The Atom

1
The Atom
Someone once posed the following question to the
great physicist Richard P. Feynman Suppose some
future disaster (nuclear war, asteroid impact,
etc.) returns humanity to stone age conditions.
If a single concept could be passed on to the
survivors to help them restart civilization, what
should that concept be? Feynmans answer was
Everything is made of atoms.
2
The Atom
An atom consists of a nucleus at the center of
the atom, and one or more electrons orbiting the
nucleus. The nucleus consists of one or more
protons, each of which is positively charged, and
may include a number of neutrons. Neutrons have
no electrical charge, but are otherwise identical
to protons.
Electrical charge is a property of matter, like
mass. There are two kinds of electrical charge,
called positive and negative.
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Each proton has a positive charge of 1.6 x 10-19
Coulombs, or 1.6 x 10-19 C Each Electron has a
negative charge of 1.6 x 10-19 C
The mass of a proton or a neutron is 1.672 x
10-24 g The mass of an electron is 9.11 x 10-28
g
3
The Atom
The smallest amount of charge which can exist is
the charge on one electron or one proton. The
next smallest amount which can exist is the
charge on two electrons or protons. Electrical
charge can only exist in integer multiples of the
charge on a single electron or proton. Charge is
said to be quantized.
Each atom normally has exactly as many electrons
in its electron cloud surrounding the nucleus as
it has protons in its nucleus. The negative
charges of the electrons exactly cancel the
positive charges of the protons, making the atom
electrically neutral.
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4
The Atom
Sometimes an atom can lose one of its outer
electrons (called valence electrons), giving it a
net positive charge equal to the charge on one
proton. Such an atom is called an ion. It is
also possible for an atom to acquire an extra
electron, giving it a net negative charge. This
is also called an ion.
When table salt, NaCl, is disolved in water, its
molecules dissociate into positively-charge
sodium ions (Na) and negatively charged chlorine
ions (Na-)
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These ions can move around in the solution,
because it is a fluid. This is how electricity
is conducted in a liquid.
Positively charged ions are called anions, and
negatively charged ions are called anions.
Pure water does not conduct electricity, because
it contains no ions.
5
Electrical Conductor
Consider a piece of copper. Each copper atom has
29 protons and 29 electrons. It is a metal, so
the atoms are arranged in a regular structure
called a crystal lattice. It is as if each atom
were connected to its nearest neighbors by
invisibile rods. No atom can move, because it is
locked into the crystal structure. They cannot
move around within the piece of copper, but each
atom can vibrate in its place.
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
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Cu
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Conductor
Copper has a special characteristic, which it
shares with silver and gold. It has one electron
in its outermost electron shell. This electron
can be knocked loose from its atom very easily.
When this happens, the resulting free electron
can move freely within the crystal lattice,
until it gets captured by another atom which has
had its electron freed.
Electrical conduction in solids is always a
result of free electrons moving within a crystal
lattice. The more free electrons




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there are in a given volume, the better the
conductor. Copper has a large number of free
electrons at room temperature, so it is a good
conductor.
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Conductor
Silver and gold have even more free electrons at
room temperature, so they are better conductors
than copper. Unfortunately, they are also more
expensive! Gold and silver have more free
electrons at room temperature because
their outer electrons are easier to knock loose,
and therefore are more likely to be freed.




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Other metals, like aluminum and iron, have enough
free electrons to be considered conductors, but
not as many as copper. Insulators are materials
which have very few free electrons.
8
The Atom
When two electrical charges, such as the positive
charge of a proton and the negative charge of an
electron, are separated by a distance r, there is
an electrical force between them which is given
by
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F
If one of the charges is positive and the other
is negative, they attract each other as shown
here. If they are allowed to move toward each
other, they do so. If they cannot move, then the
electrical force is called an electrostatic
force. If both charges are positive or if both
are negative, the force between them is
repulsive.
9
The Atom
The electrostatic force is much, much stronger
than the force of gravity. Consider two
ordinary grains of sand, 30 meters apart. The
force of gravitational attraction between them is
negligible practically zero. Because the
number of protons and electrons in each grain are
nearly equal, the electrostatic force between the
two is also negligible.

