If we rub two different non-conducting materials together, for example a rubber balloon and a woollen jumper, some of the electrons are transferred from one to the other. This leaves one with a positive charge and the other with a negative charge. This means a potential difference exists between them. We measure potential difference in volts (V), so we often refer to a potential difference as a voltage.
The unit of electrical charge is the coulomb. A current of 1 A is a flow of one coulomb per second. An ordinary battery operated torch bulb takes a current of about 0.2 A, so a coulomb is the amount of electricity that flows through the wires connected to a torch bulb in five seconds.
If we take an object and give it an electric charge we will change its potential. How much the potential changes will depend upon a number of factors including of course how big the object is and what it is made of. If the charge needed to change the potential of an object by one volt is one coulomb we say the object has a capacitance of one farad (F). A high capacitance means that a large charge will produce only a small change in potential, whereas a small capacitance means that a small charge will produce a large change in potential.
The capacitance of the Earth is very, very large indeed. This means that transferring charges to it will not significantly change its potential, any more than pouring jugs of water into the sea will significantly change sea level. So in the same way that we measure heights and depths from sea level so we measure potential from that of the Earth. We say that the Earth is at zero potential. The potential of an object is the potential difference between it and the Earth. When we earth (American ground) something we are connecting it to the Earth and so making its potential zero. We can normally totally discharge a charged object by connecting it to Earth.
The farad is a very large capacitance indeed: most objects have capacitances in the order of a few “puffs” (pF or pico-farads. A pico-farad is a million millionth of a farad.). If the capacitance of an object is very low only very tiny charges are needed to produce very high voltages.
The capacitance of a normal adult is about 160 pF. This means if we walk along a nylon carpet and pick up a static charge of a millionth of a coulomb we shall be at a potential of more than 6000 V! If we then touch something earthed such as a metal door handle we may feel a slight shock, but the charge transferred to Earth is so tiny that it will do us no harm at all.
Nothing, neither our bodies nor anything else, is perfectly insulated so the charge on any object will always eventually leak away to Earth. Dirt and damp on the surface of ceramic insulators, from overhead power lines to car spark plugs, is a particular problem.
Static electricity is discussed in greater detail on another Page of my Web Site - to link to it please click here
Static electricity (other than the special case of lightning which is considered on another Page) is not usually directly harmful to humans, but it is still dangerous in two other ways.
Inside a computer chip one centimetre square there are literally millions of very complex electronic circuits. The currents flowing in these circuits are unimaginably tiny - even the slightest static charge could give rise to a current big enough to destroy the chip. When we are working inside a computer, or any other electronic equipment, we must take every possible precaution to ensure that everything, including ourselves and any tools we are using, is earthed. Chips and electronic circuit boards containing them always come sealed in special anti-static envelopes, and you should never open the envelope until just before you are ready to use what is in it and are certain you understand all the safety precautions you need to be taking.
Air is normally a good insulator, but under certain conditions, particularly involving high voltages, a current may flow through the air. The air in the path of the electricity may be heated to a temperature high enough to make it glow - this is an electric spark. As the air is heated it expands very rapidly, faster than the speed of sound, and this sets up a shock wave, like the shockwave produced by a supersonic aircraft. We hear this as a loud crack.
If a spark, even a tiny one, passes through air which has been mixed with an inflammable gas or powder or the vapour from an inflammable liquid it may start a fire or explosion.
We all know that gas leaks are dangerous but it is easy to forget that dusts and powders and vapours from inflammable liquids are just as explosive, even in quite low concentrations.
If we put a saucer of water onto a window sill it will eventually all evaporate, no matter how low the temperature. But of course the higher the temperature the faster it will evaporate, and this is true of all liquids. If we have an open container of an inflammable liquid in a room there will always be some vapour in the room, whatever the temperature. If a spark passes this vapour may catch fire. The flash point of an inflammable liquid is the temperature at which the liquid is evaporating fast enough to support combustion, that is, if the vapour in the room catches fire the temperature of the air in the room is high enough to allow the liquid to evaporate fast enough to keep the fire burning. The flash points of many solvents used in paints and for making plastics are very low indeed, well below 0oC, and so in factories and workshops where they are being used the most careful precautions have to be taken to avoid even the tiniest spark.
Sparks can be produced by static electricity but they can also be produced by electrical equipment being turned on OR OFF - you may have noticed the spark when you turn off an electric fire. This is why if you smell gas it is as important not to turn anything off as it is not to turn anything on. You can get special spark-suppressed switches, and in most factories and workshops you must use them.