Download AET Study Group Notes: DC Basics – Matter is anything that

AET Study Group Notes:
DC Basics –
Matter is anything that occupies space and has weight.
An element is a substance that cannot be reduced to a simpler substance by chemical means. A
compound is a chemical combination of elements that can be separated by chemical means but not
physical means. The Periodic Table identifies all currently-known elements.
An atom is the smallest form of matter. An atom consists of subatomic particles that cannot exist
separately.
Protons and Neutrons are contained within the nucleus of the atom. Protons hold a positive charge
while Neutrons have a neutral charge. Neutrons are the ‘glue’ that holds the protons in the nucleus.
Electrons orbit the nucleus (analogy of our solar system) in energy levels or ‘shells’.
Electrons hold a negative charge. In an atom that is said to be ‘balanced’, there are an equal number of
protons and electrons. If an atom loses electrons or gains electrons as a result of electron exchange, it is
said to be ‘ionized’. An atom with more than the normal amount of electrons has a greater negative
charge (more electrons than protons) so it is considered to be a negative ion. Conversely, an atom with
less than the normal amount of electrons has a greater positive charge (more protons than electrons) so
it is considered to be a positive ion.
The number of electrons each shell can hold is 2n2.
The outermost shell is called the ‘valence’ shell. This is the shell that determines how well electricity can
flow. The Valence shell can hold a maximum of 8 electrons only.
The total number of electrons an atom has is called its atomic number. Atomic numbers are referenced
in the Periodic Table.
Elements with few valence electrons (1-2) are considered good conductors of electricity because that
one electron can easily be forced out of the valence shell of one atom and into one of the seven empty
holes in the next atom.
Elements with half-full valence shells (4) are not good conductors because it is not easy to force four
electrons out of the valence shell of one atom into the four empty holes in the valence shell of the next
atom. This would require more force so these elements are considered to be ‘semi-conductors’.
A force is needed to move the electrons in the valence shell from one atom to another. This force
produces a charge and is called an electro-motive force or EMF which is commonly referred to as
voltage. EMF is commonly represented in electrical circuit with the letter E. The unit of measure for EMF
is volts and is referenced with the letter V.
When working with a wide range of measurements such as volts and other electronic device
measurements, prefixes are frequently used to express very large or very small numbers in a more
manageable way. For example, 1000 volts can be referred to as a kilovolt or kV. Likewise, a fraction of a
volt, say 1,000th of a volt, can be referred to as a millivolt or mV.
The best analogy here is to think of this in terms of computer memory. Computer memory starts with a
byte. 1,000 bytes is a kilobyte, 1,000,000 bytes is a Megabyte or Mb, next is 1,000,000,000 (one billion)
bytes (or 1,000 Megabytes) is a Gigabyte or Gb. 1,000,000,000,000 (one trillion) bytes (or 1,000
Gigabytes) would be a terabyte or Tb.
Here is a list of the commonly used prefixes for large and small numbers
Pico – one billionth (1/1,000,000,000)
Micro – one millionth (1/1,000,000)
Milli – one thousandth (1/1,000)
Kilo – one thousand
Mega – one million
Giga – one billion
Terra – one trillion
Converting numbers between these prefix values is as simple as moving the decimal place of a number
three spots for every prefix unit
It is common to see voltage measurements converted and referred to using these prefixes. A voltage
measurement of .067 volts would be referred to as 67 millivolts.
In an atom with a natural or neutral state, there will be the proper amount of electrons and there will be
an equal number of protons and electrons
If the atom is disturbed and electrons are removed, there will be more protons than electrons so the
atom will have a positive charge
If this atom comes in close proximity to a material with a negative charge (more electrons than proton),
the charges will neutralize between the two atoms
In a material that is a good conductor, a charge can be created in the material using a force but when
the force is removed, the charged atoms neutralize themselves because the material is a good
conductor and electrons flow easily among the valence shells of the atoms
In a material that is an insulator, such as the rubber on the soles of a shoe and carpet, friction causes a
disruption of the atoms’ charges and because electrons can’t easily distribute themselves in insulators,
the charge in the material is held until it comes in close proximity to an opposite charge like a metal
door handle
When that happens, the charges neutralize by the transfer of electrons until the charges are equal
This phenomenon is called static electricity
Coulomb’s Law of Charges states that “charged bodies attract or repel each other with a force that is
directly proportional to the product of their individual charges and is inversely proportional to the
square of the distance between them”
This means that the two factors that determine the force with which static electricity will dissipate is the
amount of charge in the two bodies and the distance between them
The greater the charge built up in the bodies, the greater the static electricity force will be… “directly
proportional to the product of their individual charges ”
The shorter the distance between the two bodies, the greater the static electricity force will be…
“inversely proportional to the square of the distance between them”
Magnetism –
To understand the principles of electricity, it is important to understand magnetism and how it affects
electrical equipment
Many types of electronics rely on magnetism:
1.
