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An electric current is the flow of electricity (stricly electrons) through the wires and other equipment that makes up an electrical circuit. Flow of electricity is similar in many ways to the flow of water in pipes, with two important distinctions.
- Electricity must flow in a complete circuit. When considering the flow of water, we often have situations where a complete circuit is not obvious. For example, when using a hosepipe to water a lawn, we don't have to arrange for the water to make its way back to the water supply. In electrical and electronic circuits, the designer and installer almost always have to arrange a complete circuit.
- The flow of electricity can change direction very quickly. In satellite communications systems, electrical currents in the equipment are changing direction at a rate of millions of times per second.
Figure 1: Circuits in electricity and central heating
The diagram above shows the correspondence between a water flow system and an electrical system. In (a), water flows from a pump, through a boiler, through a radiator, and back to a pump. In (b) electricity flows from the battery, throuugh a microphone, through a loudspeaker and back to the battery. Note that the symbols used in this diagram (for the battery, microphone and loudspeaker) are standard, and will be understood by all electronic engineers.
So in the electrical system, the battery plays the same role as the pump does in the water system: it provides the impetus to drive the flow around the system. As in the water system, the flow is in one direction only. You will notice that real batteries have their terminals labelled `+' and '-' (positive and negative). We think of the direction of current flow as feing from the positive terminal towards the negative terminal. The example shown is of a direct currrent system; the flow of current may increase or decrease, but never changes direction.
In the electrical system, wires play the same role as the pipes do in the water system. They carry and contain the flow. Generally thicker wires can sustain a greater electrical current, in the same way that thicker pipes can sustain a greater water flow, all other things being equal.
Electrical current can be measured; the size of the current is expressed in amps. A device for measuring current is called an ammeter (not an ampmeter; electrical engineers are too lazy to pronounce the `p'. The sybmol for amps is `A', so `20A' means `20 amps'.
The amp is a good unit of measure for electrical engineers; for example the electric current supplied to a kettle might be 10A. However, in computing and communications we normally measure currents in milliamps (symbol mA). One milliamp is 0.001 amps.
2.2 Voltage
We can also give a number to the `effectiveness' of the battery. By `effectiveness' I mean its ability to produce a current. Technically, this effectiveness is called the `electromotive force', but you will probably only see this expression in old physics textbooks. These days we normally call the effectiveness of a battery or other electrical supply its voltage. This is because electromtotive force is measured in volts. The symbol for volts is `V'.
Note that it is incorrect, and misleading, to talk about the `power' or `current' of a battery. The power depends both on voltage and current. It is, however, correct to talk about the maximum current of a battery. This is the largest current it could generate, usually for a very short time. For example, a standard `AA'-size battery can generate about 2A for a short time; this is its maximum current. In all normal circumstances, the current flowing through a battery depends on the equipment to which it is connected, not on the battery itself.
Most common batteries have voltages of 1V-2V. The unit `volt' is about the right size for communications work. Most computer equipment requires an electrical supply at 5V or 12V or both. However, it is still not uncommon to deal with voltages in millivolts (mV). A millivolt is 0.001 volts.
The voltages used in communications fall somewhere in the middle of the range of voltages encoutered in the natural world. At the top end of the range we have the voltages that give rise to lighting strikes (millions of volts); at the bottom we have the voltages that arise when electrons jiggle about in wires (millionths of a volt, or smaller).
Note that there is no absolute measure of voltage. What is important is the voltage difference between two points. For example, when we speak of a 1.5V battery, we mean that the voltage difference between its two terminals is 1.5 volts. When electrical engineers say that the electrical supply to houses (in the UK) is 240V, they mean that there is a voltage difference of 240V between the `live' and the `neutral' wires.
A device for measuring voltage is called a voltmeter.
2.3 Resistance
So we have voltage, the ability of an electrical supply to generate a current, and we have current itself. If we know one of these, in any given system, we can have a stab at working out the other. In a very large number of materials, especially metals, current flowing in tha material is proportional to the voltage across it. This means that if the voltage is doubled, the current will double as a result. These materials are called conductors, as they tend to support a flow of electricity. Then we have insulators, materials that tend not to support a current however big the voltage. Common examples are air, pure water, and most plastics. If a steadily-increasing voltage is applied across one of these materials, no current will flow until the voltage reaches a certain point. Then an enormous current will flow. The point is called the breakdown voltage. A lighting strike occurs when the voltage difference between a cloud and the Earth's surface exceeds the breakdown voltage of air. The voltage is extremely large, and when a lightling strike does occur it will generate an enormous current: thousands of amps. This is why it is a bad idea to wave an umbrella during a thunderstorm; thousands of amps flowing through your body will spoil your whole day.
