ELECTRONS IN ACTION -
explanations
and experiments
A theory is acceptable
while it seems true. When contradictory evidence shows it, or
part of it, false it must be modified or scrapped.
At best a theory is an indifferent tool to explain what we cannot
comprehend in the first place.
Avoid acceptance of a theory out of sentiment, belief, faith or
because someone - who appears important - says you should accept
it.
FLUID
TO ELECTRONS
Apparatus used under
identical conditions gives identical results to those of someone
who did the same experiment elsewhere in space or time. When you
repeated Gray`s experiments you got the same results as a man who
died 261 years ago in London, England.
Theories arose from observations:- Effluvia -
A communicated electric virtue - Fluids, and today - Electrons
and ionic activity.
From the 18th
through the 19th centuries the Electricians thought
their experiments implied the existence of an invisible fluid.
Some textbooks today use
hydraulics to explain electrical terms. It is a useful analogy
and produced advances in the field of physics.
From experimental results
with electrolytes Stoney, in 1874, suggested the existence of a
particle he named THE ELECTRON.
In October 1897 J. J.
Thomson, investigating discharge through gas, discovered the
particle of negative electricity.
This was at a transition
point in Natural Philosophy because results from experiments
could no longer be explained by existing theory.
Thomson retained Dalton`s
idea of solid atoms like billiard balls. His picture was of a
lump of positive electricity with negative electrons embedded in
it like currants in a bun. There was something comfortable and
stable in this idea.
Suddenly, this vanished as Rutherford, in 1911, revealed atoms to be mostly empty space. An atom had a nucleus of positively charged protons around which tiny negative electrons moved in orbits like planets around a sun.
Bohr suggested that
electrons moved in fixed orbits. Movement from one orbit to
another either involved absorbing or emitting energy. Hence -
Quantum theory.
New experimental results
showed the electron behaved either as a particle or a wave.
`We can
scarcely describe such an entity as a wave or as a particle;
perhaps as a compromise we had better call it a `wavicle`.`
Eddington - The Nature of the Physical World (1928).
Seventy years on it is still `such stuff As dreams are made on`. Shakespeare - The Tempest.
It was a good idea but now
we need something better.
You may be pleased to learn
that we can stay with old Rutherford`s atom for our fun with
electricity and magnetism!
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ELECTRIC
CHARGE
Everything in the known
world is made from one or more of 92 basic substances called
elements.
The smallest piece of an element is called an atom.
Because there are 92 kinds
of element there are 92 kinds of atom.
Each element has a symbol -
Cu is Copper, O is Oxygen, Ne is Neon and so on.
Each element has an ATOMIC NUMBER.
This informs us of the
number of negative (-)
electrons and positive (+)
protons.
For example, Hydrogen H - Atomic Number 1 -
has 1 electron and 1 proton.
Because the negative charge
on an electron is exactly matched by the positive charge on a
proton an atom is electrically neutral.
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A substance may be made of
one kind of atom e.g. a piece of gold; or a diamond made of
Carbon atoms.
Substances made of
combinations of elements are called compounds e.g. water is a
compound of Hydrogen (H) and Oxygen (O).
The smallest piece of a
compound is a molecule.
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FOUR
STATES OF MATTER
Substances are affected by
their heat content.
Heat is a form of energy.
SOLID
At the lowest level of heat
energy the substance will be a SOLID. Its structure is a lattice
of component atoms called a crystal in which the atoms vibrate
but do not move out of position.
MELTING
Add heat and the atoms
vibrate more and more until the crystalline structure begins to
break down i.e. the solid melts and becomes a liquid.
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EXPERIMENT
- TO GROW SALT CRYSTALS
Warm a cupful of water.
Stir salt into the water until no more will dissolve.
Pour the salt solution into a flat saucer.
Put it where it is dry.
It may now take days or longer, depending on humidity of the air and temperature, to work.
The water evaporates and
leaves crystals of salt.
Examine them through a magnifier.
If you have a microscope
put a tiny drop of solution on a glass slide. Watch the edges of
it through the microscope to see crystals form.
[Salt (NaCl) is a compound of Sodium (Na) and Chlorine (Cl) atoms.
When you put it in water
the molecules came apart - dissolved - into Na atoms and Cl atoms
and formed an ionised solution which is called an ELECTROLYTE.
It reached a critical point
beyond which no more salt molecules came apart.
As water molecules in the
saucer acquired heat energy from the surroundings they escaped
from the liquid - became gaseous. The critical point was already
in existence so Na and Cl ions had less energy to keep them in
solution and they joined together into NaCl - a molecule of salt.
As more water molecules
left, so more salt molecules collected, and a crystal began to
grow.
The faster the water
evaporation the more, but smaller, the salt crystals.]
Crystals are beautiful and
have a great aesthetic appeal. The study of crystals is
CRYSTALLOGRAPHY.