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F
but suppose like charges attracted each other, so
all the charged particles in the two grains of
sand attracted each other. Now the force of
attraction between the two would be approximately
3 million tons! In the real world, like charges
repel. This means that there is an attractive
force between the opposite charges in the two
grains of 1,500,000 tons, but it is cancelled by
the 1,500,000 ton repulsive force between the
like charges. (source The Feynman Lectures on
Physics, by Richard P. Feynman, Robert B
Leighton, and Matthew L. Sands)
10
Current
Take a short length of copper wire. Lets assume
that some force is applied, causing the free
electrons to move from left to right through the
length of wire
Electron movement
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Current
Now imagine a plane cutting through the wire
somewhere between the left end and the right end.
The electrons pass through the imaginary plane
as the travel along the wire.
Imaginary plane
Electron movement
If we count the electrons passing through the
imaginary plane every second, we get a measure of
the rate at which charge is moving through the
wire. This is called current.
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Current
If 6.42 x 1018 electrons pass through the
imaginary plane in one second, then 1 C of charge
passes through the plane in one second. This is
an electrical current of one Ampere
Imaginary plane
Electron movement
A current of one Ampere is the movement of
electrical charge at the rate of one Coulomb per
second.
13
Current
Current flows through electrical conductors, like
water flowing through a pipe. It flows through
wire, through circuit elements (like the motor
below), even through the battery (which you may
think of as an electron pump). In the 19th
century, it was thought that electricity was a
fluid, literally like water, which flowed through
metal conductors as if they were pipes. This is
still a useful analogy. At the same time, it was
thought that the positive charge carriers (i.e.,
protons) were the ones that moved.
If this were the case, positive charge carriers
would flow from the positive terminal of the pump
(the battery) through the circuit, and back to
the pump to recirculate.
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Motor
14
Current
The diagram shown below is called a schematic
diagram, a circuit diagram, or simply a
schematic. The lines represent wires connecting
the battery to the motor, and the symbols for the
motor and battery are self-explanatory. The
battery applies a force to the free electrons in
the wires, which causes them to move creating a
flow of electrons, or current flowing through the
circuit.
In the 19th century, it was thought that the
mobile charges were positive instead of negative.
If this were the case, positive charge carriers
would flow from the positive terminal of the pump
(the battery) through the circuit, and back to
the pump to recirculate.
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Motor
15
Current
Later, it was discovered that electrons,
negatively charged, were the particles that were
free to move through the circuit, and that
positively charged protons were rigidly held in
place. Electrons flow from the batterys
negative terminal, through the circuit, and
return to the positive terminal. This seems
backward. It turns out that we can pretend the
current is carried by positively-charged
particles. flowing out of the batterys positive
terminal and returning to the negative terminal.

Motor
electrons
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If we use the analogy of water flowing in a pipe,
suppose we want to pump water from a tank 1 into
tank 2. Tank 2 is initially empty, containing
all air and no water tank 1 is initially full,
containing all water and no air. As we pump
water from tank 1 to tank 2, tank 2 fill with
water and the amount of air in tank 2 decreases.
At the same time, as water leaves tank 1, the
amount of air in tank 1 increases. It almost
seems if were pumping air from the tank 2 to
tank 1, instead of pumping water from tank 1 to
tank 2.
Water Flow
Pump
Tank 1 (Water)
Tank 2 (Air)
17
Current
conventional current
In an electrical circuit, we pretend current is
the flow of positive charge in the direction
opposite the electron movement. This is called
conventional current.
Motor
electrons
18
Measuring Current
To measure current, use a Currentmeter Ammeter.
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The Ammeter must be inserted in the circuit in
such a way that the current we want to measure
flows through the meter. Ideally, the Ammeter
does not resist the flow of current at all. It
measures the current, but inserting it in the
circuit as shown does not affect the circuits
operation.

Battery
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Motor
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Amperage
The word amperage does appear in technical
dictionaries, and is a valid word in that sense.
Please never use it, its like describing a
volume of water as gallonage. The correct term
is current.
20
Resistance
As weve seen, a material which has many free
electrons is a good conductor. Current flows
easily through it. If we want to cause a current
of 1 Ampere to flow through a given wire, a
relatively small Voltage (electromotive force,
the force which causes the electrons to move)
will do the job.
A good conductor resists the flow of current, but
only weakly. A small Voltage causes a large
current. A good conductor is said to have very
low resistance.




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Resistance
A material which has very few free electrons is
an insulator. Almost no current flows through
it, even if a large Voltage is applied.
A good insulator is an extremely poor conductor.
Its resistance is very, very high.




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Resistance
Material which have too many free electrons to be
considered insulators but too few to be
considered good conductors have resistance which
is somewhere between extremely low and extremely
high. Since resistance is a variable quantity,
we need a unit with which to measure it.
The unit of resistance is the Ohm. The Greek
letter Omega (W) is used as shorthand for Ohm or
Ohms.




