Ohm's law

1. Unit-3 DIRECT CURRENT CIRCUITS
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2. References
1. NEETS Module 1 – Introduction to Matter, Energy, and
Direct Current.
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3. Enabling Objectives
1. RETRIEVE or RECOGNIZE information to answer questions
about matter, principles of an electrostatic charge, and the
relationship that exists between voltage, current, resistance
and power using OHM’s Law.
2. RETRIEVE or RECOGNIZE information pertaining to
methods of producing voltage.
3. CALCULATE circuit values using OHM’s Law.
4. APPLY safety precautions associated with DC Circuits in
accordance with NAVY SAFETY PRECAUTIONS FOR
AFLOAT FORCES.
3

4. THE ATOM
An Atom is the smallest particle of an element that retains the
properties of that element.
4

5. THE ATOM
An Atom of each element are made up of still smaller units
called subatomic particles.
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6. THE ATOM
The center of an atom is called the NUCLEUS, and is made up
of Neutrons (no electrical charge) and Protons (positive
electrical charge).
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7. THE ATOM
Orbiting the nucleus are the Electrons (negative electrical
charge) which is equal to, but opposite of the positive charge
of the proton.
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8. The electrons orbit the nucleus at predetermined distances
called Shells, or Orbits.
In order for an electron to remain in a specific orbit (shell), it
must not gain or lose energy.
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9. When energy is added to an electron, it will jump to a shell that is
further away from the nucleus due to an increase in energy.
It is possible to add
enough energy to
an electron to
cause it to jump
free from its atom
and become “free
electrons”.
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10. Free electrons enable the production of voltage and transfer of
electrical energy.
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11. Valence Shell – the outermost shell of an atom. It determines its
ability to gain or lose an electron; and also determines the
electrical properties of an atom.
-
-
+ N
N +
-
-
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12. Valence Shell – the outermost shell of an atom. It determines its
ability to gain or lose an electron; and also determines the
electrical properties of an atom.
-
-
+ N
N +
-
-
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13. Materials can be classified into 3 groups based on their ability to
gain or lose electrons:
1. Conductors – have 3 or less valence electrons per atom and
release free electrons readily.
Examples: Silver, Copper, Gold and Aluminum
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14. 2. Semi-conductors – have 4 valence electrons per atom and
are neither good conductors nor good insulators.
Examples: Germanium and Silicon
3. Insulators – have 5 or more valence electrons per atom
requiring much energy to release electrons.
Examples: Rubber, Plastic and Glass
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15. An Atom is electrically neutral if it contains the same number of
protons as electrons.
PROTONS = ELECTRONS
ELECTRICALLY NEUTRAL
Ionization – process where an atom gains or loses electrons,
changing the electrical charge of the atom and becoming an
ion.
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16. Negative Ion – an atom having more negative charges
(electrons) than positive charges (protons), meaning that the
atom has gained one or more electrons.
-
-
+
+
-
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17. Positive Ion – an atom having more positive charges (protons)
than negative charges (electrons), meaning that the atom has
lost one or more electrons.
-
+
+
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18. ELECTROSTATIC CHARGES
The Law of Charged Bodies:
“Like charges REPEL each other and unlike charges ATTRACT
each other”
If the 2 bodies were allowed to
come in contact with each
other, they would become
electrically neutral.
The transfer of energy that
removes all electrical charges
is called “Equalizing”.
No equalization can occur
between bodies of like
charges.
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19. ELECTROSTATIC FIELD FLUX
Electrostatic Field – electric force surrounding charged bodies.
These lines are called
Electrostatic Lines of Force and
represent the electric forces
acting between charged bodies
within an Electrostatic Field.
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20. The Law of Charged Bodies is further defined by Coulomb’s
Law: Charged bodies attract or repel each other with a force
that is directly proportional to the product of their individual
charges and inversely proportional to the square of the
distance between them.
F = Force
Q1 x Q2 Q = Charge of Individual
F= bodies
2
d d = Distance between
bodies
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21. The charge of one electron is so small that it makes it
impractical to use as a unit of electrical charge.
COULOMB is the practical unit adopted for measuring
charges.
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1 Coulomb = 6.28 x 10 Electrons
that is:
6,280,000,000,000,000,000
ELECTRONS
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22. A positive charge would result by starting with a neutral body and
removing electrons from it.
-
-
+ +
+ +
-
-
This body would now exhibit a positive charge and now has the
ability to attract electrons because of their opposite charges.
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23. A negative charge would result by starting with a neutral body
and adding electrons to it.
-
-
+ +
+ +
-
-
This body would now exhibit a negative charge and now has the
ability to repel electrons because of their like 23
charges.

