I have a whole lot of misconceptions about electricity, and this is one of them. I don't get why the earth wire works; not how, but why. My understanding is that one end of the earth wire is connected to the metal casing of an electrical appliance, so that in case the live wire is in contact with the metal casing and there's current flowing through the metal casing, the earth wire provides a low resistance path to the Earth.
That way, there isn't a danger of getting electrocuted when you touch the metal casing even if the current leaks, because the current is flowing through the earth wire, down to the Earth.
But then I also heard that a circuit needs to be completed in order for an electric current to flow. But in the case of the earth wire, the current is just going into the ground, but it isn't going back to source of the circuit. In that case, why does the current flow through the earth wire in the first place, if it's sort of like a "dead end" and doesn't complete the circuit?
Best Answer
when you touch the metal casing even if the current leaks, because the current is flowing through the earth wire, down to the Earth.
If there is high resistance in the Earth wire then you can have a voltage on case (in the event of a fault). Generally you try and ensure that the resistance throughof the Earth wire is less than the resistance through you - so no current flows through you.
But then I also heard that a circuit needs to be completed in order for an electric current to flow
Not necessarily. If the place the electric current is flowing to is big enough that its voltage doesn't change then it doesn't need to be connected - electricity can just flow into it.
We sometimes use large tanks of salt or large blocks of metal - insulated from the actual ground to act as local Earths in high voltage experiments