This is part 1a of an amateur radio licensing tutorial course for those wishing to become licensed to operate transmitters, receivers and undertake electronic projects.
PRINT ME OUT NOW!
Back or Home
Everybody knows about atoms and electrons don't they? Well we could skip this part but of course we won't because you will likely learn something new.
All matter is comprised of molecules, which in turn are comprised of atoms, which are again comprised of protons, neutrons and electrons. A molecule is the smallest part of matter which can exist by itself and contains one or more atoms.
If you turn on a light switch for example you will see the light bulb (globe) glow and emit light into the room. So what caused this to happen? How does energy travel through copper wires to light the bulb? How does energy travel through space? What makes a motor turn, a radio play?
To understand these processes requires an understanding of the basic principles. For the light to glow requires energy to find a path through the light switch, through the copper wire and this movement is called electron flow. This is the first important principle to understand.
The word matter includes almost everything. It includes copper, wood, water, air....virtually everything. If we were able to take a piece of matter such as a drop of water, divided it by two and kept dividing by two until it couldn't be divided any further while it was still water we would eventually have a molecule of water.
A molecule of water, the smallest particle which can exist, comprises two atoms of Hydrogen and one atom of Oxygen - H2O.
An atom is also divisible - into protons and electrons. Both are electrical particles and neither is divisible. Electrons are the smallest and lightest and are said to be negatively charged. Protons on the other hand are about 1800 times the mass of electrons and are positively charged. Each are thought to have lines of forces (electric fields) surrounding them. In theory, negative lines of force will not join other negative lines of force. In fact they tend to repel each other. Similarly positive lines of force act in the same way.
The fact that electrons repel electrons and protons repel protons, but electrons and protons attract one another follows the basic law of physics:
Like forces repel and unlike forces attract.
Sounds a bit like a teenage romance - opposites attract.
When an electron and proton are brought in close proximity to one another it is the electron which moves because the proton is 1800 times heavier. It is the electron which moves in electricity. Even though the electron is much smaller, its field is quite strong negatively and is equal to the positive field of the proton.
If the field strength around an electron at a distance of 1,000,000th of a centimetre was a certain amount, then the field strength around an electron at a distance of 500,000th of a centimetre will be 1/4 as much. This is because the field decreases inversely with the distance squared. If an increase in one thing causes an increase in something else, these two things are said to vary directly. 2,000,000 electrons on an object produce twice the negative charge than 1,000,000 electrons would.
Since the electric-field strength of an electron varies inversely with the distance squared, the field strength a centimetre away would be quite weak. The fields surrounding protons and electrons are known as electrostatic fields. "Static" means stationary or not moving.
When electrons are made to move, the result is dynamic electricity. "Dynamic" means movement. To produce a movement of an electron it is necessary to either have a negatively charged field "push it", a positively charged field "pull it", or, as normally occurs in an electric circuit, a negative and positive charge (a pushing and pulling of forces).
There are more than one hundred different atoms or elements. The simplest and lightest is Hydrogen. An atom of Hydrogen consists of one electron whirling around one proton much like the moon revolving around the earth. The next atom in terms of weight is Helium (He) consisting two protons and two electrons. The third atom is Lithium (Li) with three protons and three electrons and so it goes on.
Some of the elements and their atomic weights are:
Hydrogen (1); Helium (2); Lithium (3); Carbon (6); Oxygen (8); Aluminium (13); Silicon (14); Iron (26); Nickel (28); Copper (29); Germanium (32); Gold (79); Lead (82).
Most atoms have a nucleus consisting of all the protons of the atom and also one or more neutrons. The remainder of the electrons (always equal in number to the nuclear protons) are whirling around the nucleus in different layers. The first layer of electrons outside the nucleus can only accomodate two electrons. If the atom has three electrons then two will be in the first layer and the third will be in the next layer. The second layer is completely filled when eight electrons are whirling around it. The third is filled when eighteen electrons are whirling around.
Don't think these electrons whirl around in some haphazard manner, they don't. The electrons in an element of a large atomic number are grouped into rings having a definite number of electrons. The only atoms in which these rings are completely filled are those of inert gaseous elements such as Helium, Neon, Argon, Krypton, Xenon and Radon.
All the other elements have one or more uncompleted rings of electrons.
Some of the electrons in the outer orbit of atoms such as copper or silver can be easily dislodged. These electrons travel out into the wide open spaces between the atoms and molecules and may be termed free electrons. It is the ability of these electrons to drift from atom to atom which makes electric current possible. Other electrons will resist dislodgement and are called bound electrons.
1.2 Conductors and insulators.
Materials consisting of atoms or molecules having many free electrons will allow an easy interchange of their outer-shell electrons, while atoms with only bound electrons will hinder any electron exchange
If the uncompleted ring of electrons is nearly empty, the element is metallic in character, being most metallic when there is only one electron in the outer ring.
If the incomplete ring lacks only one or two electrons, the element is usually non-metallic.
