons: either more electrons than it needs to be neutral, or fewer.
2. The positive charge in a solid sits in the protons inside atomic nuclei, and those nuclei are fixed in the structure of the material.
3. The only charge that can travel through a solid is therefore the electrons, so every explanation of earthing is written in terms of electron flow.
4. The earth acts as an enormous reservoir of charge: it can accept or supply huge numbers of electrons without its own charge changing noticeably.
5. Earthing removes excess charge only if a conductor joins the object to the ground, because electrons need free charges in order to move.
6. A copper wire, a metal chain, a metal water pipe or a person's body all provide such a path, while dry plastic, rubber or glass does not.
7. Once the path exists, electrons flow until the object has no excess charge left, and the flow then stops on its own.
8. The object is now neutral, which means the number of electrons again balances the number of protons.

Earthing a negatively charged object

1. A negatively charged object has gained electrons, so it holds more electrons than it needs to balance its protons.
2. Connecting it to earth through a conductor gives those extra electrons a route away from the object.
3. The extra electrons repel one another, so they flow from the object to the earth.
4. As they leave, the amount of negative charge on the object falls.
5. The flow stops when the electrons again balance the protons, so the object ends up neutral.
6. The electrons have not been destroyed; they are now spread through the earth, where the effect of adding them is far too small to measure.

Earthing a positively charged object

1. A positively charged object has lost electrons, so it holds fewer electrons than it needs to balance its protons.
2. It does not have extra protons, and no protons move at any stage.
3. Connecting it to earth lets electrons flow from the earth to the object.
4. Each electron that arrives cancels part of the positive charge, because it replaces a missing electron.
5. The flow stops once enough electrons have arrived for the object to be neutral.
6. Electrons therefore flow towards a positive object and away from a negative one, so you must state the sign of the charge before you state the direction of flow.

Why insulators are harder to discharge

1. Charge on an insulator cannot travel through the material, so touching an earthed wire to one point removes charge from that point only.
2. The rest of the charge stays where it was placed, which is why a charged polythene rod is still charged after being held at one end.
3. A conductor discharges completely and almost instantly when it is earthed, because its free electrons can move throughout the whole object.
4. This difference is why a metal object held in the hand never appears charged, while a plastic one does.
5. Charge does leak away slowly from an insulator in damp air, because a thin film of moisture on the surface behaves as a weak conductor.
6. Warming and drying an insulator before charging it therefore makes static effects last longer and appear stronger.