How plasma ball works? I am always confused with the concept of earthing in electricity. I have read and done few experiments and I understand that electricity needs to close the circuit. I can understand that from the context of a battery.
But what I am confused always is about earthing. From a battery we can see that the electrons can flow from the negative terminal to the positive terminal to close the circuit.
From that analogy I am unable to understand how a plasma ball works. When I read about it, the explanation given is that the electrons will flow through your body to the earth when you touch it. How does that help to complete the circuit?
I am also unable to understand the earthing in household electricity. What I have read and probably misunderstood is that the Earth is a big capacitor which can store charges. But even then how does it help to complete the circuit in the above two examples like plasma ball or household earthing?
 A: I'm assuming that "plasma ball" means these things
"the electrons will flow through your body to the earth when you touch it"
That works because the plasma ball is itself connected to earth (via the power cord), so there is a closed circuit when you touch the ball. A current can only flow if there is a continuous circuit from one power terminal to the other.
In general earthing is used in mains power circuits, to give the current a return path without going through your body, as that could be fatal. With AC equipment, earthing is also used to keep the equipment at a constant voltage level, in order to reduce induced 50Hz hum. If the equipment is battery-powered, earthing is not normally required as there are no high voltages with respect to ground. Even if the equipment generates a high voltage internally, that is not dangerous, unless the current can flow through you.
If proper precautions are used, it is quite safe to touch equipment at a potential of thousands of volts above ground. Just make sure your body cannot become part of a closed circuit, by insulating yourself from ground. Alternatively, make certain that the current through your body will be very small - it's the current that kills, not the voltage.
