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Put some small pieces of kitchen foil on the workbench. You can use small pieces of cork, instead. Rub a plastic pen with a dry woollen cloth. Rub hard for ten or twenty seconds. Hold the pen a few millimetres above the pieces of foil. They jump up and stick to the pen. Some of them may jump up and down again several times.
The reason that the pieces jump is that they are attracted by electrons on the pen. Rubbing the cloth on the pen has made electrons from the cloth transfer to the pen. We say that the pen is charged with electrons. It has an electric charge.
Some other things can be charged by rubbing. Rub a balloon with a cloth (or against your clothes). Then place it in contact with the wall of the room. It does not fall down to the floor but stays where you put it, on the wall. The electric charge has produced an electric force that holds the balloon against the wall.
KINDS OF CHARGE
You have found that:
Two charged polythene strips repel each other. They try to stay apart.
A polythene strip and an acetate strip attract each other. They try to come together.
It seems that the charge on acetate is different from that on polythene, so:
There are two kinds of charge
Two charged polythene strips repel each other, so:
Like charges repel
Two differently charged strips attract each other, so:
Unlike charges attract
POSITIVE AND NEGATIVE CHARGE
The two kinds of charge are called positive charge and negative charge. These names do not mean that positive charge has something that negative charge does not have. They just mean that the charges are of opposite kinds.
Rubbing a polythene strip with a cloth transfers some of the electrons from the atoms in the cloth on to the strip. Electrons have negative charge, so the strip becomes negatively charged. Also, the atoms of the cloth have now lost some electrons. This makes the cloth positively charged.
Rubbing an acetate strip with a cloth does the opposite. It removes electrons from the strip, leaving it positively charged. The cloth gains electrons and becomes negatively charged.
Positive and negative charges always attract each other. They try to come together. When you rub the cloth on the plastic, you separate the negative charge from the positive. It takes energy to pull them apart when they are trying to come together. This energy comes from the muscles of your arm.
Electrons are too small to see, even with a powerful microscope.
Electrons are too light to weigh. You need 1 000 000 000 000 000 000 000 000 000 000 electrons to weigh 1 kg (an amazing fact that you do not need to remember).
The most important fact about electrons is that they carry negative electric charge. The charge on a single electron is extremely small. But, if you have enough of them (as on the pen or the charged polythene), you can show the force that their charge causes. There are lots more things that we can do with electrons, as you will find out as you work through the book.
ELECTRONS AND ATOMS
Matter is made up of molecules of many different kinds. Molecules are made up of one or more atoms. Atoms are made up of electrons (negatively charged), protons (positively charged) and neutrons (uncharged).
The simplest possible atom consists of one electron and one proton. The proton is at the centre of the atom and the electron is circling around it, in orbit.
With one unit of negative charge on the electron and one unit of opposite but equal charge on the proton, the atom as a whole is uncharged.
The electron is circling at high speed around the proton, like a planet orbiting the Sun. There must be a force to keep it in orbit. In the case of a planet the force is gravity, the attraction between the masses of the Sun and the planet. In the case of the electron the force is the electrical attraction between oppositely charged bodies. The experiments on pages 3–4 demonstrated this.
OTHER KINDS OF ATOM
There are more than a hundred different elements in nature, including hydrogen, helium, copper, zinc, iron, mercury and oxygen, to name only a few.
Each element has its own distinctive structure, the atoms being made up of fixed numbers of electrons and protons.
In spite of these differences, all elements have the same basic plan. There is a central part, called the nucleus, where most of the mass of the atom is concentrated. The nucleus is surrounded by a cloud of circling electrons.
Atoms other than hydrogen have more than one proton and also some neutrons in the nucleus.
The protons in the nucleus give the nucleus a positive charge — one unit of charge for each proton. The number of electrons in the cloud equals the number of protons, so the cloud as a whole has an equal but negative charge which balances the charge on the nucleus.
The electrons of an atom are on the outside. They can be removed by friction, heating and electric fields. This is how we obtain the supply of electrons to use in the electronic circuits and devices described in this book.
Some readers may have heard of quarks and other sub-atomic particles. Detailed studies by atomic scientists have discovered that atoms are actually made up of several more sorts of particle. In electronics, however, the only particle we need to know about is the electron.
Some substances let electric charge flow through them. These substances are called conductors.
One of the best-known conductors is copper. It conducts so well because the electrons of copper atoms are able to escape easily from the atoms.
In a piece of copper, the atoms (large spheres in the drawing above) are arranged in regular rows and columns, called a lattice. The electrons that have escaped from the atoms (small circles) are able to wander about freely in the space between the copper atoms.
If we connect a battery to each end of a strip of copper, its negative terminal supplies electrons to the copper. Its positive terminal removes electrons from the other end. They are attracted by the positive (opposite) charge.