2.
3.
4.
5.
6.
Memory storage devices
Computer hard-drives
Speakers
Motors
Generators
Alternators
Magnetism is defined as the property of a material that enables it to attract iron
Metals that are relatively easy to magnetize are called ferromagnetic
Iron, Steel, and Cobalt are ferromagnetic
Magnets are made from special iron or steel alloys and are magnetized electrically
Magnetization involves inserting the material into a coil of wire and passing a heavy flow of electrons
through the wire
Magnets are classified as permanent or temporary depending on their ability to retain their magnetic
properties after the magnetizing force has been removed
Hardened steel and certain alloys that retain their magnetism are considered permanent magnets
Permanent magnets are difficult to produce because the materials natural properties oppose the lines of
force trying to magnetize it
This opposition is called reluctance and permanent magnets are said to have a high reluctance
Materials with low reluctance, such as soft iron, will retain only a small portion of the magnetism and
are called temporary magnets
High or low reluctance is one characteristic of magnetic materials; another is called permeability
Permeability refers to the ease with which magnetic lines of force distribute themselves throughout the
material
Permanent magnets produced from materials with high reluctance have low permeability, meaning the
lines of force do not easily distribute themselves throughout the material, but once they are distributed,
the material will hold the lines of force, hence holding the magnetism
Conversely, temporary magnets are produced from material with low reluctance and have high
permeability meaning the lines of force easily distribute themselves throughout the material. Since the
lines of force were easily distributed, they can easily dissipate; hence the material does not hold its
magnetism
The lines of force have a tendency to concentrate at the ends of the material and be weaker in the
middle
This is much like the earth. In fact, the earth is a magnet, with poles that have a greater concentration of
lines of force than at the middle
Magnets have been used in navigation for centuries
A magnetized material will have a tendency to align itself to the north and south poles of the earth if it is
suspended and can freely move with little resistance such as hanging from a string or floating on a liquid
The Law of Magnetic Poles states that like poles repel each other and opposite poles attract each other
Understanding how magnetism works and the properties of the earth’s magnetic field has led to the
development of modern navigation systems that can determine an aircraft’s heading much more
accurately than a wet compass (also referred to as a “whiskey compass”)
The space surrounding a magnet is called a magnetic field and is the area where magnetic forces act
Magnetic lines of force exist in this area and have the following characteristics:
1. Magnetic lines of force are continuous and will always form closed loops
2. Magnetic lines of force will never cross one another
3. Magnetic lines of force pass through all materials, both magnetic and non-magnetic
There are three types of magnetic effects:
1. Magnetic Flux – the total number of magnetic lines of force leaving or entering the pole of a
magnet
2. Field Intensity – intensity of a magnetic field is directly related to the magnetic force exerted by
the field
3. Attraction/Repulsion – the intensity of attraction or repulsion between magnetic poles may be
described by a law similar to Coulomb’s Law of Charged Bodies: The force between two poles is
directly proportional to the product of the pole strengths and inversely proportional to the
square of the distance between the poles
Magnetic Induction
Any material placed in the area surrounding a magnet has lines of force passing through the material
When the lines of force pass through the material, those lines of force act upon the material and if the
material is permeable, it becomes a temporary magnet; somewhat weaker than the original magnet
though
When removed from the magnetic field, the lines of force in the material now dissipate and the material
loses its magnetism
This is the theory of operation for relays and solenoids – a magnetic field is generated around a
permeable material creating a temporary magnet that “pulls” an electrical contact to open/close a