24. VOLTAGE
When a charge of one coulomb exists between 2 bodies, one
unit of electrical energy exists, which is called the
DIFFERENCE OF POTENTIAL between the 2 bodies.
This difference of potential is referred to as ELECTROMOTIVE
FORCE (EMF), or VOLTAGE, and the unit of measurement
is VOLT (V).
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25. Voltage - is the FORCE that causes electrons to move in an
electrical circuit and is represented by the letter “E”.
When voltage is produced, the voltage is known as a source
voltage, applied voltage, or terminal voltage. Represented by
the letters Es, Ea, Et.
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26. The chassis is considered
to be at ZERO potential
and all other voltages are
either (+) or (-) with
respect to the chassis.
When used as the
reference point, the
chassis is said to be at
GROUND POTENTIAL,
referred to as GROUND.
The ground is a common connection point in the chassis.
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27. VOLTMETER – device used to measure values of voltage.
Must be connected across the difference of potential and will
display the difference of potential between the 2 points
selected.
To obtain a voltage reading, the circuit MUST be energized.
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28. METHODS OF PRODUCING VOLTAGE
1. By FRICTION (Static Electricity) – occurs by rubbing certain
materials together.
It is the least used of the 6 methods of producing voltage
because it is very difficult to maintain a steady difference of
potential or control the quantity of electrical charges.
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29. 2. By PRESSURE (Piezoelectricity) – occurs by squeezing
(compressing) crystals of certain substances.
When a crystal is compressed by a mechanical force,
electrons tend to move in one direction creating an electric
difference of potential between the two opposite faces of the
crystal.
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30. 3. By HEAT (Thermoelectricity) – occurs by heating the
junction of 2 dissimilar metals.
In some metals (copper) electrons tend to move away
from the hot end toward the cooler end, while in other metals
(iron) electrons tend to move toward the hot end.
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31. 4. By LIGHT – occurs when light strikes certain materials
causing valence electrons to be dislodged from atoms near
the surface of the material.
Devices that use
photosensitive material
to produce voltage are
called Photovoltaic Cells.
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32. 5. By CHEMICAL (Electrochemical action) – occurs when
certain substances are exposed to certain chemicals.
When 2 dissimilar metals or metallic materials are
immersed in a solution that produces greater chemical action
on one material than on the other, voltage will exists between
the two.
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33. 6. By MAGNETISM (Electromagnetic) – occurs when 3
conditions are met: a magnetic field, a conductor, and relative
motion between the two.
Moving a conductor through a magnetic field causes
electrons to be forced to one end of the conductor creating a
difference of potential between the 2 ends of the conductor.
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34. CURRENT
Voltage is the force that CAUSES electrons to move.
1. Random Drift – the effect of free electrons in a conductor
moving about in a haphazard manner without a voltage (force)
applied.
2. Directed Drift – results when a
voltage is applied to the conductor,
causing the electrons to be
repelled by the negatively charged
terminal and attracted to the
positively charged terminal of the
battery.
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35. 3. The directed drift or movement (flow) of electrons through a
conductor is called ELECTRICAL CURRENT, represented by
the letter “I”.
4. When 1 coulomb of electrons pass a given point in 1 second,
one unit of current is said to flow. This unit is called the
AMPERE, represented by the letter “A”.
1 Coulomb moving in 1 Second
is equal to: 1 Ampere
6,280,000,000,000,000,000 Electrons
Given Point in 1 Second
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36. As the voltage increases, more electrons will be forced to
move through the conductor, so that the current increases.
Voltage Current
The relationship between current and voltage is:
Current is DIRECTLY PROPORTIONAL to Voltage.
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37. 5. The device used to measure current is the AMMETER. It
indicates current flow in conductors that have a positive and
negative potential applied (complete circuit).
The meter MUST be connected so that all the current at the
point to be measured will flow through it (series / in line).
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38. 6. The Electron Flow Theory states that electrons flow from
negative to positive, this is the theory the Navy uses.
(a) Sometimes current is said to flow from positive to
negative, this is known as the Conventional Flow Theory.
ELECTRON FLOW THEORY
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39. 6. Electrical current is classified into 2 types:
(a) Direct Current (DC) is one way current flow, such as from
a battery.
(b) Alternating Current (AC) is 2 way current flow, such as
from a standard outlet in the wall.
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40. RESISTANCE
a. Resistance (R) – is the opposition a material offers to current
flow.
(1) Resistance is mainly based on the number of free
electrons a material has available.
(a) A “conductor” has many free electrons able to move
when voltage is applied, thus low resistance to current flow.
(b) An “insulator” has almost no free electrons and requires
a very large voltage to cause any electrons to move, thus has
a high resistance to current flow.
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41. Resistance
Opposition to current flow.
Voltage Current
Force that causes electrons to move. Movement (flow) of electrons
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42. (2) There are 4 factors affecting resistance:
(a) Type of material from which the material is made.
(b) The Length of the material. The amount of resistance of
a material is directly proportional to its length.
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43. (c) The Cross-Sectional Area of the material.
High R High I
Low I Low R
(d) The Temperature of the material. In some materials, the
resistance decreases with an increase in temperature
(Negative Temperature Coefficient), while in others, the
resistance increases with an increase in temperature (Positive
Temperature Coefficient).
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44. (3) Current flow is INVERSELY PROPORTIONAL to the
resistance in a material.
I – Current
I=1/R R – Resistance
(a) When voltage is applied to a material, free electrons want
to move as quickly as possible.
(b) Any resistance in a circuit will restrict the number of
electrons that can move, causing the flow to decrease, thus,
decreasing the amount of current.
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45. (4) The unit of measurement used to specify the amount of
resistance is the Ohm.
(a) The symbol used to represent the term “Ohm” is the
Greek letter “Omega” - Ω .
(b) One Ohm is the value of
resistance present in a 1 Volt 1 Ohm
material that allows just ONE
ampere of current to flow
when there is ONE volt
difference of potential applied.
1 Amp
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46. (5) The resistance of an electrical component is measured
utilizing an OHMMETER.
(a) Resistance readings are taken only when the circuit is
DE-ENERGIZED (turned off).
(b) The Ohmmeter is connected across the material to be
measured. In parallel with the component.
(c) If an Ohmmeter is giving erratic readings or a negative
reading, voltage may be present.
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47. (6) One other term to be familiar with is Conductance (G),
which is the inverse of resistance, or the ability to allow current
flow.
(a) Mathematically, the formula is:
G=1/R also R=1/G
(b) The unit of conductance is MHO (Ohm backwards); the
symbol used to represent conductance is the Greek letter
Omega upside down - .
1) Another term for MHO is SIEMENS.
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48. POWER
a. Power (P) - is the rate at which work can be or is being done.
(1) Work is done whenever a force causes motion or
movement.
(a) Work is a product of the force applied times the distance
moved:
W=FxD
(b) In electricity, voltage (E) is the force and current (I) is the
movement. When voltage causes current to flow, electrical
work is being done.
(c) Rewriting the Work Formula using electrical terms:
P=ExI
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49. (2) Current must flow to have power.
(a) When voltage exists but current does not flow, (no load is
connected to the voltage source), no work is done.
(3) The unit of measurement for power is the WATT,
represented by the letter “W”.
(a) One Watt of power represents the work done when a
force of one volt caused one Ampere of current to move past a
point in one second.
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50. (4) As current passes through a device, the resistance of the
device causes some of the electrical energy to be converted
into heat energy.
(a) The heat energy is given off or DISSIPATED to the
surrounding area, this is called Power Dissipation.
(b) Electric power conversion to heat is desirable, such as in
toasters, griddles, and heaters.
(c) May be undesirable by-product, such as in computers,
motors and generators.
(5) The amount of power a device can dissipate before
overheating or before damage occurs in the Power Rating of
the device.
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51. RELATIONSHIP
a. The relationship between voltage, current and resistance is
expressed by Ohm’s Law as: Current (I) in a current is directly
proportional to Voltage (E) and inversely proportional to
Resistance (R).
I=E/R
b. This relationship between voltage, current, and power is
actually called the Basic Power Formula, which means: Power
(P) is directly proportional to Current (I) and Voltage (E),
generally written as:
P=ExI
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52. USING OHM’S LAW
a. When calculating for voltage, current, resistance, or power in
electrical circuits, if any 2 values are known, the other values
can be found using Ohm’s Law or the Basic Power Formula:
(1) Ohm’s Law can be rearranged to solve for voltage or
resistance:
For Voltage:E=IxR
For Resistance: R = E / I

