It has become obvious from many of he questions in 'Interchange' that one of the most confusing aspects of electricity is that of polarity. In this short series I hope to attempt to clarify this complex and often confusing subject.
In order to understand what electric current actually is, we need to get down to absolute basics.
All materials, including ourselves, are made up of atoms. These atoms in turn consist of a nucleus, surrounded by a cloud of sub-atomic particles called electrons, Electrons, which orbit the nucleus much as communications satellites now orbit the Earth, are not only the carriers of electricity but their interactions are the cause of all chemical reactions. There is a major difference between the atoms of electrically conductive materials such as metals and the atoms of insulating materials like plastic or glass. In insulators, the electrons are bound closely to the nucleus, whereas in metals the electrons in the outer orbits of the atom are only loosely held and are referred to as 'free' electrons. These free electrons are able to wander from one atom to another and it is their movement which causes the effect we know as electricity.
In your vehicle's battery, the chemical reaction between the acid and the lead plates causes the negative plate to have more free electrons than the positive plate. This difference in the number of free electrons is known as the 'potential difference' or more commonly the Voltage of the battery.
When the two plates are connected via a load to form a circuit, the extra electrons in the negative plate will flow around the circuit to the positive plate. This is measured in Amps and is known as electric current.
If you've followed this article up to now, you are probably saying "But I thought current flowed from Positive to Negative". This is a serious anomaly which requires some explanation. When early scientists investigated the phenomenon of electricity, they realised that something was flowing from one electrode to the other, and tried to determine what it was. Experiments with metal rods immersed in a conducting liquid (or electrolyte) showed that metal was removed from one rod and added to the other. This is the process we now call electroplating. It was therefore deduced that the metal carried the electricity through the electrolyte and that the direction in which the metal moved was also the direction of current flow.
The apparent source of the current was named the Positive pole and the destination was named the Negative pole.
We now know this to be incorrect, but by convention we still refer to current as flowing in this direction. This is therefore referred to as 'conventional current'. In fact history has now come full circle in that the major carrier of electricity is now considered to be the hole left by an electron, rather than the electron itself.
To explain this strange concept, the usual analogy is that of a doctor's waiting room. Several patients may be sitting in seats waiting to see the doctor. When the first patient is called in, he leaves an empty seat. The next patient then moves into that seat leaving his seat free, which is then occupied by the next patient in line, and so on until the last seat is left empty. The point of this analogy is that the patients move very slowly, but the empty seat appears to move rapidly from one end of the room to the other. This transfers to electricity in that we know that current flows very quickly, yet electrons themselves move quite slowly from atom to atom. The empty seat in this case represents the 'hole' which is the carrier of electric current.
For many purposes polarity is unimportant. Conventional light bulbs, for example, work by the heat generated when electricity is passed through a resistance and as such the direction of current flow is irrelevant.
There are other items however where the polarity matters. Some electric motors, for example, will turn in the wrong direction if the supply is reversed.
Modern electronics in particular requires the polarity to be correct. Semiconductor components such as transistors depend on the properties of individual electrons, and as such the direction in which they flow is crucial.
Appliances such as radios and portable television sets contain many semiconductor components and can be seriously damaged if the polarity of the supply is reversed. One of the more important components as far as vehicle electronics is concerned is the semiconductor diode. This component has the function of passing current in one direction only, and is therefore extremely useful in battery charging circuits.
Diodes form an integral part of the modern alternator, where they allow current to flow into the battery for charging, but prevent the battery from discharging back through the alternator coils.
If for any reason the battery were to be connected in reverse, current would be able to flow through the diodes, possibly causing severe damage to the alternator.
Great care must therefore be taken when charging a battery in situ, and particularly when jump starting one vehicle from another.
So far we have only talked about direct current or D.C., where the current constantly flows in one direction.
Direct current was used in the early days of domestic electricity supplies, but had one major problem. Sending the electricity down long cables resulted in serious losses of power, which meant that each town required it's own generating station.
It was an American named Tesla who realised that there was a solution to this problem. That solution was alternating current or A.C.
The advantage of A.C. is that using a combination of wire coils it is possible to transform the power to a higher or lower voltage as required. We know this arrangement of coils as a transformer.
Using transformers it is possible to generate electrical power at a very high voltage and at these high voltages a large amount of power can be supplied by a relatively small current. By reducing the current in this way electrical power can be carried over long distances by reasonably sized cables and with very little loss.
This high voltage is transformed back to the nominal 230 volts A.C. at a sub-station close to the consumer.
It is only A.C. that has made networks such as the National Grid possible.
In A.C. power systems, instead of the current flowing continuously in one direction, it reverses many times per second. In Europe this reversal of current takes place 100 times per second, which means that the current completes a full cycle fifty times every second. The voltage plotted against time describes a curve known as a sinusoid, as it is similar to the Sine function in mathematics. In Europe the nominal voltage is now 230 volts R.M.S, the R.M.S. suffix meaning that the power generated is equal to that provided by a 230 volt D.C. supply. The A.C. supply actually swings between plus and minus 325 volts.
It would appear at first sight that if the current is constantly reversing, the polarity must be unimportant. Unfortunately the A.C. mains supply is more complex than it appears. To make the generation and distribution of the mains supply even more efficient, the power is generated using what is known as a three phase system.
The full complexity of three phase power is beyond the scope of this article, but essentially there are three 'live' poles, known in the U.K. as the Red, Yellow and Blue phases. In addition to the three phases, there is a fourth 'pole' called Neutral. This fourth pole is connected to Earth at some point, normally at the local substation.
In industrial applications it is normal to take power between two of the Live phases to provide a supply of approximately 400 volts.
Domestic supplies are however taken between one Live phase and Neutral, and this is where polarity becomes important.
The Live wire in a domestic installation carries the high voltage whilst the Neutral line stays at or near Earth potential at all times. In practice, variations in the power being take from each phase cause the Neutral voltage to vary, but it normally stays within a couple of volts of Earth.
Most equipment sold in the U.K. only has single-pole switching. This means that the on-off switch is only connected in the Live wire to the equipment. This is perfectly safe provided everything is connected correctly.
Reversed polarity will mean that the switch is now in the Neutral line, and even if the equipment is switched off, the Live line is still connected.
This sets up the situation that even though the equipment is apparently turned off, inadvertently touching either the Live or Neutral wire will result in an electric shock.
Perhaps more seriously, most installations only have fuses in the Live side of the supply. In the event of a fault, even if the fuses blow, they will not isolate the equipment from the mains, allowing the fault condition to continue with possibly disastrous results. In a domestic situation, a polarity reversal is unlikely to occur unless some rewiring has been carried out. In a motor-caravan however, you are totally dependent on the power hook-up being correct, and that is by no means certain.
In the U.K. virtually all sites use the 16 amp 'ceeform' plug for mains hook-ups. These are an industrial power connector, and there should be no problems with polarity if the hook-ups have been correctly wired.
In most other European countries, standard 'continental' plugs are the norm, and these have the disadvantage that they are reversible.
In order to ensure that the plug is connected correctly, some form of polarity indicator is required. This will be the subject of the next article.