An electric current is produced when electrons move through a substance. Cooper wire is often used to carry electrical current. Electrons normally revolve about the nucleus of each atom of cooper in the wire, but when electrical pressure ( that is the voltage) from a battery or generator is applied, some of these electrons are forced out of their orbits and pass from atom to atom along the length of the wire. These electrons are called free electrons and come from the outer orbit of the atom.

The amount of current depends on the number of electrons passing a given point in a circuit each second. It is measured in amperes (A) using an instrument called an ammeter. An ammeter must be connected in series with other devices in a circuit. The letter I is used to represent the amount of current in a circuit.

•  A direct current (DC) is produced when free electrons move in only one direction in a conductor.

• Pulsating direct current is a current  in one direction which regularly varies in intensity.

• Alternating Current (AC) is produced when the current regularly changes its direction and intensity.

The voltage produced by a DC source causes electrons to flow through a circuit from one terminal of the source to the other.  It was once thought that the current flow was from the positive terminal to the negative terminal.  This is referred to as conventional current flow.  Although it was later discovered that the current actually flows in the other direction, this conventional current flow is still used today. 

When an electrical current flows in a conductor two effects are always present. These are heat and magnetism. As current moves through the conductors, work is being done by the electrons to overcome the resistance of the conductor’s material and heat is generated as a result.

The amount of heat energy (measured in joules) depends upon the amount of current and the resistance of the conductor.

There is always a magnetic field surrounding a conductor that has current flowing through it. The larger the current, the greater the magnetic strength. This effect can be used to create an electromagnet. To intensify the magnetic field we wind the wire into a coil. To further increase the strength of the magnetic field we place an iron core inside the elecromagnet.

Another effect of electric current can be a chemical effect. When current flows through certain gases or liquids there is a chemical reaction. For example, if a metal object is palced in a solution of silver nitrate and a current is passed through the soluiton, the silver is deposited on the metal object. We also utilise this effect when we recharge a lead-acid battery.

The property of a material which causes it to oppose the movement of lectrons is called resistance. All materials have some resistance. Materials which offers little resistance to electron movement are called conductors. Those whic offer high resistance are called non-conductors or insulators.

Resistance is represented in formulas by the letter R and is measured in ohms. The symbols for ohms is the Greek letter Omega (Ω). The instrument used to measure resistance is called ohmmeter. The voltage source must be disconnected from the circuit when using an ohmmeter.

Conductors are substances that allow the passage of electric current and their resistance is usually low.  They are different  from other materials because their atoms have fewer than four electrons in their outer orbit.  These electrons are not tightly bound to the nucleus of the atom and can easily become free electrons when a voltage is applied to the conductor.  Most conductors are metals.

Insulators are non metallic substances that do not allow the passage of elctric current easily and have a very high resistance.  They have more than four electrons in the outer orbit of the atom, so these electrons are tighly bound to the nucleus. When a voltage is applied to an insulator, very few electrons become free and there is little or no current flow. There is no such thing as a perfect insulator because all insualting materials will allow some current to flow if the voltage is high enough.  However, under normal conditions we can assume that insulators allow no current flow.

Semiconductors are substances that have only four electrons in the outer electron orbit.  Under certain conditions they act as conductors, but under different conditons they act as insulators

Conductors:

• Copper
• Brass
• Gold
• Solder (tin and lead)
• Aluminum
• Silver

Semiconductors:

• Germanium
• Silicon