Introduction
when an amber rod is rubbed with fur, the rod has the amazing characteristic of attracting some very light objects such as bits of paper and shavings of wood.
a substance such as amber or glass when given a vigorous rubbing was recognized as being electrified, or charged with electricity.
In the year 1733, Charles Dufay made an important discovery about electrification. He found that when a glass was rubbed with fur, both the glass rod and the fur became electrified.
he concluded that there must be two exactly opposite kinds of electricity.
Benjamin Franklin is credited with first using the terms positive and negative to describe the two opposite kinds of electricity. Those bodies which were not electrified or charged, he called neutral.
Static Electricity
One of the easiest ways to create a static charge is by friction.
When a hard rubber rod is rubbed with fur, the rod will accumulate electrons given up by the fur, as shown in the figure. Since both materials are poor conductors, very little equalizing current can flow, and an electrostatic charge builds up. When the charge becomes great enough, the current will flow regardless of the poor conductivity of the materials. These currents will cause visible sparks and produce a crackling sound.
Nature of Charges
When in a natural or neutral state, an atom has an equal number of electrons and protonsAn atom becomes a positive ion whenever it loses an electron and has an overall positive charge. Conversely, whenever an atom acquires an extra electron, it becomes a negative ion and has a negative charge.
In most solids, the transfer of charges is by the movement of electrons rather than ions. The transfer of charges by ions will become more significant when we consider electrical activity in liquids and gases.
Charged Bodies
In the atom, the negative electrons are drawn toward the positive protons in the nucleus. This attractive force is balanced by the electron's centrifugal force caused by its rotation about the nucleus.
The law of charged bodies may be demonstrated by a simple experiment.
Coulomb's Law of Charges
Charged bodies attract or repel each other with a force that is directly proportional to the product of their individual charges, and is inversely proportional to the square of the distance between them.
Unit of Charge
1 Coulomb = 6,280,000,000,000,000,000 electrons
Or, stated in reverse terms, one electron has a charge of about 0.00000000000000000016 coulombs.
Electric Fields
The space between and around charged bodies in which their influence is felt is called an electric field of force. It can exist in air, glass, paper, or a vacuum. Electrostatic fields and dielectric fields are other names used to refer to this region of force. Fields of force spread out in the space surrounding their point of origin and, in general, diminish in proportion to the square of the distance from their source.
The field about a charged body is generally represented by lines which are referred to as electrostatic lines of force.
Conduction of Electricity in Solids, Liquids and a Vacuum
Solids
Electric current is the movement of valence electrons. Conduction is the name of this process.
The exception to this is graphite (one of the forms of the element Carbon). Carbon is a non-metal which exhibits some electrical conductivity.
Liquids
The only liquid elements which conduct are the liquid metals.
Non-metals such as water, alcohol, ethanoic acid, propanone, hexane and so on, are all non-conductors of electricity.
it is possible to make some non-conducting liquids conduct electricity, by a process called ionization.
Taking water as an example, that water is considered to be a non-conductor of electricity. This is due to the presence of a minute concentration of H+ and OH– ions in the water.
However, electrons cannot flow through water. Covalent substances do not conduct an all-in solution. Ionic substances are able to conduct electricity when they are dissolved in water.
When the ionic solid is dissolved in water the ionic lattice breaks up and the ions become free to move around in the water.
An electrolyte is a liquid that can carry electric current through it. Ionic solutions and ionic melts are all electrolytes.
Electrolysis describes the process which takes place when an ionic solution or melt has electricity passed through it.
Gases
Gas in its normal state is one of the best insulators known. However, in a similar way as a liquid, it can be forced to conduct electricity by ionization of the gas molecules.
once the applied electric field approaches the breakdown value, The breakdown process forms a plasma that contains a significant number of mobile electrons and positive ions, causing it to behave as an electrical conductor. In the process, it forms a light-emitting conductive path, such as a spark, arc or lightning.
Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized" from their molecules or atoms. Plasma can be formed by high temperature, or by application of a high electric or alternating magnetic field as noted above. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions and hence carry the bulk of the current.
Vacuum
Remember that a conductor is "something through which electricity can flow," rather than "something which contains movable electricity." A vacuum offers no blockage to moving charges. Should electrons be injected into a vacuum, the electrons will flow uninhibited and unretarded. As such, a vacuum is an ideal conductor.