The flow of electrons along the copper strip is called an electric current. The flow is from negative to positive.
DIRECTION OF FLOW
As explained above, an electric current is a flow of negative charge (electrons) from negative to positive.
In electronics, we usually think of the current as flowing from positive to negative. Although this is not what actually happens, most people like to think of it in that way. This idea of a current flowing from positive to negative is known as conventional current.
In the rest of this book, when we say 'current' we mean conventional current, flowing from positive to negative.
The best conductors are metals. Copper is the most commonly used conductor because it conducts electric charge better than any metal, except silver. But silver is too expensive to be used. Copper wires are used in almost all electronic equipment. The tracks on a circuit board are also made of copper.
The next best conductor is aluminium. This is often used in power lines, because of its lightness and cheapness. It is not as strong as copper, so a few strands of steel wire are included when making the cable.
Carbon is a non-metal but it has important uses as a conductor. It does not conduct charge as well as the metals do. Rods of carbon are used for making certain kinds of electric cell. Carbon is also used in making resistors (p. 43).
Solutions of salts in water are reasonably good conductors. Much of the human body consists of such solutions, so the body is a reasonable conductor of electricity. This is why we must be very careful when handling electrical equipment and working with electricity in the laboratory. Even quite a small current through a part of the body can paralyse the nerves and may kill you.
Electricity can also cause unpleasant burns.
These substances contain few or no free electrons, so they are not able to conduct electric charge. We sometimes call them non-conductors.
Insulators included substances such as:
Many types of plastic, including polyvinyl chloride (PVC), which is used for insulating electrical cables and wires.
Glass and many ceramics.
Cells and Batteries
Cells are the most compact sources of electric power. They produce electricity as a result of chemical actions inside the cell. When the cell is made, it is packed with chemical compounds that are ready to react together. As current is drawn from the cell the chemical reaction occurs. Current can be obtained from the cell until the chemicals have completed their reaction and no more of the original compounds are left.
There are several different types, based on the chemical reactions that drive them:
Type of Features Examples of Uses Cell
Zinc- Cheapest type. Produces Electric torches, carbon 1.5 V. The voltage falls handlamps, doorbells, slowly during the life of security alarms. the cell. May leak when old and damage the equipment by corroding its metal parts.
Alkaline Holds 3 times more Electronic equipment charge than a zinc- such as clocks, remote carbon cell of the same controllers, electronic size, but is more toys. Also used for the expensive. same purposes as zinc- Produces 1.5 V. carbon cells and gives Can supply a larger superior performance. current than a zinc- carbon cell. Voltage steady during life of cell; falls sharply at the end (keep a spare handy). Does not leak.
Zinc- Heavy duty cell in As for zinc-carbon cells. chloride standard sizes. Produces 1.5 V. Holds more charge than zinc-carbon cells and is cheaper.
Excerpted from Electronics by Owen Bishop Copyright © 2011 by Owen Bishop. Excerpted by permission of Newnes. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Posted May 13, 2011
Very sorry I bought this. Consists exclusively of fluffy up in the air theory with absolutely no real life application.
I could kick myself for not reviewing the sample more closely. I was taken in by the intriging chapter titles. They are all fluff.
Costs WAY WAY TOO MUCH.
1 out of 1 people found this review helpful.Was this review helpful? Yes NoThank you for your feedback. Report this reviewThank you, this review has been flagged.
Posted October 18, 2012
Posted January 3, 2013
Posted October 18, 2012
I awoke laying on the cold white floor of my cell. A single shaft of sunlight streamed through the solitary window on the east side of my concrete prison. I rose slowly to a sitting position and stared groggily at the wall across from me, the wall on which i had been counting my days as prisoner with hatch marks made with a fork i stole from a meal tray. I yawned and stood shakily to my feet and stumbled over to the window and looked out onto the glorious pure white world i would never be part of again. I squinted in the early morning sunlight that bathed the world outside my cell in a warm golden glow. I closed my eyes and tried to remember the days before i became a Condemned, playing in the washed out streets with my sister. Eating a rare delicacy, ice cream, with my father before HE became a Wanderer. It was a bittersweet feeling that lasted only a moment, before it was interupted by a horrible ear splitting, blood curdling shreik. The cry was followed by a series of pleas for mercy and the a despairing moan. I stood there frozen with fear my heart racing. Slowly i turned my head and looked at the hatch marks on the wall. My fear was confirmed, it was my hundreth day in this world of white concrete nothingness, my last day on Earth, the Day of Feasting....**Thanks for reading! Tell me what you think and if i should continue!~Candace/Amanda**Was this review helpful? Yes NoThank you for your feedback. Report this reviewThank you, this review has been flagged.