LIQUID
The atomic components of a
liquid are able to move around. In its liquid state a substance
will take on the shape of its container and is able to freely
change its shape when poured into another container.
Vaporisation - when liquid becomes gas.
Adding heat energy to a
substance in its liquid state makes the components move faster.
They begin to escape from the surface.
GAS
In its gaseous state a
substance can take in more heat energy and occupy more space.
When confined this is manifested as an increase in pressure.
PLASMA
In the states of Solid,
Liquid and Gas, atoms remain intact.
If sufficient energy is
added to a gas its atoms break up. This is called ionisation. A
plasma is an electrically conducting confusion of detached
electrons and protons.
IONS
Ionisation is a condition
in which some electrons attach to an atom - making it
negative OR detach from an atom - making it positive.
These `affected` atoms are called ions.
THAT WAS:-
SOLID TO LIQUID (melting)
LIQUID TO GAS (vaporisation)
GAS TO PLASMA
(ionisation)
TAKE AWAY HEAT - that is REMOVE ENERGY - AND THE PROCESS REVERSES:-
PLASMA TO GAS (neutralisation)
GAS TO LIQUID (condensation)
LIQUID TO SOLID
(freezing)
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CONDUCTORS
Atoms of substances which
conduct electricity have electrons, in the outer orbits, which
may be detached - by heat energy or electrical repulsion.
These detached electrons
are called FREE ELECTRONS.
Imagine free electrons
inside a conductor as being similar to a mist of negative charge.
As a whole, a conductor is
electrically neutral. The negative mist of free electrons move
randomly amongst neutral and positively charged atoms.
THE
MOVEMENT OF FREE ELECTRONS UNDER THE INFLUENCE OF AN ELECTRIC
CHARGE IS CALLED ELECTRICAL CONDUCTION. THE MOVEMENT IS CALLED AN
ELECTRIC CURRENT.
INSULATORS
In an insulator there are
few free electrons to permit electrical conduction. Therefore there is little or no current flow
in an insulator. Electric charge either of ions or electrons on
the surface of an insulator remain stationary i.e. STATIC
ELECTRICITY.
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Combinations of insulators
and conductors direct electrical charge along paths to undertake
some task.
E.g. A copper wire is
covered with a plastic insulator.
CIRCUITS
In the experiment in which electrical conduction was discovered the free electrons went to one end of the conductor - Gray`s rod in the cork and, later, his insulated packthread; similarly in our experiment with a wire.
Charge was detected by the
electroscope or a pile of foil or paper bits.
To use electrical
conduction, for example, to give us light from a filament, we
need to keep the free electrons moving. In other words, to keep
them in circulation.
To do this we need to
maintain an imbalance of charge so the free electrons can
continuously move from negative to positive.
This imbalance can be
produced chemically in cells and batteries. Or mechanically by
rotating a magnetic field in the vicinity of a conductor in a
power station, a portable generator, car alternator or bicycle
dynamo.
The free electrons must be
able to move away from the imposed negative charge through the
conductor, do their work, e.g. heat a lamp filament, and reach
the attracting positive charge.
There must be a complete
path, A
CIRCUIT, along which
free electrons can move.
EXPERIMENTAL
SUGGESTION -
There are hundreds of circuits .
To make a start and get some experience AND stay safe:
use a couple of torch cells to make a battery in a cardboard tube (3 volts).
Use 2.5v or 3 volt bulbs and covered wire.
If you can get lampholders
and switches, fine, but you can connect to the bulbs by wrapping
wire around the body and fixing a drawing pin in a board for the
other connection. Switches can be made by opening a paper clip
into a long S-shape.
S
Anchor one wire and the
clip at one end with a drawing pin into a board and the other
wire under a drawing pin so that when you press down the long S
it makes contact.
You can try bulbs one after the other in a series circuit; OR alongside each other in a parallel circuit; and in series parallel.
With the battery connected
and switch on remove various bulbs to see if the circuit is
broken and so stops the flow of free electrons.
A TESTER
Connect one wire from a bulb to one terminal of the battery.
Connect a wire to the other terminal of the battery.
Connect a wire to the other side of the bulb.
As yet the circuit is not
complete.
Touch the ends of the two
free wires to light the bulb to check that the tester is OK.
You can use it to sort
insulators (which do not conduct) from conductors.
Suspect fuses - switch off any
power supply then
remove the fuse- it can then be checked. Tester bulb stays off =
fuse blown.
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FRICTIONAL
EXPLANATIONS
It is not known how, when a
plastic tube is rubbed, charge is generated. Mechanisms have been
proposed but it remains an unsettled question.
Surfaces are very
complicated structures. Fractal Geometry of Surfaces gives a
sense of the mysterious akin to that of the `wavicle`.
But we do know that rubbing
a plastic tube with a cloth generates an electric charge with one
polarity on the tube and its equal but opposite polarity on the
cloth.
TUBE CHARGE
A negative tube charge is a surplus of electrons, somehow removed from the cloth.
