Industrial Electricity EEM 105

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Industrial ElectricityEEM 105

• Chapter 2
• Electron Theory and Ohms Law

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Matter

• Anything that occupies space and has weight is
  called matter.
• All liquids, gases and solids are forms of matter
  in different forms.
• Matter is made up of smaller units called atoms.

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Atoms

• An atom resembles the solar system with the sun
  in the center, around which the planets rotate.
• In an atom there is a relatively large mass at
  the center which is called the nucleus.
• Electrons revolve in orbital patterns around the
  nucleus.

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Electrical Charge

• A material has an electrical charge when it
  attracts or repels another charged material.
• A material may have a positive or a negative
  electrical charge.
• Materials with similar charges will repel each
  other.
• Materials with dissimilar charges will attract
  each other.

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Protons and Neutrons

• Part of the nucleus is made up of protons.
• Protons have a positive charge.
• The rest of the nucleus is made up of neutrons.
• Neutrons are electrically neutral.

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Electrons

• One or more electrons revolve around the nucleus
  of an atom.
• Electrons are negatively charged and are lighter
  in weight than protons.
• An atom will contain the same number of electrons
  and protons.

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Current

• Electrons in motion results in an an electrical
  current.
• Copper wire is often used to carry electrical
  current.
• Each atom in the copper wire has electrons
  revolving around its nucleus.

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Current

• When pressure (voltage) from a battery or
  generator is applied, the electrons are forced
  out of their circular orbits and are passed from
  one atom to another along the length of the wire.
• The greater the number of electrons passing a
  given point in a circuit, the greater the
  intensity of the current.

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Current

• Intensity of the current is measured in amperes
  (A).
• An ammeter is used to measure current.
• The amount of current in a circuit is represented
  by the letter I.
• Ammeters are connected in series with other
  devices in a circuit.

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An ammeter can be used to measure current.
An ammeter displays current measured in amperes
(or milliamperes or microamperes). Its pointer
moves in front of a graduated scale.
The rotation of the pointer is proportional to
the current that is flowing through the meter.
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Current Types

• Direct current (dc) is the movement of electrons
  in 1 direction in the conductor.
• Pulsating direct current is a dc current that
  varies in intensity at a regular interval.
• Alternating current (ac) is a current which
  changes in direction and intensity at a regular
  interval of time.

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Voltage

• A closed circuit and a source of energy are
  required to produce an electrical current.
• Electrical pressure, known as voltage or
  potential difference, is obtained from a variety
  of sources
• Generators, batteries, photo-electric cells.

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Voltage

• The letter E is used to represent a voltage.
• The volt (V) is used to express the quantity of
  electrical pressure. The voltmeter is used to
  measure voltage.
• Voltmeters are connected in parallel with other
  devices in a circuit.

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Digital voltmeter
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Polarity

• All dc sources of electrical pressure have 2
  terminals to which electrical devices are
  connected.
• One terminal is the positive terminal and the
  other is the negative terminal.
• This is called electrical polarity.
• Electrons flow through the device from the
  negative terminal to the source of the positive
  terminal.

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Resistance

• The property of a material which causes it to
  resist the flow of electrons is called
  resistance.
• All materials have some resistance.
• Materials that offer little resistance are called
  conductors.

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Resistance

• Materials that offer a high resistance are called
  insulators.
• Resistance is measured in ohms. The symbol for
  ohms is the Greek symbol omega ?.
• The instrument used to measure resistance is the
  ohmmeter.
• Electrical power must be disconnected in a
  circuit when using an ohmmeter.

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Ohms Law

• Ohms law and formula shows the relationship of
  current, voltage and resistance.
• In any electrical circuit the current is directly
  proportional to the voltage applied to the
  circuit and is inversely proportional to the the
  resistance in the circuit.
• Note that both resistance and voltage affect the
  current.

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Ohms Law

• When resistance is constant, the current can be
  changed by changing the voltage current will
  increase when the voltage is increased.
• When voltage is constant, the current will
  decrease when the resistance is decreased.

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Ohms Law

• I E/R
• I intensity of current in amperes
• E quantity of electrical pressure in volts
• R amount of resistance in ohms

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Ohms Law

• Two other forms of Ohms Law are
• E IR
• R E/I

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Ohms Law

• Example
• If a voltage of 24 volts appears across a
  resistance of 4 ohms, what is the current through
  the resistance.
• I E/R 24 volts/4 ? 6 amperes.

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Ohms Law

• Example
• Find the voltage that appears across an 8 ohm
  resistance if the current through it is 10
  amperes.
• E IR (10 amperes)(8 ?) 80 volts.

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The End