Ohm's law

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Unit-3 DIRECT CURRENT CIRCUITS

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References

1. NEETS Module 1 – Introduction to Matter, Energy, and Direct Current.

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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.

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THE ATOM

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

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THE ATOM

An Atom of each element are made up of still smaller units called subatomic particles.

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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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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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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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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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Free electrons enable the production of voltage and transfer of electrical energy.

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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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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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-

-

-

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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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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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An Atom is electrically neutral if it contains the same number of protons as electrons.

Ionization – process where an atom gains or loses electrons, changing the electrical charge of the atom and becoming an ion.

PROTONS = ELECTRONS

ELECTRICALLY NEUTRAL

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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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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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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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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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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 =Q1 x Q2

d2

F = Force

Q = Charge of Individual bodies

d = Distance between bodies

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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.

1 Coulomb = 6.28 x 1018

Electrons

that is:6,280,000,000,000,000,00

0ELECTRONS

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A positive charge would result by starting with a neutral body and removing electrons from it.

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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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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 charges.

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-

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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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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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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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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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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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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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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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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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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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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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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.

35Given Point in 1 Second

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”.

6,280,000,000,000,000,000 Electrons

1 Coulomb moving in 1 Second is equal to: 1 Ampere

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Voltage Current

As the voltage increases, more electrons will be forced to move through the conductor, so that the current increases.

The relationship between current and voltage is:

Current is DIRECTLY PROPORTIONAL to Voltage.

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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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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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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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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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Voltage Current

Resistance

Movement (flow) of electronsForce that causes electrons to move.

Opposition to current flow.

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(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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(c) The Cross-Sectional Area of the material.

(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).

High ILow R

High RLow I

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(3) Current flow is INVERSELY PROPORTIONAL to the resistance in a material.

I = 1 / R (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.

I – CurrentR – Resistance

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(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” -

(b) One Ohm is the value of resistance present in a material that allows just ONE ampere of current to flow when there is ONE volt difference of potential applied.

1 Volt

1 Amp

1 Ohm

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(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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(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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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 = F x D (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 = E x I

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(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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(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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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 / Rb. 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 = E x I

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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 = I x RFor Resistance:R = E / I

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(2) The Basic Power Formula can be rearranged to solve for voltage or current:

For Voltage: E = P / IFor Current:I = P / E

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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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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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SUMMARY AND REVIEWEnabling 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.