Elements with a ring about half completed will exhibit both metallic and non-metallic characteristics; carbon, silicon, germanium and arsenic are examples. These elements are called semi-conductors. Hah! that's where that name came from.
If the free electrons are numerous and loosely held the element is a good conductor. On the other hand, if there are few free electrons (as in the case the electrons in the outer ring are tightly held or bound) the element is a poor conductor. If there are virtually no free electrons, the element is a good insulator.
The semi-conductors of course exhibit conductivity midway between that of good conductors and good insulators.
Some examples of good conductors:
Silver; Copper; Aluminium; Gold
Some examples of good insulators:
Glass; Mica; Rubber
1.3 Sources of electromotive force (EMF):
Electromotive Force - the electron-moving force in a circuit that pushes and pulls electrons (current) through the circuit. To produce a drift of electrons, or electric current, along a wire it is necessary that there be a difference in "pressure" or potential between the two ends of the wire. This potential difference can be produced by connecting a source of electrical potential to the ends of the wire.
As I will explain later, there is an excess of electrons at the negative terminal of a battery and a deficiency of electrons at the positive terminal, due to chemical action.
Then it can be seen that a potential difference is the result of the difference in the number of electrons between the terminals. The force or pressure due to a potential difference is termed e.m.f.
An emf also exists between two objects whenever there is a difference in the number of free electrons per unit volume of the object. If the two objects are both negative, current will flow from the more negatively charged to the less negatively charged when they are connected together. There will also be an electron flow from a less positively charged object to a more positively charged object.
The electrostatic field, i.e. the strain of the electrons trying to reach a positive charge or from a more highly negative charge is emf.
It is expressed in units called volts. A volt can be defined as the pressure required to force a current of one ampere through a resistance of one ohm.
To make this easier to visualise, consider the water pressure (volts) required to pass a litre of water (current) through a copper pipe of a certain small diameter (resistance).
Also try and visualise water going through other pipes of varying diameters (smaller to larger in size). Either the water pressure required would vary or the volume delivered would vary, or both. You have just grasped the basics of ohms law, where E = Volts; I = current in amperes and R = reistance in ohms:
R = E/I ; I = E/R ; AND ALSO, E = I * R
This emf can be generated in many different ways.
(i) primary cells - composition, construction;
A very common method of producing emf is by the chemical action in a cell (strictly speaking two or more cells form a battery). Take a typical flashlight cell in common use in many small appliances. It most likely (in its rudimentary form) consists of a zinc can (the negative terminal), a carbon centre rod with a copper cap (positive terminal), and a black, damp, pastelike substance called an electrolyte between them.
These materials were selected from substances such that electrons a pulled from the outer orbits of the molecules or atoms of the positive carbon terminal chemically by the electrolyte and deposited on the zinc can. The massing of these electrons on the zinc produces a backward pressure of electrons, or an electric strain, equal to the chemical energy in the cell. The cell remains static at 1.5V until it is connected to some load.
Once connected, the electrons flowing through the circuit start to fill up the deficient outer orbits of molecules of the positive rod in a continuous stream, a bit like bumper to bumper traffic in peak hour. It is very important to understand that this motion produces an equal amount of current throughout the circuit at the same time.
Apart from the common carbon-zinc cell we also have alkaline cells 1.2V (twice the energy capacity), the mercury cell 1.34V (long working) and, the nickel-cadmium or Nicad 1.25V which is rechargeable.
Ionisation -When an atom loses an electron, it lacks a negative charge and is therefore a called a positive ion. In most metals the atoms are constantly losing and gaining free electrons. In this condition the metal is a good conductor. When gas is ionised under certain conditions this too becomes a good conductor. Examples would be lightning, neon lights and fluorescent lights.
Ionisation forms an important part of electronics and radio.
(ii) secondary cells - capacity (ampre hours);
The wet cell, lead-acid storage battery is in near universal use an automotive battery. The cell delivers about 2.1V and of course is rechargeable. This particular battery is made of coated lead plates immersed in a solution of sulphuric acid (sulfuric in USA) and water. The acid content of the dielectric varies with the state of the charge, which may be determined by measuring the specific gravity of the electrolyte. A reading of about 1.27 indicates a full charge while a reading of 1.15 or below indicates the cell needs recharging.
This cell may be fast charged for a 12V battery PROVIDED that care is taken to let escaping gases free themselves and PROVIDED the electrolyte temperature is below 50oC or 125oF.
Note that an automotive battery was specifically designed for rapid charge and rapid discharge. For example, starting an auto can cause currents well in excess of 500 amperes to flow (the reason jumper leads use thick wires). This is the principal purpose of the battery. It was never designed for continuous use such as running a radio or headlights in a stationary position for an extended period of time (hah! flat battery).
Similar types of cells are SLA (sealed lead acid) which may be used for emergency stand by power.
