Over 150 years ago Michael Faraday and others
developed most of the theory of how electricity
works.
Which was done by using the fact that all
of matter comes in two distinct types, we
now call charge, where like charge types repel
and opposite charge types attract.
(these types were categorised much earlier
still, over 260 years ago by Benjamin Franklin
as plus and minus types)
Just after Franklin designated the polarity
types, The force of attraction between charges
was worked out in 1767 by a brilliant French
physicist by the name of Charles-Augustin
de Coulomb, which is now called coulombs force
law after him.
It is one of the fundamental forces of nature.
It tells us that for any two chosen charged
particles, the strength of the force between
them is proportional to the product of the
charges and inversely to the square of the
distance they are apart.
Through experiment it was also discovered
that charge comes in discrete one sized packets
only, which as it turns out, is exactly equal
to the amount of charge that is on an electron,
which are the outer oribital particles of
atoms.
So all collections of charges are multiple
units of this single electron charge.
This elemental charge, is equal to 1.6*10^-19c.
The Protons within the nucleus of atoms also
carry this exact same size charge but with
the opposite plus polarity.
So that every +charged proton in the nucleus
of an atom is balanced by the same number
of -charged electrons that spin around it.
Its this strong charge attraction of the tiny
distances between these two particles within
atoms that keeps most of our universe neutrally
charged.
So to harness this fundamental force of attraction
between unlike charges all we need to do (as
engineers and hobbyists) is produce large
separated collections of minus and plus charges
and then by using the attractive coulomb forces
between them, build useful circuits.
For building such circuits, there happens
to be something very helpful about copper
and other metal atoms.
Their outer valance shell electrons are very
easy to dislodge.
Just the heat of room temperature alone with
copper for example, can knock off a significantly
large number of electrons from the parent
atoms.
All copper metals then, have many of these
dislodged electrons within them.
Some describe this as the free electron sea
or cloud.
These free electrons (free as in free from
their parent atoms) are all evenly distributed
throughout the metal because of their mutual
charge repulsions.
So this property of having huge numbers of
free electrons in the metal allows us to use
the metal copper as an ideal material for
the conduction of electrical energy.
The traditional fundamental base unit for
all electrical calculations is the coulomb
name after Charles-Augustin de Coulomb.
It represents 6.24x10^18 discrete elemental
electron charges.
(This is a huge number of charges)
With the coulomb defined we can now describe
the most important definition in electronics,
like so.
The number of coulombs that pass through any
cross sectional area of a wire within a period
of one second, is a rate of charge flow given
in coulombs/second.
It has its own symbol “I”.
Now one coulomb passing in one second is also
a standardised unit called the Amp.
But I prefer to stick with c/s.
So for example, a current 'I' of 2 amps is
the same as saying 2 c/s.
Using c/s instead of Amps is just so easier,
when thinking and learning about charge flow
and energy.
Current can be thought of as a flow of charge,
a little bit like water currents are the flow
of water molecules.
Water molecules tend to flow from areas of
high gravitational potential energy to low
gravitational potential energy.
Electric currents flow from high electric
potential to low electric potential.
And the greater the difference between the
high and low potential, the more current that
flows.
I'll cover the definition of electric potential
in another lesson.
Now lets think about current directly from
the charges point of view.
We are going to connect the terminals of a
battery together with a length of copper wire.
Now if we look at this from a negatively charged
electrons point of view, we say that the higher
potential is when the electron charges are
at the negative terminal and the lowest potential
is for charges when they've reached the plus
terminal.
So then immediately the connection is made,
due to the coulomb forces between like charges,
the negative terminal will release its closest
and highest potential layer of electrons out
of the battery into the wire.
Likewise at almost the same instant in time
the next layer of free electrons that are
part of the copper wire will also be repelled
by this first layer entering the wire and
then the subsequent layers will repel each
of their immediate neighbours to also move
and so on and so on.
So that this wave of nudged layers of neighbouring
charges propagates instantly to the final
layer of electrons at the other end of the
wire, which enter into the attracting positive
terminal.
So these charges are all moving from a high
potential to the lowest potential.
The time taken between the first nudged charges
from the minus terminal over to the final
nudged charges on the plus terminal is not
completely instantaneous, but almost.
For copper its about 98% the speed of light.
Note: This is how fast energy propagates.
We are not referring to charge velocity here.
Now this layer of electrons from the external
copper wire that enter via the positive terminal
into the battery, these charges will all now
be at zero potential energy.
Now we can think of the internal chemistry
of the battery as moving all these charges
through the battery against the repulsive
coulomb forces of the opposing battery terminals.
The work done to do this then is supplied
by the batteries electrolytes and hence gives
all the charges that get moved through the
battery back up to a higher potential energy,
ready again to nudge their neighbouring charges
through the external circuit.
This will then cycle around continuously as
a steady flow of charge (eg current) until
either the wire is disconnected or the batteries
chemicals become exhausted.
So batteries are in fact energy providers.
They do not produce any extra charges, they
simply cycle or transport energy that is all
used up by the circuit, by the use of this
instantaneous nudging of all the neighbouring
charges.
These are charges (as electrons) that already
exist within the copper wire and the charges
within the battery, continually cycling around
and around the circuit as a current.
