Download Basic Electrical Ideas Understanding electrical ideas is

Basic Electrical Ideas
Understanding electrical ideas is something that anyone who wants to be a radio
amateur really does have to take some time to come to terms with.
You may know already that it is small 'particles' called electrons that as they move
carry an electrical charge with them. To describe an electron turns out to be a
problem. We won't go into that here but do what scientists tend to do when they are
faced with such a situation. They describe the problem in terms of a 'model'.
Models are descriptions of perhaps complicated things in language and pictures that
are simple yet provide an insight into how things work. In radio, models can be used
to great effect to get over basic ideas that work in the real world without sowing the
seeds of misunderstanding. However, with all models it is important not to believe
that they are the last word in understanding, better models come along that
eventually give a deeper understanding. All models are imperfect and to some extent
have to be taken with a pinch of salt.
Electrons and Electric charge
Trying to describe electric charge is like trying to describe to a person that has been
blind from birth what the colour red looks like. We know it is a definite colour but to
appreciate it you have to be able to see it. What is known is that electric charge is to
be found as a property of the electron and other particles to be found inside atoms.
To start thinking about electrons it is perhaps easier to think of them as tiny particles
that orbit around the nucleus of the atom. In reality it is not quite so straightforward
but particles are close enough for beginning to understand radio.
We know that there are two kinds of charge that are known as negative and positive
because they behave differently, for example if we pass positively and negatively
charged particles between the poles of a magnet. They take oppositely curved paths.
Electrons have the type of charge that is known as “negative”. This affects how
electrons behave.
Electrons are so tiny (smaller even than atoms) and carry the smallest amount of
negative charge. Because the amount is so tiny we tend to think about great crowds
of electrons, which when added together have enough negative charge for us to think
about more easily and also measure with the instruments that we come across in
amateur radio.
The unit of electrical charge (i.e., what it is measured in) is the coulomb. It takes
about 100000000000000000000000 electrons (that's 23 zeroes) to have enough
charge to make a Coulomb.
All this seems perhaps a bit pointless and nothing to do with amateur radio, but yet it
has! If you look around the 'shack' of most radio amateurs you will see various kinds
of meters. A lot of these meters are designed to respond to 'electric current'. Electric
currents are essential in radio. Without them it would not be possible to speak to
people over the radio. Clearly an understanding of what a current is must be
important.
Electric Current
In a metal, some of the electrons are free to move around. They are so small that
they can easily fit between the metal atoms. In a wire the electrons move around at
random. Putting a battery across the ends of the wire causes the electrons to move
in a particular direction i.e., from the most negative end of the wire to the more
positive. This slow drift of the electrons is called “a current”, like the flow of water in a
river.
In the diagram the atoms in the wire are represented by the blue circles. The
electrons are small red dots with
arrows pointing in the direction
they are moving in. Notice that
the spaces in between the atoms
are large enough to allow the
electrons to drift between the
atoms.
+
_
Why do they move towards the
left side of the diagram?
When a battery is attached to a
wire it sets up what is called an
electric field within the wire. This
is a type of force field which
affects anything with an electric
charge on it. The electric field moves the electrons around the wires in the circuit. The
electric field direction is from the positive of the battery to the negative.
Do the electrons move quickly? Surprisingly the answer is No. The electrons drift along
quite slowly, usually only a few millimetres per second even with a large current. What
does travel very quickly around the circuit is the electric field that causes them to move.
The field travels at a speed very near to the speed of light. The result of this is that as
soon as a switch is turned on all the electrons in the circuit begin to move at once, even
though they move slowly.
Electric fields have a direction which runs from positive to negative, so the electrons are
forced in a direction that runs opposite to the electric field. They are pulled by the
positive and pushed by the negative sides of the battery.
Understanding what happens when a battery is used in a circuit is fine but in radio
circuits the type of current needed is quite different. To produce a radio wave we need
an electric current that is constantly changing direction, moving back and forth in a wire.
The electrons are not drifting along but vibrating back and forth rapidly.