(iii) batteries - connection of cells, total voltage/capacity;
Now when two or more cells are added together, in series, they form the "official" battery. In fact the voltages add together. This is why some flashlights or portable radios comprise four cells to make up 6V (4 * 1.5V). An automotive battery comprises 6 cells in series so we get 6 * 2.1V = 12.6 but in actual practice we get 13.8V for a fully charged auto battery.
On the other hand if we put two batteries or cells in parallel we would get the same voltage but twice the capacity, i.e. twice the energy available to us.
It is this series/parallel combination for example which allows solar cells to deliver nominal votages and currents such as 6V @ 150 mA or whatever you require.
(iv) charging methods and rate of charging;
Now here is an area where you can buy a lot of arguments. Me, I'll take the coward's approach and give you an overview. My simple reasoning is this:
1. There seems to be as many
opinions on the tpoic as there are people.
2. What was considered all the rage a few years ago is now possibly incorrect.
Does that sound all too familiar?
Here goes (these are generalised statements):
I think you have probably had more than enough so far. Now! - PLEASE email me your pithy comments and your overall impressions. Totally, I think this course will run to over 600 printed pages. Consider buying a suitable binder.
NEXT AND IN COMING WEEKS:
(a) Cells (continued)
(v) internal resistance;
(vi) typical volts per cell;
(b) Mechanical generators/alternators - basic principles of conversion of mechanical energy to electrical energy;
(c) Other sources (e.g. photo-voltaic
or solar cells).
1.4 Direct Current (DC) circuits:
(a) Electrical units;
(i) unit of
(ii) unit of current flow;
(iii) unit of electrical pressure;
(iv) unit of electrical resistance;
(v) unit of electrical power;
(vi) relationship between units and calculations;
(ii) resistance in parallel;
(iii) resistance in series and parallel;
(iv) calculation of total value;
(v) practical resistors, power rating, colour codes;
(vi) power dissipation, formulae, calculations.
1.5 Electric and magnetic fields:
(a) Magnetism, permanent magnets;
(b) Electromagnets, relays;
(c) Electromagnetic induction.
1.6 Alternating Current (AC):
(a) Generation of sine wave;
(b) Sine wave amplitude values;
(i) root-mean-square (RMS);
(iv) peak to peak;
(c) Frequency and period relationship (calculations);
(d) Frequency and wavelength relationship (calculations);
(e) Harmonics and harmonic relationships (calculations).
1.7 Radiofrequency (RF) ranges - LF, MF, HF, VHF, UHF and SHF.
(a) Unit of capacitance;
(b) The capacitor - concept of energy stored in an electrostatic field;
(i) factors which determine capacitance;
(ii) dielectric losses;
(iii) working voltages and characteristics of dielectrics;
(iv) types of capacitors and typical applications;
(v) capacitors in series (calculations);
(vi) capacitors in parallel (calculations);
(c) Alternating voltage applied to a capacitor;
(i) phase relationships of voltage and current;
(ii) capacitive reactance - formula (calculations);
(iii) effect of frequency and capacitance on reactance;
(d) Ohm's Law in capacitive circuits (calculations);
(e) Time constant of a CR network - formulae - calculations - applications.
(a) Unit of inductance;
(b) The inductor - concepts of induced EMF - self inductance - mutual inductance;
(i) factors which determine inductance;
(ii) types of construction;
(iii) types of core, permeability;
(iv) types of practical inductors and typical applications;
(v) inductors in series (calculations);
(vi) inductors in parallel (calculations);
(c) Alternating voltage applied to an inductor;
(i) phase relationships of voltage and current;
(ii) inductive reactance - formulae - calculations;
(iii) effect of frequency and inductance on reactance;
(d) Time constant of an LR network - formulae - calculations - applications.
1.10 Alternating current circuits containing L, C and R:
(i) circuits containing reactance and resistance - formula - calculations;
(ii) Ohm's Law for combined LCR circuits;
(b) Tuned circuits;
(i) resonance - concept - formula - calculations;
(ii) series tuned circuit - impedance at resonance;
(iii) parallel tuned circuit - impedance at resonance;
(iv) quality factor (Q);
(v) L/C ratio;
(vi) resonance curves and selectivity.
(a) Principles of operations;
(b) Construction, types and typical applications;
(ii) eddy currents;
(iii) copper loss;
(d) Turns ratio - formula - calculation;
(e) Voltage ratio - formula - calculation;
(f) Current ratio - formula - calculation;
(g) Power ratio - formula - calculation;
(h) Efficiency - formula - calculation;
(i) Auto-transformers - applications;
(j) Characteristics of different types of core materials;
(k) Shielding - methods, purpose.
The piezo-electric effect: characteristics of quartz crystal and typical
The detailed syllabus I wish to use is crown copyright. Some details can be found here:
Back or Home
Copyright © 1999 Ian C. Purdie. All Rights Reserved.
This Website and all the content contained herein are protected under national and international copyright laws.
Email to: Ian C. Purdie
This site was entirely written and produced by Ian C. Purdie
Created:4th July, 1999 and Revised: 21st August, 1999