Overview of electrical current

- a tutorial, overview or summary of electrical current.

One of the most basic concepts in electronics and radio is the flow of current. Wherever one looks the flow of current is central to whatever is being achieved. Whether it is in a personal computer, audio amplifier, radio circuit or any other aspect of electronics the flow of current is of fundamental importance.

To understand what current is, it is necessary to look at the composition of the materials around us. All matter is made up from molecules. These are the smallest quantity of a substance which can exist and display its physical and chemical properties. There is an almost infinite variety of molecules, from the simplest ones like oxygen and nitrogen, the gases which are common in the air, to much more complicated ones like those found in plastics or in organic substances. However all of these molecules are found to be made up from even smaller particles known as atoms.

Unlike molecules there is a limited number of types of atom. Currently 102 are known. Hydrogen is an example of an atom, and so is oxygen, although when these exist in the air they both combine with another atom of the same type to form a molecule. In other words the oxygen we breathe in the air consists of a molecule containing two oxygen atoms.

Make up of atoms

Atoms are exceedingly small and cannot be seen under a microscope. However their behaviour and their properties have been studied and much is known about them. It is known that atoms are made up from even smaller particles - electrons, protons and neutrons. It is found that some of these particles are charged. An electron has a negative charge, and a proton has an equal but positive charge. A neutron has no charge. It is also found that atoms are electrically neutral. In other words the number of positive charges counteracts the number of negative charges.

Their relative weights are also known. Neutrons and protons are the same, but electrons have a much smaller weight and this can normally be ignored.

In an atom there is a central nucleus which contains neutrons and protons, and the electrons appear in orbits around the nucleus. The factor which differentiates one type of atom from another is the number of protons and neutrons which make up the nucleus. Some including hydrogen are very simple, whereas others which are much larger and heavier are far more complicated.

Conductors and Insulators

It is found that some materials conduct electricity well (conductors) whereas others do not conduct it (insulators). It is found that metals conduct electricity well, as do materials like carbon, and water solutions containing acids or salts. Many materials do not conduct electricity, including ceramics, plastics, gases in the air etc..

The ability of a material to conduct electricity is governed by ease with which electrons can be detached from the atom to move through the material to another atom. In some substances there is a continual movement of electrons through the structure from one atom to another. If an electrical pressure is applied across the material, electrons will drift into one end and out of the other. This movement is called an electric current.

Units of current
One way of visualising a current is to compare it to water flowing in a pipe. Turning on a tap is equivalent to throwing a switch in an electrical circuit.

The amount of water flowing in the pipe is of importance, and so is the amount of current flowing in an electrical circuit. Current is measured in Amperes or amps for short. The unit is named after the famous French scientist Andre Ampere who performed much early work on electricity.

Many circuits have currents of an amp of more flowing in them. A mains driven kilowatt heater element will take about four amps. However many electronic circuits take much smaller currents. To be able to refer to these smaller currents more easily the units milliamps (mA) meaning a thousandth of an amp, or microamps ( A) meaning a millionth of an amp are used.

Most small electronic circuits will use currents in the region of a few milliamps, although complete units will take more. A CB radio may take an amp or so, most of this current will be used in the output stage which feeds the radio frequency power to the antenna.

Effects of current

Another effect which can be noticed is that a magnetic field is built up around the conductor. If a current is flowing in conductor then it is possible to detect this. By placing a compass close to a wire carrying a reasonably large direct current, the compass needle can be seen to be deflect. Note this will not work with mains because the field is alternating too fast for the needle to respond and the two wires (live and neutral) close together in the same cable will cancel out the field.

The magnetic field generated by a current is put to good use in a number of areas. By winding a wire into a coil, the effect can be increased, and an electro-magnet can be made. Relays and a host of other items use the effect. Loudspeakers also use a varying current in a coil to cause vibrations to occur in a diaphragm which enable the electronic currents to be converted into sounds.

Summary