Above, it was mentioned that the electrons actually drift along quite slowly. What would
happen if the battery were to be suddenly disconnected and then turned around so that
what had been positive was now negative and vice versa? The electrons would drift the
opposite way. If this were to happen over and over again, the electrons would not really
go anywhere, they would simply stay where they are and vibrate back and forth. This
leads to some very odd, but very important types of behaviour.
In a radio, rather than keep swapping the battery around, a special circuit is used to
make the electrons vibrate. This is known as an oscillator and is a most important type
of radio circuit. This will be returned to later. For now, it is convenient to consider the
electrons just moving in one direction.
Potential Difference
Whenever anything moves, energy is always involved. Trying to pin down exactly what
energy is, needs some thought. Perhaps a good place to begin is by thinking that
'energy is what makes things happen'. This isn't perhaps how a scientist would phrase
it but it will do for a start. By “make things happen”, this really does mean anything from
opening and closing your eyelids to producing light and heat in a star. Moving electrons
in a circuit requires energy. This energy can come from a battery or when the circuit is
driven from the electrical mains and a power supply.
Energy does have some important properties; you can't make it, and you can't destroy
it. However, you can pass it around and eventually fritter it away as heat. It also comes
in a number of different forms. Heat, light, sound and electricity are forms of energy that
we commonly come across.
When thinking about radio circuits there are some important features of energy that
have a direct bearing on what we can and cannot do.
Sources of energy
Batteries are convenient sources of energy that we can use in amateur radio. However,
a battery doesn't make energy, it stores it. The chemicals that react together in a
battery release energy when they react together. The chemical energy in the battery is
transferred to electrical energy in the wires.
What does this mean? The electrical energy appears in the first instance in Energy in radio
the form of an electric field in the wires. If the electric field surrounds the wave
electrons in the wire (and there are lots of these) it makes them move around
the circuit from the negative terminal of the battery to the positive. This drift of electrons
is called “A Current”.
The current is able to transfer this electrical energy to components such as bulbs that it
happens to flow through. In a radio, this energy transferred by the current is eventually
released as a radio wave from the antenna. The radio wave can carry this energy and
transfer it across the earth or out into space.
A radio receiver works in the opposite way. It collects radio waves and transfers their
energy back into a current. It is the current that can be used to reveal the messages
contained in the radio wave.
Antenna
Loudspeaker converts
electrical to sound energy
Resistance
Above, it was pointed out that it requires energy to make electrons flow through a wire
or any other kind of material.
All materials have a property called “Resistance” which is a type of opposition that the
material offers to the electrons passing through it. However, it has to be appreciated
that resistance is only one particular type of opposition to current: it results in the
production of heat in the material. There are other types of opposition that are not
caused by the conducting material itself but are caused by magnetic and electric fields
that can exist in certain types of components. You will meet these at Foundation and
Intermediate level.
Resistors are a particular kind of component that are designed to have a given
resistance. This is often shown by means of coloured bands around the component.
The various colours indicate the value of the resistor i.e., how much resistance it has
got.
What is resistance measured in? The unit of measurement is the Ohm. The symbol for
this is the Greek letter omega 'Ω'.
The resistors found in radios are usually from a few hundred ohms to several thousand
or even more.
The picture shows a few resistors with
coloured bands to indicate their value.
How to read a resistor colour code is
dealt with at Intermediate Level.
As yet there has been no attempt to try and explain exactly what resistance is, so it’s
back to a model.
When electrons flow through a material they require energy to push them along. The
reason for this is that energy is transferred to heat all the time the current passes
through the material. In very simple terms, it might be imagined that the electrons
'collide' with atoms as they wend their way from one end of a conductor to the other.
The collisions result in the electrons losing energy as heat. In an electric fire this effect
is what makes the elements glow red hot. It is important to realise that the real story is
more complicated than this and not required for the purposes of amateur radio. In a
high value resistor these collisions are more frequent than in a low value resistor
carrying the same current. If one resistor has more resistance than the other, it results
in greater heat production than the other.
Anthony G0WFG