Electricity holds all matter together from moon rocks to cucumber sandwiches! In fact, it is precisely this property of holding things together which kept electricity a secret until the last two hundred years of human history. This is because electricity, in the guise of electric charge, comes in two forms, which the early pioneers of electrical research could have decided to call Ying and Yang or Tarzan and Jane but which, for no particularly good reason, they decided to call "positive" and "negative" (although there's nothing inherently positive or negative about them.) These two, differing forms of electricity have a strong attractive force. So strong in fact, that they are rarely seen separated and it is only when these charges are separated that electricity manifests itself.
The atom stays together because of the mutual attractive force between the positively charged nucleus and the negatively charged electrons, in the same way as the solar system stays together because of the gravitational attraction which exists between the sun and the planets. Although, it is worth noting that the electric force is many times stronger than the gravitational force.
In an undisturbed atom, the number of negative electric charges in the electrons is exactly balanced by the positive charge of the nucleus: there is thereby no overall, electric effect. As stated earlier, it is for this reason that electricity was so elusive until the last few centuries. One exception, which has been known since antiquity, is that, if a piece of amber is rubbed with a piece of silk, it can be used to pick up small objects and even cause sparks to jump to other objects when brought close. The fact the word electricity stems from the ancient Greek word for amber (electron) demonstrates just how ancient this manifestation really is! But what is happening? We now know that when the amber is rubbed with silk, some of the electrons in the atoms of the rock are stripped off and are transferred to the silk. This causes the amber to have a surfeit of positive charge and demonstrate some of the effects we associate with electricity. The atoms of the amber (and the atoms of the silk) are said to have become ionised: a few of the surface amber atoms have become positively ionised and a few of the atoms of the silk have become negatively ionised. The effect is only temporary however, and the ionised atoms of the amber will gradually re-attract (or lose) electrons via the air and return to the neutralised state.
Remember that the amber needed to be rubbed to make it electrically charged. This is a general rule: some force is required to put and maintain electric charges in an unbalanced state and - left to their own devices - all the charges will combine under their own attraction and neutralise. The force which exists between unbalanced charges is known as an electric field. The measurement of the electrical pressure which exists between unbalanced charges (you can think of this as the electrons' "pressure to return home") is termed potential difference or pd for short. The flow of electrons as they are drawn towards the positive potential across the electric field is termed an electric current.
In some elements and materials, some electrons are relatively loosely bound to their atoms so that there is a constant exchange of electrons as they "hop" orbits with adjacent atoms; a bit like an ultra-microscopic, three dimensional barn dance in which couples exchange partners but no-one is ever on their own! When such a material - which is called a conductor of electricity - is left to its own devices, this atomic barn dance is a random affair and there is no overall movement of electrons one way or the other. However, if the conductor is subjected to an electric charge so that there is an electric pressure to be resolved, electrons will start to flow towards the positive charge and their will be a gradual drift of partners from the negatively charged end to the positively charged end. This is termed electron drift. For this drift to be sustainable the electrons who are permanently lost to the dance at the positive end must evidently be replenished at the negative end. But it would be a great mistake to believe that, when an electric current flows, the electrons arriving at the positive terminal are the same ones which have recently left the negative terminal for, although electricity flows at an astoundingly fast speed of about 300,000 kilometres a second, the individual electrons only drift at a very slow speed of a couple of centimetres per minute.
Materials known as insulators, have electrons which are tightly bound to their atoms. Even under the application of an electric field, insulators will not permit their partners to exchange in the dance (an atomic foxtrot perhaps?) so that there is no overall drift of electrons and therefore no electric current flow. All metals are relatively good conductors. Glass, paper and many plastics are insulators.
Under their natural inclination, electrons will always flow towards and neutralise the positive charge. Imagining our barn-dance again, it was noted that, as a current flows, electrons were absorbed at the positive end and would need to be replaced at the negative end. In order to sustain an electric current there must be certain devices which replenish this supply of electrons and therefore do the unusual job of moving electrons from the positive terminal to the negative. Batteries and electric generators do this unusual job and they are said to provide an electromotive force (or EMF). This figure illustrates current flow, electron flow and the battery providing EMF against the usual neutralising flow of electrons toward the positive terminal of the battery.
You'll see in the figure above how the current is shown flowing from the negative terminal of the battery to the positive, and this is indeed the case. Unfortunately however, much of the work done on electricity was done before the existence of the electron was discovered and the experimenters and theorists guessed that electricity was due to a flow of positively charged "somethings" from the positive terminal to the negative rather than the other way around. (Well they had a 50/50 chance of getting it right!) By the time the true nature of electricity revealed, it was too late to re-write all the theory books, so - to this day - current (sometimes called conventional current) is assumed to flow the opposite way to the way it really does.
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