Electrical Conductors

Electrical Conductors

Some materials like metal allow electricity to pass through them easily and are known as ‘electrical conductors’. Only materials which act as good electrical conductors are suitable for use in the parts of plugs and wiring where the electricity is required to flow.The parts of any appliance or electrical connection device where electrical flow is undesirable (the parts we touch) has to be made out of a material which does not act as an electrical conductor, nowadays usually plastic,. The filament in a traditional light bulb is itself an electrical conductor. For the scientifically minded the function of an electrical conductor is to allow movement of the charged particles ‘electrons’, or (in the case of the cationic electrolyte of a battery, or the proton conductor of a fuel cell), ‘protons’, from one end of the wire to another.

All electrical conductors harbour electrical charges which are capable of moving along the electrical conductor. This occurs when an ‘electric potential difference’ exists between separate points on the length of the electrical conductor i.e. either end of a cable. The value of the ‘potential difference’ in the points on the electrical conductor is measured in volts and the flow of the charge in amps. This is what is meant when we refer to electric current ‘flowing’ along an electrical conductor. In most electrical conductors the direct current operates in a predictable relationship with the voltage as long as the temperature stays constant and the electrical conductor stays in the same shape or ‘state’ (state meaning solid, liquid, gas or plasma). All electrical conductors installed in the home for the purpose of conducting mains electricity are solid. Serious difficulties arise if any electrical conductor melts and turn into liquid and the prospects of a domestic electrical conductor evaporating into gas let alone turning into plasma are remote

Types of Electrical Conductors

Silver is the best electrical conductor but is expensive so copper is the most commonly used choice for wiring in standard installations. Silver however is used as a thin plating in situations (such as in satellites) to reduce losses at high frequency operation. Gold is sometimes used as an (albeit expensive) electrical conductor in some surface to surface connections. But some non metallic electrical conductors like plasma, salts, graphite and conductive polymers are also available. But unless the electrical conductor is made of superconducting substances all materials used as electrical conductors offer some electrical resistance. The result is that they all get warm.

The capacity of any material to act as an electrical conductor is restricted by its ability to dissipate the heat generated in the process of carrying the electricity. So best design of an electrical conductor has to allow for the temperature at which it’s expected to operate as well as the strength of the electric current. The capacity of an electrical conductor to transport the electricity is partly determined by how able it is to dissipate this heat. And this is a particularly important consideration in printed circuits where the electrical conductors are small and permanently operating in close proximity to one another. The heat generated in the electrical conductors could melt the tracks. Most materials which are good electrical conductors are also good thermal conductors although some of the non metallic materials such as graphite are exceptions to the rule.

Contrary to popular belief water itself does not conduct electricity. It’s the things dissolved in it which do. Pure water doesn’t conduct electricity at all and clean water is a poor electrical conductor. We do not however recommend that readers use a hairdryer in a bath. Although water itself has few of the necessary ions in it to enable it to conduct electricity the various impurities present do he job instead. Pure water is virtually unknown outside a specialist environment and only a very few invisible impurities dissolved in the water make the solution we still call ‘water’ into an electrical conductor. And if salts in particular are present electricity will flow through easily. Sodium itself is a highly reactive metal.

Electrical Resistance

The resistance of any electrical conductor is determined by the material it’s made of and its size. The resistance is inversely proportional to the cross sectional area. So a thick copper wire acts as a lower resistance electrical conductor than a thin copper wire. And a long copper wire is a more  resistant, electrical conductor than a short one. Complications arise where Alternating Current (AC) flows through the electrical conductor. The resistance exhibited will be higher than would otherwise be anticipated. And if two electrical conductors carrying electricity are positioned close to each other a ‘proximity effect’ results in further resistance. This effect can particularly marked in electricity sub stations.

The International Standard by which all electrical conductors are compared is ‘Annealed Copper’. Owing to the ease with which it can be connected clamped and soldered, copper remains the first choice for light load wiring.  It is however becoming more expensive. Oxygen Free High Conductivity Copper is available for use where an electrical conductor needs to be welded or brazed. By weight aluminium (but not anodized aluminium which is not conductive at all) is a better electrical conductor than copper and is sometimes used because it’s cheaper but its properties can cause problems in some buildings. Aluminium can form a resistive oxide in the connections causing the devices and terminals to heat up and can ‘creep’, resulting in the connections on connecting devices loosening. Aluminium also speeds up the rate at which connections loosen unless devices designed for use with it are used. Aluminium is however the most commonly used electrical conductor for high voltage transmission where it is combined with steel reinforcement.

An electrical conductor’s ampacity is the amount of current it is capable of carrying and is in proportion to its resistance. Higher resistance electrical conductors can carry a larger current. For a ‘bare’ electrical conductor, (one with no covering on the wiring) the limit is reached when the wire melts. But apart from fuses most electrical conductors in use are operated at a fraction of their capacity. And the safety limit on domestic wiring is determined by the temperature at which the plastic coating would melt, usually around 60 degrees Centigrade. If any material has an electric field applied to it and an electric current is induced which flows in the same direction the material it’s classified as an isotropic electrical conductor. If the current induced goes in the other direction the material is an anisotropic electrical conductor.