A positive charge is due to electrons leaving the tube and going to the cloth.
Polarity of charge depends
on the materials used for the tube and the cloth.
The movement of free
electrons in a conductor helps explain what happens in the
experiments.
Let`s look at just one to
help you work out the others:-
THE
ELECTROSCOPE
The tube is an insulator -
hence no free electrons - so generated charge stays put.
Tube Negative
Near the electroscope, free
electrons in the feather move away from negative on the tube and
repel free electrons in the cotton thread into the stand and into
the earth.
The electroscope is now
positive. So we, now, have a negatively charged tube near to a
positively charged feather.
Opposite charges attract so
the feather moves to the tube.
Tube Positive
The tube attracts free
electrons in the feather, thread, stand and from the earth.
This time the feather is
negative.
Opposite charges attract so
the negative feather moves to the positive tube.
Electroscope Insulated
The same thing, except no
free electrons are conducted into or from the earth. The
electroscope becomes positive at one end and negative at the
other depending on tube charge polarity.
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HOW FAST IS ELECTRICITY?
Faster than your
reflexes as I discovered in childhood when I tried to beat an
electrified cattle fence.
A thought experiment
Imagine a length of
pipe just over 2cm inside diameter. The pipe is full of marbles
2cm diameter. Each marble is touching its neighbour.
A marble represents a free electron. You push a marble into one
end of the pipe. All move along the pipe a distance of one marble
diameter 2cm and a marble falls out of the other end.
The pipe is 186,000 miles long (it is a thought experiment -
forget any practical difficulties). Over the period of a second a
marble is pushed into the pipe.
Each marble moves 2cm per
second. 186,000 miles away a marble falls from the pipe.
Similarly, free electrons
drift at the rate of a few centimetres per second but the
electrical effect travels at the speed of light.
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FRICTIONAL
- LIGHTS AND SPARKS
The end of the metal rod
has a sharp edge. It could be turned to a conical point in fact.
A current of free electrons moves towards or away from the charge
on the plastic tube.
Charge is concentrated at
the sharp end of the rod.
There are always ions in
the atmosphere - from local and cosmic radiation.
Ions near the rod are
attracted to or repelled from the end by the intense collected
charge. (Direction depends on the polarity of ions and rod).
Ions collide with air
molecules and a chain reaction of ion production ensues. This
ionisation emits electromagnetic radiation. Some of the light you
see as a glow or brush discharge. The ultra violet aids more
ionisation and the radio frequencies you can hear on a nearby
radio.
EXPERIMENT - Radio e/m radiation
Rub the tube near a radio. (DON`T TOUCH THE AERIAL - YOU COULD DAMAGE SOLID STATE COMPONENTS)
Listen to a range of
crackles as air is ionised by charge concentrated on the tube.
AN
ELECTRIC SPARK
The ionisation of molecules
increases as the number of free electrons increases when the tube
charge is increased.
Think of the situation of
rubbing the tube and increasing its charge. More and more free
electrons are moving away from this increasing charge to the far
end of the rod.
(The same is true if the
tube is positive and free electrons are moving away from the end
of the rod so increasing the positive charge there).
The early stages of
ionisation produce glows. As the number of ions increases the
glow is brighter and streaks of light appear in it - this is
CORONA and BRUSH DISCHARGE (ST. ELMO`S FIRE).
Ions collide with ions and
air molecules thus ionising them - the process is called an
AVALANCHE.
Free electrons on the move
from negative to positive - electrical conduction - is an
electric current. Likewise, ions on the move - electrical
conduction - is an electric current.
In the corona, conducting
pathways of charged ions materialise. Once a conducting path
appears other ions will rush to travel along it. The path is more
intensely ionised and most of the ions now travel along it This
avalanche of ionisation is called a SPARK.
Crackles from sparks are
sonic waves produced by heated air.
This gave Gray, Franklin
and others the feeling that the greater sparks and crackles of
thunderstorms may involve electricity.
THE NEON
I will assume the tube
charge is negative.
The free electrons in the
wire near the tube concentrate on a bulb electrode as they are
repelled by the charge on the tube.
The few ions already in
existence in the Neon gas will cross the gap between electrodes -
i.e. set up an electric current.
Free electrons on the other
electrode are now repelled - down the wire along your skin and
into the earth.
Ionisation of gas molecules will increase with current flow. When the molecules of a gas are ionised they emit light.
(The light is
characteristic - a fingerprint - of the gas, and spectroscopic
analysis enables gases to be identified by their spectral
`fingerprints` - in stars for example).
If you, now, assume the tube charge to be
positive you can work out that the same process occurs but with
electrons moving in the opposite direction.
CONCLUSION
If you have worked as far as this you have gained a lot by your own efforts.
From now on when you come
across things electrical you will have a good idea as to what is
happening. Congratulations!!!
There are lots of books to
take you on from here and to start putting some numbers on it.
My aims are to get you
started or explain a few things to you who are already on the
way.
