AEA Some elementary electrical instruction updated 18/08/2007
01 The purpose of this essay is to acquaint any reader with a small amount of understanding of electrical theory. Ordinary readers of newspapers are constantly assailed with total rubbish. The System International (SI) system of units is regularly abused; the commonest example being the use of a lower-case "m" when an upper-case "M" should be used. All sorts of hyperbole and total rubbish is used when talking about high voltages. I hope to allow a thinking reader to become a little more knowledgeable. If you are an electrical engineer, study what I have written below and shout if you see any error. Call me on: CDCNottm@AOL.com
02 If the reader of this essay aspires to become a scientist. one of the first rules is to be a sceptic. Listen to what you are told but never totally believe it. There have been some hard theories over the centuries that seemed to be correct at the time but later research showed the world that what used to be FACT, wasn't. Do this to me, only half-believe what I say, and look for flaws in my reasoning. As regards units of measurement, the National Physical Laboratory at Teddington publish all sorts of pieces of paper to enlighten enquirers. And you are going to have to be a real genius to prove them wrong.
03 A volt A volt is the unit of electrical pressure. Voltage is the force that drives current through an electrical conductor, such as a piece of copper wire. Sometimes "voltage" is given the title "electro-motive force", or EMF. Voltage DOES NOT GO THROUGH anything, despite the way that many newspapers talk, It is simply the unit of force, or the pushing effect of electricity.
04 An amp An "amp" is the common word used, but it is an abbreviation for an "ampere". The amp is the unit of electric current. It is the current that GOES THROUGH a wire, and it needs a force (voltage) to push it through the wire
05 An ohm An ohm is the unit of resistance to the flow of current. It is the property of an electric conductor to resist the flow of current through the conductor. It is analogous to friction when considering water flowing through a pipe. When a current flows through a conductor, several things happen. The most basic thing is that heat is generated. A magnetic field is also generated by the current (more on that later). One of the most common uses of heat that is generated by the flow of current, is the FUSE. A very thin wire is used as a safety device to stop excessively high currents flowing when a fault occurs. The thin wire has resistance and when an excessive current flows, the fusewire heats up and its resistance suddenly rises higher than its original value, The higher the resistance the more heat is generated. Positive feedback ensures that the wire fuses (melts) rapidly. When the wire melts, the flow of current ceases. And the fuse wire is contained so that the molten metal doesn't set fire to anything. With extremely high fault currents, the wire does more than melt -- it explodes. Hence the bang you may hear when a fuse blows.
06 A watt is the unit of power. "Power" is the rate of dissipating energy. So power times time gives energy. A watt-hour is a unit of energy. If you multiply the voltage (in volts) across a circuit, by the current (in amps) going through the circuit, you get watts. I must now mention that there are other things than wires that pass an electric current, so I will now use the word "conductor". A wire is a conductor, but so are many other things. The human body, a bucket of water, a piece of damp ground; all these things are "conductors". You can have good conductors and you can have bad conductors. The very best conductor is a piece of silver; the thicker the piece of metal the better it is as a conductor. The worst conductor is a vacuum. The worst conductor, of course, is the best insulator. A conductor has "conductance". An insulator has extremely low conductance. There isn't a word for the quality of insulation like there is for the quality of conductivity. Perhaps there should be one.
07 A joule A joule is a basic unit of energy. "Energy" is what costs you money. There is a specific amount of energy in a gallon of petrol or a cubit foot of gas, et cetera. There are other units of energy that were used in the past, but I will avoid those. It is, of course, possible to convert one unit into another. A joule is the same as a watt-second (i.e. a watt being dissipated for one second. A "watt-hour is a watt being dissipate for one hour, or ten watts for a tenth of an hour). A kilowatt hour is one watt being dissipated for a thousand hours, or a thousand watts for one hour. I use the word "dissipate" as that is the best way of thinking about what happens to the energy that leaves a conductor when an electric current is passed through it. Generally when energy dissipates, the conductor gets hot and loses that heat into the air. I say "generally" because there are other ways of transferring energy. Energy cannot be made or destroyed, it can only be changed from one form into another.
08 At this point I will introduce the
multipliers (or dividers) of electrical units:
k (kilo) this lower case letter indicates a thousand times the
quantity of the unit
M (Mega) this upper case letter indicates a million times
m (milli) this lower case letter indicates one thousandth
µ (mu) this Latin letter indicates a millionth (generally only
used in electronics)
G (giga) this upper case letter indicates one thousand million times
Newspapers like the Telegraph and most of the others abuse the system and use
"m" as a so-called multiplier meaning a million. They should us "M".
Even that august journal Quality World (who should know better) abuse the system.
