Basic radio theory, circuits and calculations

Molecules, atoms, and electrons

All matter is comprised of molecules; the molecule is the smallest quantity of a substance which can exist and still display all the physical and chemical properties of that substance.

Molecules are made up of smaller particles called 'atoms' of which there are over 100 different types. All molecules consist of various combinations of these atoms, for example, two atoms of hydrogen and one of oxygen form one molecule of water, and sulphuric acid is made up of two atoms of hydrogen, one of sulphur and four of oxygen.

Atoms are so small that they cannot be seen under the most powerful microscope but their constitution is of vital importance in electrical and communication engineering.

Atoms consist of a relatively heavy, positively charged core or nucleus (red in the animation), around which a number of much lighter negatively charged electrons move in one or more orbits (blue in the animation).

Diagram 2.1 The structure of an atom

One type of atom differs from another in the number of positive and neutral particles known as 'protons' and 'neutrons' which make up the nucleus and the number and arrangement of the negative 'electrons' which are continually orbiting round the nucleus. Some atoms are extremely complex, having a large number of electrons in several orbits, and others are quite simple.

Direction of conventional current flow >>>>

<<<<< Direction of actual electron flow

Diagram 2.2 Electron flow through a conductor

The above animation shows the electrons (yellow) flowing through a conductor. The conventional flow of electrons is from a positive charge towards a negative. This direction of flow was assumed, long before there was any method to check the actual direction the electrons moved in. We now know the electrons do actually flow from the negative to the positive. Whenever current flow is discussed, it is safe to assume the use of the 'conventional flow direction' is what is meant.

Under some circumstances it is possible to detach an electron from an atom, particularly when its outer orbit contains only one electron. In other atoms it is virtually impossible to detach an electron.

Additional explanation of electron flow through a conductor.

We all know that the speed of light in a vacuum is 3 x 10⁸ms^-1. We should also know that the speed of electricity passing through a piece of wire is similar, but how does electricity actually travel through wire?

Electric current flow relies on what are called free electrons. These move in a wire a very short distance and collide with another electron, which in turn moves a very short distance and collides with yet another electron. Given that there are many billions of electrons per cubic centimetre there will be many billions of collisions every second.

What do we mean by moving a very short distance? This distance will be microscopically small only resolvable by using some form of electron microscope.

So we have to ask does a specific electron actually move from one end of a piece of wire to the other? The answer is yes, but not at the speed of light, it drifts very slowly. How I hear you ask? If there are very fast collisions and very slow drift this appears to be a contradiction in terms! Think of it like this, you step forward 2 paces and then back 1. The result is moving a total of 3 paces, but forward movement is only 1 pace.

So the electron bounces back and forth at great speed, but only moves forward at a slow rate. This rate is known as drift velocity and is very slow indeed. For a piece of copper wire 1.0mm diameter carrying a current of 5A the drift velocity is just under 0.5mm per second, or 1.8 metres per hour.