circuit
When the magnetic field is removed, the temporary magnet can no longer hold the attraction and the
electrical contact is released to its normal “resting” position
Magnetic Shapes
Magnets are categorized into three basic shapes: bar magnets, horseshoe magnets, and ring magnets
Bar magnets are typically used in laboratories to study and better understand magnetism and its effects
Ring magnets are typically used in computer memory devices and to shield electrical instruments form
stray magnetism
The most frequently used magnet used in electrical and electronic equipment is the horseshoe magnet
Horseshoe magnets are very similar to bar magnets, only they are bent to form a horseshoe shape
This brings the poles closer to each other and generates a greater magnetic strength
Electrical Charges –
There are two types of voltage – Direct Current (DC) and Alternating Current (AC)
Direct current has a constant current with electrons flowing in only one direction
Alternating current continuously reverses the polarity of the electrical charges to generate an oscillating
effect
There are six general means of producing EMF: magnets, friction, pressure, heat, light, and chemical
action
1. Magnets have a natural positive and negative charge that attracts and repels electrons. Placing a
magnet near a conductor can ‘force’ the electrons to move but the magnet or the conductor
must be in motion to produce the electricity. An electromagnet works on the opposite principle
of using an electrical charge to generate a magnetic field. This is how a solenoid or relay
operates
2. Friction builds up a charge (think static electricity or lightning) but is difficult to harness or
control. By rubbing certain materials together, electrons can be forced to move leaving atoms
with either an abundance of electrons in its valence shell (positively charged) or missing
electrons in their valence shell (negatively charged). When this happens, the electrical charge is
built up until becomes close enough to a material with an opposite charge that it can ‘arc’
through the air and the electrons from the positively charged atoms jump over to the negatively
charged atoms.
3. Electricity can be produced by applying pressure to the surface of crystals such as quartz.
Applying pressure to one surface creates opposing charges on opposite surfaces. This is called
the piezoelectric effect. Alternating the pressure (or vibrating the crystal) will produce
alternating current which is commonly used in microphones, oscillators, radio receivers, radio
transmitters, and sonar equipment.
4. Electricity produced by heat is called thermoelectric voltage and is generated by heating one
end of a metal rod. Depending on the type of metal, the electrons will move either away from or
toward the heated end. This rod would be considered a thermocouple and is typically used in
measuring temperature by monitoring the voltage produced by the metal rod. In most metals,
including copper, electrons move away from the hot end toward the cooler end. Some metals,
such as iron, the opposite takes place; electrons move from the cool end toward the hot end.
5. Light has energy so when it hits the substance, it can dislodge the electrons on the surface,
thereby creating an electrical charge. This is called photoelectric voltage. Metals tend to be
more ‘photosensitive’ and are more susceptible to electron movement when hit by light. Silver
oxide and copper oxide are two compounds that are common used to create ‘photoelectric
cells’, also referred to as solar cells because the sun produces the most powerful photoelectric
energy and is a form of natural energy.
6. Chemical energy is produced when two dissimilar substances (commonly metals) are immersed
in a solution that creates a greater chemical action on one substance than the other. This
creates a difference of potential, or a positive charge on one substance and a negative charge on
the other. Connecting a conductor between the two substances allows the electrons to flow,
thereby creating a current flow. Chemical action can be accomplished using either ‘wet cells’ like
lead-acid batteries or ‘dry-cells’ like nickel-cadmium (NiCd) or nickel-metal-hydride (NiMH)
batteries. These dry-cells are not actually dry; rather, the substances are immersed in an
electrolyte mixed with other materials to forma paste.