There are a number of other letters that are used that I will not introduce here.
09 This essay is only intended to cover electrical theory, but as the whole of engineering science can be tied together, some of the electrical units tie in with units of other disciplines. You will see on you gas bill these days, the kilowatt hour (kWh) being quoted. And as energy is what you pay for, two kilowatt hours of juice will cost twice as much as one kilowatt hour. It is possible to convert kilowatt hours into other units like therms. (A therm is the unit that the Gas Board used to use to denote the energy available in a quantity of gas. That unit is no longer used.
10 A calorie I strongly suggest that the use of this unit is avoided. There are several different calories and it is better to forget about the word. But like a lot of pseudo-science, you will see the word used in newspapers. Again I say, avoid newspapers if you wish to improve your scientific education. In things like global warming, science correspondents are generally to be relied on, but if you read a story of someone getting electrocuted, the science is almost always rubbish. I'll cover electrocution below.
11 A horsepower This is another unit that is a bit risky to use. It originated as a measure quoting the strength of a motor. In the electrical field, a horsepower is 746 watts. I don't know what size of horse actually develops one horsepower. Certainly a pony and a Clydesdale generate enormously different amounts of power. It is said that a normally fit man can produce about one seventh of a horsepower for a period of time. So you can get a very rough idea of the strength of a horsepower.
12 A warning I have just referred to my Collins English Dictionary; a superb book covering the world-wide use of our language. But I saw the words "energy or power" in a definition. And as I keep stressing, energy and power are different things. So even high academia can sometimes get it wrong.
13 Some calculations
Probably the simplest electric circuit consists of a battery and a single
conductor. When you start to do calculations, a single piece of conductor
is generally called "a resistor". When a circuit is drawn on paper, the
wires that are shown joining the resistors, are assumed to have no resistance.
There are, of course, many other components with different names. But I
will just concentrate on resistors. And as the name implies, these
components have resistance Everything has resistance, even the very best
conductors, such as a piece of silver or a copper wire.
14 The most basic of electrical
calculation is referred to as Ohm's Law. It is a very simple law, but it
the most important one to know. For complicate historical reasons, certain
letters were used to represent the volt, the amp and the ohm. I will not
use these old symbols; I will use "V" for "volt", "A" for "amp", and "R" for
"ohm" (Resistance). I've avoided "O" for ohm as the letter and the
numeral are so easily confused.
Ohm's Law states that: A = V / R (the old letters were I = E / R).
I have used a slash to indicate division rather than the symbol
"÷",
as that symbol is beginning to become obsolete. Now when two symbols
are drawn without any operator between them, they are considered to be
multiplied, It is unnecessary to use an "x" (An "operator" is things
like + - x and ÷ . The name also
applies to many other mathematical symbols use in higher maths)
15 I'll now introduce the words "serial" and "parallel". These words are used to show how several resistors may be connected together. Two resistors in series are connected so that the current that flows through the first resistor is the same current that flows through the second resistor. Think of an electrical circuit as being like a flow of water, with the wires being the pipes and the resistors being small sealed tanks that allow water to flow in and out of them. This analogy is a useful one that is often used to visualise the flow of electric current. There are a lot of parallels between the flow of water and the flow of electric current.
16 If two resistors are in parallel, they are normally show side by side with the wires connected in a manner that allows you to see that the current splits into two parts, each part going through one of the resistors. The current that goes through each resistor depends on its ohmic value. When the current leaves the two resistors, it unites into one to return to the battery.
17 A circuit is a closed ring with the current that leaves the battery being the current that returns to the battery. For historical reasons the direction of current flow has changed. Whether you consider it flowing from positive to negative, or negative to positive, doesn't really matter. Take your choice. The actual current being electrons (sub-atomic particles that have a negative charge) leaving and entering atoms, the direction of flow could be argued as "negative to positive". But it doesn't really matter. Forget about it! What is important is to know that what leaves the battery is the same as what re-enters the battery.
18 As I can't draw pictures with this software package I will have to use words. Nowadays most electrical components are drawn as boxes. In the past resistors used to be shown as a zigzag line with the value being indicated alongside the zigzag. In modern circuits the ohmic value is drawn inside the box. If you have a battery and two resistors connected in series, you will have a line leaving the battery leading to a box that is connected to the next box, and a further line will show the return path back to the battery.