Just think of the charges as simply a convenient
medium for the transportation of the batteries
energy.
Energy is in fact transported within all electrical
circuits close to light speed, via this basic
mechanism of neighbouring charges repelling
each other.
For each complete cycle between the terminals
of a battery, all of the initial potential
energy given out by the battery per charge
is used up, expended within the wires and
components of the circuit.
If we wanted to, we could just as easily have
talked about this cycle of current in terms
of the movement of positive charges instead
of negative ones.
We can look at it from either point of view
because in all circuits minus charges move
one way and plus charges at the same time
will move in the opposite direction.
Here we just chose to describe it from the
electrons point of view.
Its just a lot easier to describe electrons
moving than it is to describe the absence
of them (eg holes) moving the other way.
This can be hard to fathom when you first
come across it so, this analogy can help,
you can think of this complete current cycle
(the internal charge movement within the battery
and the external charge movement through the
external wire) as one single cylindrical cylinder
filled up with touching marbles (charges),
with a roller (the battery) at one end.
This roller as our battery is moving the stack
of touching marbles through the circular cylinder
around and around.
We will always make sure that our roller maintains
a steady marble current (For example 5 marbles
per second), by turning the roller's handle
at a constant rate of revolutions.
This roller will keep on moving the touching
marbles around all the while it is touching
the side of the marbles that get rolled by
it.
So this means when one marble is moved they
all will move at the exact same moment in
time and at the same marble flow rate (or
marble current).
So, lets say then that a marble is moved by
the roller out through the plus side (eg.
A positive charge leaves the positive terminal)
Now since all marbles are touching that means
a marble will at almost the exact same moment
in time enter the minus side of the roller.
(eg.
A different positive charge at the other end
will enter the minus terminal).
Now all the while the roller rolls at a steady
rate, this current of revolving marbles will
continue.
So, a push on the first marble will get that
force transported in almost instant time to
the end marble which, lets say is 1000 miles
away.
This is very much like how energy is transported
through wires using repelling charges instead
of touching marbles.
If all the marbles were perfectly rigid this
energy transport would (just like charges
within a wire) be close to the speed of light.
What sometimes is surprising, when you first
find out, is the fact that the speed of individual
electron sized charges, average forward progress,
through the wires of all circuits is very,
slow.
Their progress is faster (although still slow)
through thinner wires than wider wires.
All charges in wires at room temperature or
above, will be vibrating and so will be moving
fast in random directions, but their average
forward progress is between about 2, to 20cm
per hour.
So, lets now pretend our external copper wire
consists of a fat section of wire into a thin
section back into a fat section.
Now we can appreciate due to the cross sectional
area of these different sections of wire,
that far more charges will be present in the
wider sections than the thinner section.
Now because all the charges in our copper
wire are again similar to touching marbles,
if at one end we have a current of 1 coulomb
per second, that will mean that the other
end must have the same number of coulombs
moving each second as well (eg the current
is same throughout the circuit).
This means that the thinner wire which we
know holds a lot less charges per cross sectional
area will have its charges moving faster because
the coulomb per second rate is the same everywhere
in the circuit (eg pushing the first layer
of marbles at a given rate of marbles per
second or current will mean last layer must
also have that same rate of current.)
So for any fixed current value individual
charges will move faster through thin wires
than through fat wires.
Current through thin wires is analogous to
having a thin pipe connecting two buckets
of water.
We push down the layer of water in one of
the buckets at some fixed rate of push, with
some kind of plunger.
All of the water molecules will move together
at the given plunge rate, so this means the
second bucket must also fill up at exactly
the same rate as this one is emptying.
For the connecting pipe in the middle the
water molecules will be moving though much
faster then the molecules do in the two buckets.
So with this thin connecting pipe, the water
is forced to flow through it faster, because
the whole system maintains the same volume
of water flowing per second throughout.
Eg Push down at 1 cubic cm per second and
the other bucket must fill up at 1 cubic cm
per second.
Since the cross sectional area of the pipe
is smaller than the cross sectional area of
the buckets the water will move faster through
the pipe to maintain the same cubic cm per
second volume flow rate.
From this analogy, it is also interesting
to note, that because the charges are moving
faster through thin wires, they will be hitting
the sides far more often.
Which means more of the batteries energy will
be lost through these extra collisions.
So you can now also begin to see the reasoning
how this gets us towards introducing a concept
of resistance.
I'll cover both voltage (which is the energy
per charge used by a circuit) and resistance
in more detail in another lesson.
So, there are then three important points
to remember to fully grasp the concept of
charge and energy conduction in copper wires
and through batteries.
1.
Coulomb/seconds => called the current.
Represents a huge number of individual charges
(6.241x10^18) passing through a cross sectional
area per each second.
2.
Speed of an individual charge => on average
speed is very slow about say 2cm/hour but
if for example wire is half as thin then speed
will go up accordingly eg 4cm/hour.
This nicely gets us to think about the concept
of resistance.
3.
Energy movement => Due to consecutively spaced
layers of slow moving charges that can all
repulse their neighbours due to the coulomb
forces between them.
Allows for the conduction of energy through
the circuit in an instant.
The action of touching marbles is an easy
analogy for this.