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Resistance
Water flows through a pipe with no effort at all,
right? It takes the same amount of force to move
water at a certain rate through a soda straw or a
firehose, right? Of course not!
Small pipe requires more pressure
Large pipe requires less pressure (for the same
flow rate)
A pipe has resistance to the flow of water. A
small pipe has more resistance, a large pipe has
less.
24
Resistance
Any wire also has resistance to the flow of
charge. The resistance depends on the material
the wire is made of. Copper is better than steel,
for instance. It also depends on the length and
thickness of the wire.
Long, thin wire has higher resistance
Short, thick wire has lower resistance.
A pipe has resistance to the flow of water. A
small pipe has more resistance, a large pipe has
less.
25
Resistance
Resistance is not just an unwanted property of
imperfect electrical conductors, its actually
useful. Resistance is deliberately incorporated
into many electrical circuits to control the flow
of current. Circuit elements called resistors,
which are available in values from fractions of
an Ohm to many megohms (millions of ohms) are
very common. The schematic symbol for a 10 W
resistor is shown below.
10 W
R1
26
Voltage
To make water flow through a pipe from one place
to another, a force must be applied. This could
be the force of gravity, making the water flow
from a higher elevation to a lower elevation, or
it could be a force applied by a pump to make the
water flow from a lower elevation to a higher
elevation.
Each gallon of water in Tank 1 weighs 8.35
pounds, so gravity exerts a force of 8.35 pounds
per gallon to move water from the higher tank to
the lower tank.
Tank 1
Tank 2
27
Voltage
To move water from the lower tank to the higher
tank, a force must be exerted on the water which
more than cancels the force of gravity. This
force must be at least 8.35 pound per gallon.
This force may be exerted by a pump, pumping the
water from the lower tank to the upper tank.
Each gallon of water in Tank 1 weighs 8.35
pounds, so gravity exerts a force of 8.35 pounds
per gallon to move water from the higher tank to
the lower tank.
pump
Tank 1
Tank 2
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Voltage
Lets say that that Tank 1 is higher than Tank 2
by 10 feet. The work required to pump one gallon
of water from Tank 2 to Tank 1 is given by
This also happens to be the potential energy of
each gallon of water in Tank 1. The pump
increases the potential energy of each gallon of
water it moves from Tank 2 to Tank 1 by 83.5
ft-lb.
pump
Tank 1
Tank 2
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Voltage
The answer is 4.34 psig.
Tank 1
Tank 2
30
Voltage
How much must the pump increase the pressure of
the water in order to move it from Tank 2 to Tank
1? By at least 4.35 psi! This is the pressure
required to overcome the force of gravity, and
increase the potential of each gallon of water
moved from Tank 2 to Tank 1 by 83.5 ft-lb.
pump
Tank 1
Tank 2
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Voltage
Next, imagine a system consisting of a reservoir,
a pump, a turbine, and piping to connect them as
shown below Water is pumped from the reservoir,
acquiring potential energy as it passes through
the pump. It flows through the pipes to the
turbine, where it produces work (the turbine may
be connected to a generator).
After passing through the turbine and expending
all its potential energy, it empties back into
the reservoir.
Pump
Turbine
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Voltage
Now imagine large block of copper. It contains
many free electrons, but there is no force to
make them move. There is no net movement of
charge, no current, and no work is done.
The copper block is like a reservoir of free
electrons. There is no turbine, and no pump.
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Voltage
Add a motor, connected to the copper block by two
wires, as shown below. The lines represent
wires. There is still no force to make current
flow, so no work is done. The motor does nothing.
The copper block is like a reservoir of free
electrons, the wires are like pipes, and the
motor is like a turbine. The only thing missing
is an electron pump.
Motor
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Voltage
The electron pump is a battery, represented
here by its schematic symbol. A generator is
another type of electron pump. It raises the
potential energy of each unit of electrical
charge which flows through it, in the same way as
the pump raises the potential energy of each
gallon of water it moves.
A 1 Volt batter raises the potential energy of
each coulomb that passes through it by 1 Joule.
Battery

1 Volt
Motor
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Voltage
The pump raised the pressure of the water by an
amount which was proportional to the potential
energy increase. Voltage is the electrical
equivalent of pressure it is often called
electromotive force or emf, the force which
causes electrical current to flow.
Battery

1 Volt
Motor
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36
Voltage
It isnt actually necessary to have a copper
block to serve as a reservoir of free electrons.
Any piece of wire has plenty of free electrons
for that purpose. In this analogy, the copper
block really serves as the reference potential,
as the potential energy of each gallon of water
in the pump/turbine/reservoir system was
measured relative to the water level in the
reservoir.
Battery

