If, on a dry day, you shuffle your feet on a carpet and then touch a doorknob, you will often feel a shock. The shock is caused by static electricity suddenly leaving your body. Other examples of the effects of static electricity include lightning, hair standing on end, the ‘ozone smell’, and the spontaneous explosion of grain silos.
A plastic rod can be given a static charge by rubbing it with fur. Similarly, a glass rod can be charged in the same manner. If one of the rods is suspended from a thread, and the other is brought near, the rods will attract each other. However, two charged plastic rods or glass rods will repel each other.
Benjamin Franklin was among the first to propose the viewpoint that there are two types of electric charge. He called one type of charge positive and the other negative, arbitrarily defining the type of charge on the glass rod to be positive, while the charge on the plastic rod became negative. He explained in terms of an “electric fluid” that flowed between the objects.
We now know that the “electric fluid” consists of electrons, which by Franklin’s choice turn out to have a negative charge. As a result, we still speak of charge moving from negative to positive, which is contrary to our usual analogy to the flow of water from high to low. A bit awkward, but everything still works in the end.
When two objects are touched together, electric charge is transferred between them. Using an electroscope you can measure this charge. After many experiments, it was found that the total amount of electric charge is constant. Electric charge is not created or destroyed – merely transferred from object to object, as would be expected for Franklin’s “electric fluid”. This is called the law of conservation of electric charge.
Lightning is the result of a static discharge between clouds and the ground. This provides dramatic evidence for the production of enormous amounts of static electricity. An average bolt of lightning carries energy equivalent to perhaps 10 pounds of TNT. No wonder we are easily startled by thunder!
On a smaller scale, one of the most important aspects of working with modern microelectronics is to ensure that static charges are isolated from integrated circuits, so that static discharge does not destroy the electronic devices. This is accomplished by wearing a conductive antistatic strap (often a wristband) which is grounded so that static charges are drained off harmlessly.
Although we have been describing problems caused by static electricity, it also provides us with a useful tool for modern technology. Xerox machines, electrostatic air filters, paint sprayers, and many other devices are based on the controlled use of static electricity. Static electricity also provides the most common motive power for nanomachines, such as the matrix mirrors which are the basis for modern projection televisions.
In summary, static electricity is a very common phenomenon. It can be both a source of trouble and a friend for civilizations at all levels of technological development. Despite being studied intensively for thousands of years, the mechanisms and properties of static electricity can still fascinate and amaze.