electric-circuitselectricityelectrons
I made a simple bulb-battery circuit and then I cut one of the wires and attached both ends to cemented floor, the bulb didn't glow, this means cemented floor is a poor conductor of electricity. Then how does earthing work ? This idea of activity came from when I got a shock being barefoot but got no shock from same source with slippers on.
So,
How can electrons pass through insulator like cemented floor during earthing ?
Because of electromagnetic forces, all of the electrons in the wire are displaced towards A with a certain velocity causing a positive current towards B.
The electrons have a small drift velocity, not moving much.
Although your light turns on very quickly when you flip the switch, and you find it impossible to flip off the light and get in bed before the room goes dark, the actual drift velocity of electrons through copper wires is very slow. It is the change or "signal" which propagates along wires at essentially the speed of light.
A single electron does not go from A to B. Think of it as each electron pushing the next one, and the signal travels with the velocity of light ,maximum, down the wire.
This drift of electrons heats up the highly resistive filament wire in the bulb and makes it glow.
True.
But I don't understand why they are able to move back and forth. Specially if length of the wire was large, say 3 * 10^8 meters, then would the movement of electrons on one end of the wire be "in sync" with movement of electrons on the other end?
Why not? When the field changes at A and B the change propagates by electrons moving back and forth over an average position. Similar to water waves, the atoms do not move much from their position, the energy is transferred atom by atom. In the case of electric fields and electrons the field is built up at the microscopic scale by the motion in situ of the electrons, in a sinusoidal way.
Very long wires enter the realm of special relativity and the limit of the velocity of light in transferring effects of fields.
If the neutral wire was not grounded, the electricity would be still delivered to the devices, but the standard earth protection system would not work.
There are systems like SWER (Single-Wire Earth Return) where the electricity is delivered by one wire with the return current flowing through the earth. They are not very efficient and they are mostly used to power some remote loads, in many cases consuming relatively low power.
SWER transmission lines, as all long haul power transmission lines, operate at high voltages (tens of kilovolts) and therefore carry relatively low currents, so the losses due to the earth resistance are not very high. Besides, the connections to the ground are done with long rods reaching ground waters, so it is not just dirt and stones. After the step down transformer, where the current increases, the electricity is carried by two wires.
The conductivity of the earth is roughly the same for DC and low frequency AC currents. In both cases through, you need to go deep under ground to achieve low resistance.
Grounding of the equipment housing keeps its potential at near zero level even when it comes in contact with a live (hot) wire. This system is well described on numerous websites. The illustration below will hopefully make it easier to understand.
Here red is a line wire, blue - neutral and green - ground. For more details, you can check out this post.
Under ideal conditions, connecting the housing to the neutral wire (grounded at the main panel) would work fine as well. But in the real world it is not such a good proposition: besides some more subtle reasons, an accidental break of the neutral wire, leading to a device grounded in such fashion, will bring the housing of the device to the line voltage, which could be lethal. Grounding the housing by a separate ground wire adds redundancy and therefore is more safe.
Best Answer
Earthing something means dumping the electron flow into the earth. Since the earth is so big, it can absorbe/give a practically infinite amount of charge without changing potential, this means that you can treat earth as a reservoir of ready to use electrons.
If you plug the phase of your home power line into the ground (without safety devices in the middle), you are actually dumping the electrons in the earth. (In reality -since we use AC- you are repeatedly dumping and taking back electrons 50 times per second).
Note: the other wire of the power line that gets to your home is connected to earth at the nearest distribution node.