Read the text. In those substances called conductors, electrons in the outer orbits of the atoms are free to move at random from one atom to another neighboring atom

Electric current

In those substances called conductors, electrons in the outer orbits of the atoms are free to move at random from one atom to another neighboring atom. This is because the atoms are so close that their outer electron orbits touch and the electrons in these orbits occupy a neutral zone where they are equally attracted by the other nuclei. These electrons are called free electrons.

In conductors, since the number of free electrons is enormous and they move at random, we can consider the number of electrons moving in one direction equal to the number moving in opposite direction. Therefore there is no apparent movement of electrons. Now, if, owing to an external cause, there is an excess of electrons at one end of a conductor and deficiency of electrons at the other end, then the free electrons in the conductor will be subjected to two electrostatic forces: a force of attraction towards the positive end (the end with a deficiency of electrons) and a force of repulsion from the negative end (the one with an excess of electrons). The random movement of the free electrons will continue, but in this case there will be a movement or flow of electrons towards the positive end.

An excess of electrons at one end of a conductor and a deficiency at the other end can be obtained bу connecting the ends of the conductor to the terminals of a battery or a generator. In fact the property of batteries or generators is to drive free electrons they have inside them continuously from one terminal to the other against the electrostatic forces. This property of generators or batteries to tend to make an electric current flow is indicated by saying that they have an electromotive force (e.m.f.). On the other hand, for an electric current to flow in a conductor that is connected between the terminals of a generator or a battery, there must be a potential difference or voltage between the two terminals. The potential difference is expressed by the work done per unit charge when it passes between the two terminals.

The electric current in a conductor is actually a flow of negative charges; but by tradition an electric current is considered as a flow of positive charges from the positive to negative terminal. Although this traditional idea is wrong it is convenient for everyday use. Electric current cannot flow easily in all substances. There are many substances in which the electrons move from atom to atom with difficulty. These substances are known as insulators. All substances, however, offer some opposition to the flow of electrons, and there is no very clear dividing line between conducting materials and insulating materials. Conductors include all metals and alloys, carbon and graphite; typical insulators are rubber, plastic, dry air, oil, glass, paper and cotton. The opposition to electric current flow in a body when there is a potential difference between the ends of a body is expressed by its electrical resistance. In order to compare the resistance of different substances, it is necessary to consider the resistance offered by a sample of fixed dimensions. It is usual to choose a cube as the sample body, and the dimensions of the cube will depend on the system of units we employ. The resistance between opposite faces of this sample cube is called resistivity.

When the resistivity of a substance is known, the resistance of any body composed of that substances can be calculated. In particular we must bear in mind that the resistance of a wire is directly proportional to its length, inversely proportional to its cross-section, and directly proportional to the resistivity of the substance forming the wire. Now we can say that when a given potential difference voltage is applied across the ends of the wire, an electric current always flows along it, and the value of this current depends on the resistance of the wire.