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Static Electric Charge
 All matter is made of atoms. All atoms contain particles which possess electric charge. Protons possess positive charge. Electrons possess negative charge. N.B. This is, of course, just a convention... there's nothing particularly positive about a proton nor negative about an electron! The unit of measurement of electric charge is the Coulomb after Charles-Augustin Coulomb, French physicist and engineer, (definition of the Coulomb) If a body has equal quantities of positive and negative charges, it is said to be electrically neutral. If a material allows electrons to move through it easily, it is a conductor. All metals are conductors because they have free electrons (that is, electrons which are not bound to a particular atom). A material through which electrons cannot easily flow is an insulator. It is perhaps time to ask ourselves: what do we mean by the words "electric charge"? It is found that certain particles attract or repel each other. This behaviour is "explained" by saying that they possess charge. In other words, charge is a property of certain particles which causes them to attract or repel. It is found that similar charges repel and opposite charges attract. Also, a positively or negatively charged object will weakly attract an object which is neutral. This occurs because the charged object has the effect of "rearranging" the charges inside the originally neutral object. When two different insulators are rubbed together, electrons can be transferred from one insulator to the other. The body which has gained electrons has a negative charge and the one which lost electrons has a positive charge of equal magnitude. This process is called charging by friction. A body can also be charged by simply placing it near a charged body. This process is called charging by induction. This is the process of "rearranging" the charges mentioned above. This diagram represents the situation when a piece of metal is charged by induction. Free electrons in the metal move towards the inducing charge, leaving a deficit of electrons at the other end, hence one end negatively charged and the other positively charged. Electrons can be drawn up from the earth to neutralize the positive charge on the right hand end of the metal. If the earth connection is then removed we have this situation. If we now remove the inducing charge, we find that the piece of metal now has a charge equal but opposite to the charge on the inducing plastic rod. The situation is different when an insulator is charged by induction, as shown by this diagram. The atoms (or molecules) in the balloon near the inducing charge are "stretched out" (their electrons spend a bit more time in their "orbits" near the inducing charge than at the other side. The atoms (or molecules) in the balloon near the inducing charge are "stretched out" (their electrons spend a bit more time in their "orbits" near the inducing charge than at the other side. Diagram not to scale.(I assume you guessed that!) Electric charges which are not moving are called static charges. The only charges which can move freely through metals are negative charges carried by electrons. A flow of electric charges is called an electric current.
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