A voltage source is a device which is capable of supplying and maintaining voltage
To be a practical source of voltage, the potential difference cannot be allowed to dissipate – when one
electron leaves, another must be supplied to take its place
Electrons will flow out of the negatively charged area due to the Electro-Motive Force ‘pushing’ the
electrons away from the positively charged area
This is why electricity (or electron flow) is said to ‘flow from negative to positive’
The volume of electrons flowing across the atoms is used to determine current
Current is commonly represented in a circuit by the letter I
Current is measured in Amperes which is represented by the letter A
1 ampere of current flow equals one coulomb of electrons passing a point in one second when an EMF
of 1 volt is applied
A Coulomb is equivalent to 6.24 x 1018 electrons
This means that just one volt is all that is needed to push 6.24 x 1018 electrons past a point in a circuit in
one second
Scientific Notation is used to express large numbers like this to a more manageable number that is
easier to write, understand, and reference
The first number is called the coefficient and is the number that will be multiplied to create the much
larger number
10 is called the base – this number will be multiplied by itself a certain number of times before being
multiplied by the coefficient
18 is the exponent which is the number of times the base will be multiplied by itself before being
multiplied by the coefficient
If the EMF increases, the resulting current flow will also increase; therefore, voltage (EMF) and current
are directly proportional in an electrical circuit
Resistance is the opposition to the flow of electrons and is represented by the letter R
Resistance is measured in ohms which is represented by the Greek letter Omega
A resistor is made of a semi-conductor substance that limits electron flow
Conductors also exhibit a small amount of resistance
This resistance is very small and varies based on material type, cross-sectional area, length, and
temperature
Because resistance in conductors is so small, it is typically considered negligible and not factored into
circuit calculations
In an electrical circuit, an increase in resistance slows the flow of current while a decrease in resistance
allows more electrons to flow freely
This means current and resistance are inversely proportional in an electrical circuit
A simple DC Circuit consists of a voltage source (typically a battery), a load (resistance), and the
conductors that connect them
If there were no load resistance in the circuit, electrons would be able to flow freely from the negative
lead of the power source to the positive lead without being slowed
This high rate of electron flow would quickly produce substantial heat to the point the conductors would
burn or the battery would heat up and catch on fire
Some type of load is necessary to prevent this thermal breakdown of the circuit
The simplest form of an electrical circuit is a battery connected to a lamp
The lamp serves as the resistive load and the battery is the voltage source
In most circuits, it is undesirable to have it operate at all times so a control mechanism is often used
The simplest type of control mechanism is a simple switch
A switch will have a center node which is connected to the part of the circuit that will always be used
such as the voltage source
Sometimes the voltage source will be switched between two different parts of the circuit or the switch
could be used to simply open the circuit by connecting nothing to the normally closed contact and
connecting the load to the normally open node
When the switch is turned “ON”, current is allowed to flow through the circuit and illuminate the lamp
DC Circuit Calculations –
It is important to understand the relationship of voltage, current, and resistance in a circuit and to know
how to calculate each
The basis for this is called Ohm’s Law
Ohm’s Law states that the current in a circuit is directly proportional to the applied voltage and inversely
proportional to the circuit resistance
This can be expressed as a formula: I = E / R where I represents current measured in amps, E represents
voltage measured in volts, and R represents resistance measured in ohms
Using Ohm’s Law, if two quantities are known, the third can be determined using simple algebra
An example would be if a 1.5V battery were connected to a lamp that has 5 ohms of resistance, we can
substitute the appropriate values and calculate the current. I = 1.5 / 5 or 0.3 amps
The formula for Ohm’s Law can be restructured using basic algebra principles to be able to easily
calculate voltage if current and resistance are known (E = I * R), or to calculate resistance if voltage and
current are known (R = E / I)
Another concept that must be understood is power
Power is the rate at which work is being done
When an EMF forces electrons to move, work is being done
The basic unit of measurement for power is the watt
Power is represented by the letter P and the formula for power is P = I * E
When determining power, voltage (E) and current (I) must be known
If only voltage (E) and resistance (R) are given, Ohm’s Law can be used to determine current (I), then the
Power Formula can be used to determine Power (P)
A power rating of a device, such as a lamp or electronic circuit, is sometimes given as a watt rating
An example would be a light bulb which might have a power rating of 40W, 60W, 75W, 100W, or 150W
This power rating indicates the rate at which the device converts electrical energy into another form of
energy such as light, heat, or motion
Examples:
Light bulbs convert electrical energy to light energy
An oven or electric heater converts electrical energy to thermal energy
An electric motor converts electrical energy to mechanical energy
In applied avionics (systems engineering), the power of avionics equipment is typically specified and the
equipment may be able to operate at different voltages
In this case, as voltage varies and power remains constant, the current will vary with the voltage
It is important to determine current to ensure the current-carrying capacitor of the conductor is not
exceeded as this could result in overheating and ultimately a fire