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53. (2) The Basic Power Formula can be rearranged to solve for
voltage or current:
For Voltage: E=P/I
For Current: I = P / E

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54. b. By combining formulas, a relationship can be found between
any 3 values:
c. There are 12 relationships available through combining
formulas.
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55. d. Ohm’s Law has been used to determine the effects current
flow has on the human body and to determine safe limits for
safety programs throughout the world.
(1) It has been determined that 0.1 Amp can KILL a person.
(2) At times, the resistance of the human body can be as low
as 300 Ohms due to moisture, sweat, or even fatigue.
(3) The voltage that can produce 0.1 Amp through a body at
300 Ohms is 30 volts:
0.1 Amp = 30Volts / 300Ohms
(4) The Navy has DIRECTED that a system that is greater
than 30 Volts is a dangerous High Voltage system requiring
special safety precautions.
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56. SUMMARY AND REVIEW
Enabling Objectives
1. RETRIEVE or RECOGNIZE information to answer questions
about matter, principles of an electrostatic charge, and the
relationship that exists between voltage, current, resistance
and power using OHM’s Law.
2. RETRIEVE or RECOGNIZE information pertaining to
methods of producing voltage.
3. CALCULATE circuit values using OHM’s Law.
4. APPLY safety precautions associated with DC Circuits in
accordance with NAVY SAFETY PRECAUTIONS FOR
AFLOAT FORCES.
56