19 To know how much current that the
battery has to deliver to two resistors in series, it is necessary to calculate
the equivalent resistance of the two resistors in series. You simply add
the value of the resistors to get the overall value of the circuit. The
wires are considered to have no resistance. If the internal resistance of
the battery is to be shown, it is generally indicated as an external resistor
with a note to indicate that it is not a real resistor but just a representation
of the internal resistance of the battery. If you want to calculate the
equivalent resistance of two resistors in parallel, the formula is
1/R = 1/R1 + 1/R2
where R is the equivalent value
of the the two resistors in parallel
and R1
is the value of the first resistor, and
R2
is the value of the second resistor
20 Multiple series/parallel resistors use the same rules for deducing the overall value of the circuit. Work in steps with each group at a time, replacing two or more resistors with one that is the equivalent of the group. If you are clever with algebra, you could work out a complicated formula for every complicated circuit
21 I'll briefly talk about DC and AC. Direct Current (DC) flows in one direction. Alternating current (AC) continually reverses direction. Batteries provide DC. Both DC and AC have their specific uses. Most houses these day are fed with AC because transformers cannot work with DC.
22 I said above that an electric current generates a magnetic field. If a current passes through a wire, a magnetic field surrounds the wire. When indicated on paper, a curled arrow is used to show the magnetic field AROUND the wire. Now think about a coil of wire with a current passing through it. As each single strand has a field around it, the many strands alongside each other form a magnetic field that adds up in strength. The overall effect is that a magnetic field passes through the centre of the coil.
23 If a piece of magnetic material, like a rod of iron, is placed within the coil, the magnetic field becomes concentrated within the iron. I talk about "iron" as it is the most common magnetic metal. There are other materials that are occasionally used for very special reasons, like nickel or cobalt. Even liquid oxygen can be magnetised. But iron is by far the most common magnetic material. Even different sorts of iron have to be considered for different uses. Soft iron magnetises easily but doesn't hold its magnetism, while a special hard iron alloyed with a small quantity of other metals, like cobalt, is used to make a permanent magnet.
24 I said I'd mention electrocution. To be electrocuted, a current has to flow through the heart muscle. When a sufficiently high current flows through the heart, the muscle fibrillates. This is an uncontrolled beating of the heart that leads to physical damage to the muscle. If a high current was passed between the ears and the brain was fried, you may be dead, but strictly speaking you would not have been electrocuted. Rather an academic use of words I must admit.
25 If you ever see an electrician working on a live installation, he is likely to have one hand in his pocket. This is because a common way of getting a serious shock, is for current to flow from one hand, up the arm, across the chest, and down the other arm. And, of course, the heart is en-route across the chest.
26 This sparks would probably be standing on an insulated surface so no current would flow down through his feet to earth. He may know that his dry leather shoes are a good insulator instead.
27 In almost all electrical distribution systems these days, one of the two poles will be earthed. As the body of the earth is a very good conductor, this provides a "live" and a "neutral" at the house. Think of the substation at the end of your road as if it were a battery. I know a battery is DIRECT CURRENT (DC) and the mains power is ALTERNATING CURRENT (AC), but for the purposes of thinking about what happens, you can ignore that difference. At the house, if you touch the neutral conductor, you will feel nothing. If you touch a live wire and are connected to earth (even by a very bad conductor like slightly damp shoes whilst standing on concrete), YOU WILL GET A SHOCK. That shock could be lethal!
28 In most hardware shops these days you can buy a "Powerbreaker". This is a clever device that was developed in the 1960s that gives a great deal of protection against the most common way of electrocution. You plug the Powerbreaker into the 13A socket and plug your lawnmower (or whatever tool you are using) into the Powerbreaker. The principle of the device was known back in Victorian times, but a practical one would have been about a foot cubed. (It is surprising to many people how sophisticated electrics were in those early days. Remember that London had an underground railway in 1900, and there's a lot of technology required to build and power a tube train, let alone the rails and the tunnel.
29 The Powerbreaker is more correctly known as an RCD (residual current device) or an RCCB (residual current circuit breaker). The gadget monitors the current that goes down the live wire to the tool, and compares it with the current that returns back up the neutral wire. If the two currents differ by more than a very small amount, the gadget trips the supply.
30 Electric lawn mowers are a case in point. It is all too easy to mow the cable and connect the body of the mower to the live wire. And without some protection, the operator standing on wet ground is at serious risk of electrocution. But with the RCD, any fault current means that the live and neutral currents differ because some of the current passing down the live wire goes to earth instead of returning via the neutral wire. This difference causes the device to trip. The gadget works so fast that you may get a shock but only be aware that the power has failed. A truly marvellous safety device.
31 Many modern electrical installation have a RCD built into the consumer unit so that half of the house wiring is protected. The lighting circuits are generally not included in the RCD protection, so that when the device trips, all the lights don't go out. Some larger buildings has an RCD on every individual 13A circuit so that only that small part of the building goes dead in the event of a fault.
32 Give me a call on CDCNottm@AOL@com if you have any questions.