The last few days have been particularly bad. Everytime I have stepped out of my car and put my hand on the door handle to close it, I have got that irritating shock. I always know that I am going to get it but then there seems no way out. And then a few days back I actually saw a spark when my fingers reached close to the door handle! And I recalled my high school Physics and remembered that a spark( caused by the electrical breakdown of air) needs atleast a 1000 volts!!!! Does this mean that when a person comes out of a car he has more than a 1000 volts of charge???? If it is true then isnt that dangerous??? How much charge does a person walking on a carpet acquire??? In today’s VERITAS we will recall static electricity and answer all these above questions.
Lets take an ordinary piece of cloth. It is electrically neutral.
That means that the number of electrons and the number of protons are equal. Now lets take two pieces of cloth of different materials ( say one of silk and the other of wool). Now place them together so that they touch. SOme materials hold their electrons tighter than other materials. So if one piece of cloth touches another then the
proton(nucleus) of one material may come close to the electron of the other material and may try to pull it. The cloth that holds its electrons weaker may loose some electrons to the cloth that holds the electrons stronger( or closer). So one piece of cloth soon has more electrons than the other. So one piece of cloth becomes electrically positive and the other becomes negative. If you rub the two pieces of cloth with each other then more electrons and protons come in contact with each other and the more they pull at each other and more charge accumulates. If both the materials are insulators of electricity then they do not transmit charge. They store charge. So we have a charge accumulation without any current. This is static electricity.
Now all this we have studied in high school. But how much is it?
If you walk rubbing your socks on a carpet you can acquire 20,000 volts in 20 minutes. On a winter day if you sit in a car for an hour and keep rubbing against the car fabric you could acquire 10,000 to 30,000 volts!
And I am not talking about crazy amounts of rubbing. Just that much rubbing that happens when you drive for an hour! And then when you come out of the car you are ****charged**** and when you touch the door handle there is a shock as 20,000 volts of electric potential get discharged!!!
So we are not playing with a 10 volt pappu charge. We are talking about thousands of volts!
So how does a spark happen? When your fingers get charged to a huge amount and you place your fingers near the handle an intense electric field is build up between the fingers and the handle. The air molecules feel this huge electric field. Electrons are ripped from the air atoms because of this field. A electron that is ripped from the atom moves at an incredible speed guided by the electric field. On its way it hits more atoms and smashes more electrons from the atoms.
It creates a cascade of electrons which keep smashing against atoms.
Some electrons are recaptured and they loose energy by emitting light.
And we see a brilliant spark! So a huge amount of charge is needed to create this electrical breakdown of air and cause ths spark. And rubbing against the car fabric gives you that much charge!
But is it dangerous? You may be charged to 20,000 volts but when you touch the car door handle you are discharged in a fraction of a second! You are discharged in about a millionth of a second or less. You could have been killed if it took a second or more. But it cannot because the 20,000 volts of potential difference is not maintained. In a house circuit the voltage of 220V is maintained and touching the wires for even 1/2 a second can kill you. So static voltage though it is so high cannot be dangerous because it gets discharged in a millionth of a second…. too little time for the body to experience anything significant. Only your fingers get hurt because of the spark. And there is a way to get over this pain: Dont touch the door with your fingers. When you get out of the car touch the door first using your car key. The spark will flow from the car key to the door and not from your fingers and you will be discharged in no time and after that you can touch the door with your fingers. So it is not the electrical discharging that hurts…. it is the spark that hurts because of the heat it produces in such a short time.
And using the car keys to discharge will make it much less painful.
Go, wondrous creature! mount where Science guides:
Go, measure earth, weigh air, and state the tides:
Instruct the planets in what orbs to run,
Correct old time and regulate the Sun;