1 Volt
Motor
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Voltage
The block can be replaced by a wire connecting
the motor to the negative terminal of the
battery. All voltages in the system are measured
relative to this wire, which is said to be at
ground potential 0 Volts. This is shown by the
ground symbol.
In some cases, but not all, the ground point in
an electrical system is literally ground a
copper stake driven into the earth.
Battery

1 Volt
Motor
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Ground
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Control
The motor can be stopped by breaking one of the
wires, as shown here. This stops current from
flowing from the battery to the motor. The
circuit is said to be open, because it is no
longer a closed loop.
Current only flows if the circuit is closed, not
if it is open.
Battery

1 Volt
Motor
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Ground
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Control
The analogy to this in the pump/turbine/reservoir
system would be to inerrupt the flow of water by
cutting and capping the pipe. This could
actually be done anywhere along the pipe, because
capping it anywhere would prevent water from
flowing anywhere.
Pump
Turbine
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Control
Instead of cutting and capping the pipe, we could
install a valve. If the valve is open, water
flows, and work is done. If the valve is closed,
water does not flow and no work is done.
Pump
Turbine
Valve
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Control
A more convenient way of controlling the motor is
the use of a switch, which is a small piece of
electrically conductive material which can be
inserted to close the circuit, or removed to open
it, usually by operating some sort of lever.
When the conductor is inserted, the switch is
said to be closed. When it is removed, the
switch is open. A closed switch is like an open
valve. An open switch is like a closed valve.
Battery

1 Volt
Motor
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Ground
42
Power
Now, suppose the valve is open, the pump develops
a pressure of 10 psig, and the flow rate is 600
gallons per minute (10 gallons per second).
Pump
Turbine
Valve
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Power
Add a standpipe as shown, made of transparent
material so the height of the water in it can be
seen.
standpipe
Pump
Turbine
Valve
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Power
The weight of the water in the standpipe will
always be equal to the upward force exerted on it
by the pressure of the water in the pipe. If the
upward pressure is 10 psig and the crosssection
of
the standpipe is 1 inch, the upward force is 10
pounds. The water will rise just high enough in
the standpipe so that its weight is also 10
pounds.
standpipe
Pump
Turbine
Valve
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Power
The standpipe serves as a simple pressure gauge,
but it also indicates the potential energy of the
water in the pipe. In this example, the height
of the water column is 277 inches. The cubic
Inch of water at the top of
the column weighs .0361 lb., and is at a height
of 10 feet above the reservoir (lets say the
pipes height is equal to that of the reservoir),
so its potential energy is 0.361 ft-lb.
standpipe
Pump
Turbine
Valve
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Power
The gauge pressure of this top cubic inch of
water is zero, but its potential energy is .361
ft-lb. The potential energy of the bottom cubic
inch is also .361 ft-lb, because its gauge
pressure is 10 psig. Each
Cubic inch of water at a pressure of 10 psig
possesses this amount of potential energy, so
each gallon which flows out of the pump carries
83.45 ft-lb of potential energy.
standpipe
Pump
Turbine
Valve
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Power
As water flows under pressure from the pump to
the turbine at 600 gpm, it carries energy from
the pump to the turbine at a rate of
standpipe
Pump
Turbine
Valve
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Power
So water flowing under pressure of 10 psig at a
rate of 100 gpm carries energy at a rate of 188
Joules per second, which is the same as 188 Watts
of power, or 0.25 horsepower. The power carried
by the flowing
water is proportional to pressure (potential
energy per unit volume) times flow rate.
standpipe
Pump
Turbine
Valve
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Power
In our electrical circuit, voltage plays the role
of pressure Potential energy per unit charge.
In fact, a voltage of 1 Volt means that each
coulomb of charge carries 1 Joule of energy
Battery

1 Volt
Motor
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Ground
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Power
If charge flows at a rate of 1 Coulomb per
second, a current of 1 Ampere, carrying with it 1
Joule of energy per coulomb, energy is
transferred to the motor at a rate of 1 Joule per
second, which is the same as one Watt of power.
If we let E represent voltage
I represents current, and P represents power,
then
Battery