Note that the breakdown voltage is smaller if the size of the piece of material gets smaller. This is one of the factors that limits how small electronic components can be made.
For conductors, the relationship between voltage and current is very simple: it is given by Ohm's law:
V = I x R
Where `V' is the voltage between any two points, `I' is the current that flows between those points, and `R' is the resistance. The resistance of a material is its tendency to restrict (or resist) the flow of electricity. If resistance is higher, then a larger voltage is required to cause a given current to flow. Resistance is measured in ohms. The normal symbol for ohms is the Greek letter `omega'. However, as this document is being presented on the Wordl-Wide Web, and many Web browsers do not have the capability to display Greek letters in English text, I will follow the more modern convention of using the letter `R' to stand for `ohms'. So 100R = 100 ohms.
Any electrical equipment, including wire, has a resistance. Except in the dark basements of physics laboratories there is no known substance that has zero resistance. This means that no current, however, small, will flow in a system unless there is some voltage present. It is important to understand that all electrical and electronic components have a resistance, and it is very often necessary to know what that resistance is. Also, the resistance of electrical cable, although small, accumulates when the run of cable is long. For example, if a telephone cable has a resistance of 0.01R (0.01 ohms) per metre, then a kilometre of this cable as a resistance of 10R. This is an appriciable resistance compared to some electronic components, as the following example will demonstrate.
2.4 Example
The following example shows how we can use the concepts of voltage, current and resistance, along with Ohm's law, to work out why a communications system does not work. 
Figure 2: Ohm's law example
Please take the time to follow this example, even if you aren't keen on arithmetic. There is nothing more complicated than multiplication and division in it, honestly.
In the figure above, a piece of equipment is transmitting digital signals to a receiver, which is some distance away. The transmitter generates a voltage of 1V to represent a binary `one', and no voltage (zero volts) to represent a binary `zero'. What voltages should the receiver accepts as meaning a `1' or a `zero'?
First, it is important to realize that the receiver should accept a range of voltages for each of `zero' and `one'. This will prevent a false reading being obtained when electrical interference is present. Common-sense would suggest that, as the transmitter sends 1V for a one and 0V for a zero, the receiver should accept that anything above 0.5V is a 'one' and anything below 0.5V is a zero. So far so good. Now any electrical interference will have to be very large to cause an error. Will this system work? Consider the case where the cables is 1 kilometre (1000 metres) long. The total amount of cable is 2000 metres. As the resistance of the cable is 0.01R per metre (see above), the total resistance in the system is 2000 x 0.01R for the cable, plus 100R for the receiver. This is a total resistance of 1020R. So when the transmitter is sending a `one', the current flowing is 1/120 A, or 0.0083A. We know this because Ohm's law says V = I x R, so I = V / R. V = 1 volt, and R = 120 ohms. So what voltage is there across the receiver when the transmitter is sending a `one'? By ohm's law again, with a current of 0.0083A and a resistance of 100R, the voltage is 0.83V. Since this is greater than 0.5V, this will be read correctly as a binary `one' by the receiver.
Now consider the case where we extend the cables from 1 kilometre to 10 kilometres. The resistance in the system is now 20000 x 0.01R for the cable, plus 100R for the receiver as before. So the total resistance is 300R. Thus the current flowing when the transmitter sends a `one' is 1/300 A, or 0.0033 A. Now the voltage measured across the receiver is 0.0033 x 100, 0.33 V. Oh dear. This is less then 0.5 volts, so will be read incorrectly as a `zero'. This example illustrates what can happen when systems are designed without paying attention to basic electrical theory.
In this case the way to solve the problem is to adjust the receiver (if possible) so that any voltage over, say, 0.1V is a `one' and below 0.1V is a zero. This will then correctly read the voltage of 0.33V as a `one'.
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