Electric charge

The Greeks were the first to discover electricity about 2500 years ago. They found that when a piece of amber was rubbed with other materials it would attract small objects such as dried leaves, or straw. The Greeks word for amber is electron. The word electric was derived from it and meant "to be like amber."

There are numerous materials which posses a similar property. The explanation of this property is not as clear as the explanations in mechanics, for example. In mechanics we see the objects, we can measure and “feel” the force acting on it. Everything is in a macroscopic scale.

The phenomena of attracting by amber other materials is somehow mysterious. There is no way to observe any macroscopic change of amber after it is rubbed. We must use our imagination and propose some “mechanism” or model for such behavior. This is the usual method applied in Physics and other Sciences to explain phenomena which cannot be observed directly. We propose a model based on our previous knowledge of a broad class of similar phenomena. If a broad range of observed effects could be explained with the assumed model, the model is accepted.

In the case of the described above effect of one material attracting another the electric charge was introduced to explain it. It was assumed that each object (body) contains large amounts of electric charges. Two types of these charges are present in each body. One type is called positive and denoted with “+”, the other is called negative and denoted with “-“.

In the normal state in every object there are equal amounts of positive and negative charges, therefore the object seems to be neutral (without electric charge) to the observer.

If, by some means, rubbing for example, the equilibrium is disturbed, the object behaves as if it possessed a net electric charge equal to the difference between the positive and negative charges present in it.

In 1909 in a wonderful experiment Robert Millikan determined that there was a smallest “unit” charge, so that the charge is “quantized”. He received the Nobel Prize for his work. This smallest charge is equal -1.6 x 10 ^-19 C. C stands for Coulomb which is the SI unit of electric charge. This is the amount of electric charge present on a single electron. The positive counterpart of it is a charge of +1.6 x 10 ^-19 C present on a proton.

An electric charge is an intrinsic property of electrons and protons, it automatically accompanies those particles.

Any net electric charge present in nature is always a multiple of the unit charge associated with a electron (only the signs may vary). The effect of the existence of electric charges can be observed everywhere. If you walk on a carpet in dry weather and then you bring your finger close to a metal doorknob you produce a spark between the finger and the metal. If you use a plastic comb in the dark standing in front of a mirror you can see small sparks. An effect of the same nature, but on a huge scale is lightning. There are many wonderful experiments which can be performed in a classroom or at home, which demonstrate the existence of electric charges and help to understand the physics of electricity.