1 Volt
Motor
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Ground
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Magnetics
Consider a bar magnet, a bar of magnetized iron
or other magnetic material. It has a north
pole and a south pole, and it is surrounded by
a magnetic field.
The magnetic field can be illustrated by
imaginary lines, called magnetic flux lines,
shown at left.
S
N
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Electromagnetism
If a current I flows through a conductor (a wire)
a magnetic field results. Magnetic flux lines
surround the wire as shown. If the current flows
in the direction indicated by the arrow, the
magnetic flux will also have the indicated
direction. The flux is proportional to the
current doubling I doubles the flux.
I
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Magnetics
If the wire is coiled , as shown below, the flux
lines from the coils turns sum. Consider a
small vertical slice of a two turn coil
The same current flows through both turns (after
all, they, are the same wire) and in the same
direction. The flux lines from the first turn
combine with the flux lines from the second turn,
so the flux is doubled. Three turns triples the
flux, and so on.
Coiled wire
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Vertical slice of a two-turn coil
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Magnetics
For a multiturn coil, the flux lines arrange
themselves as shown below. This is similar to a
bar magnet. Because a vacuum (or air) has low
permeability (that is, its harder to set up
lines of flux in air, vaccum,
plastic, copper, aluminum, etc., than in iron,
steel, nickel, etc.) the flux density within the
coil is less than the flux density within the bar
magnet.
N
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The flux lines go from the north pole to the
south pole (outside the coil).
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Magnetics
In the same way that electric current flows more
easily through some materials and with greater
difficulty through others, magnetic flux is set
up more easily in some materials and less in
others. For this reason, electromagnets are
often wound on a core of material which
supports flux better than air or vacuum. Such
materials include iron and steel. An
electromagnet wound on a straight bar or rod of
magnetic material, like this is called a
solenoid. It may may be used as a mechanical
actuator, by exerting a force on a lever or other
simple machine which may be turned on or off by
turning the solenoid current on or off.
A solenoid may be used to actuate a set of
electrical contacts, in effect opening or closing
a switch. The solenoid, or coil, and contacts
are known together as a relay.
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Relay Logic
A simple motor control, implemented with switches
and a relay, is shown below. The STRT pushbutton
is normally open, and closes when pressed. The
STOP pushbutton is normally closed, and opens
when pressed.
Pressing STRT allows current to flow from V to
V-, through STOP, STRT and relay coil M. This
energizes the relay coil, causing the
normally-open contacts labelled M to close,
starting the motor
STRT
STOP
M
V
V-
M
M
L1
Motor
L2
L3
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Relay Logic
If STRT is now released, one of the M contacts
still provides a path for current to flow through
the relay coil, so all the M contacts remain
closed and the motor continues to run.
Pressing STOP interrupts the current flowing
through the coil, causing the M contacts to
open and the motor to stop. Releasing STOP
closes the STOP pushbutton, but now STRT and M
are both open, so the relay remains deenergized
and the motor remains stopped.
STRT
STOP
M
V
V-
M
M
L1
Motor
L2
L3
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Relay Logic
This is a very simple logic circuit, implemented
with relays and contacts. This is called relay
logic. If STRT has been pressed more recently
than STOP, the motor runs. Otherwise, the motor
does not run.
Relay logic may be used to implement much more
complex controls than this, such as the one in
Figure 1-2 of the text.
STRT
STOP
M
V
V-
M
M
L1
Motor
L2
L3
59
Relay Logic
Sometimes it becomes necessary to modify the
logic of a control circuit. We might need to add
a limit switch to stop the motor when something
driven by the motor moves outside its safe range.
This would mean rewiring the control circuit.
If the control logic is implemented using either
a programmable logic device or a computer and
software instead of relays, we just change the
program (and add a couple of wires for the limit
switch)
STRT
STOP
M
V
V-
M
M
L1
Motor
L2
L3
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Programmable Logic Controllers
A PLC implements the control logic using a
computer and software, so the first part of a PLC
is the processor, or CPU (central processing unit)
CPU
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Programmable Logic Controllers
A CPU cant do anything without memory, so
CPU
Memory
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Programmable Logic Controllers
Without a power supply, we cant turn it on
Power Supply
CPU
Memory
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Programmable Logic Controllers
Next, we add an input module, to connect
switches, buttons and contacts to
Power Supply
The input module reads the state (open or closed)
of the contacts connected to it, and converts
these states to logic signals the CPU can use.
CPU
Input Module
Memory
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Programmable Logic Controllers
Finally, we add an output module. This responds
to signals from the CPU by energizing or
deenergizing relay coils or heating elements,
lighting or extinguishing lamps, or generally
turning things on or off.
Power Supply
M
CPU
Input Module
Input Module
Memory
Input Sensors
Output Loads
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Programmable Logic Controllers
The input sensors and output loads may be
referred to collectively as field devices or real
world devices
Power Supply
M
CPU
Input Module
Input Module
Memory
Input Sensors
Output Loads