Transcript Chapter 2 - Cengage Learning

Chapter 2
Basic Electricity
Objectives
• Upon completion of this course, you will
be able to:
– Briefly explain the atomic theory and is
relationship to physical objects and electron
flow
– Explain the flow of electrons and how it is
accomplished
– Explain electrical potential, current flow, and
resistance and how they are measured
Objectives (cont’d.)
– Explain electrical power and how it is
measured
– Explain Ohm’s law
– Calculate the potential, current, and
resistance of an electrical circuit using Ohm’s
law
– Calculate the electrical power of a circuit and
the Btu/hour rating of an electrical resistance
heater
Key Terms
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Alternating current
Ampere
Atom
Compound
Conductor
Current
Direct current
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Electric energy
Electric power
Electric pressure
Electricity
Electrode
Electrolyte
Electron
Key Terms (cont’d.)
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Element
Field of force
Free electron
Insulator
Kilowatthour
Law of electric charges
Matter
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Molecule
Neutron
Nucleus
Ohm
Ohm’s law
Power factor
Proton
Key Terms (cont’d.)
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Resistance
Seasonal energy efficiency ratio (SEER)
Static electricity
Volt
Voltage/Potential difference/
Electromotive force
• Watt
Introduction
• Electrical energy
– Used to maintain desired temperature
• Control systems used in the heating, cooling,
and refrigeration industry
• Basic principles of electricity
– Must be understood by technicians
• To correct problems
Atomic Theory
• Key terms:
– Matter: substance of which a physical object
is composed
• Composed of fundamental substances (i.e.,
elements)
– Atom: smallest particle of an element that
can exist alone or in combination
– Molecule: smallest particle of a substance
that has the properties of that substance
Atomic Theory (cont’d.)
• Atom structure:
– Nucleus: made up of protons and neutrons
– Electrons: orbit the nucleus
– Protons: positive charge
– Electrons: negative charge
– Hydrogen atom: simplest atom
• One proton and one electron
Atomic Theory (cont’d.)
Figure 2.1 Atomic structure of a water molecule
(one atom of oxygen and two atoms of hydrogen).
(Delmar/Cengage Learning)
Positive and Negative
Charges
• Atoms can lose a few electrons for short
periods
– Electrons in outer orbits of some materials
can be easily knocked out of their orbits
(i.e., free electrons)
• Conductors: materials with free electrons
• Atoms can acquire additional electrons
– Become negatively charged
Positive and Negative
Charges (cont’d.)
• Electricity:
– One atom is charged and there is an unlike
charge in another atom
• Electrons can flow between the two
• Law of electric charges:
– Like charges repel and unlike charges
attract
Positive and Negative
Charges (cont’d.)
Figure 2.2 Like charges repel and unlike charges attract each other.
(Delmar/Cengage Learning)
Flow of Electrons
• Accomplished by several means:
– Friction: produces static electricity
– Chemical: produces electricity in a battery
– Magnetic (induction): produces electricity in
a generator
– Other methods are also used
Static Electricity
• Oldest method of moving electrons
– Permanently displaces an electron from an
atom
– Prolonged or steady flow of current is not
possible
– When charges are equalized, flow stops
– Usually caused by friction
Electricity Through Chemical Means
• Battery produces
electron flow by a
chemical reaction
– Causes transfer of
electrons between
two electrodes
Figure 2.3 Construction
of a dry cell battery.
(Delmar/Cengage Learning)
Electricity Through Magnetism
• Conductor cuts through a magnetic field
– Causes a displacement of electrons
• Examples: alternator, generator, and
transformer
• Used to supply electricity to consumers
– Flow of electrons produces magnetism
• Used to cause movement, or thermal energy,
which in turn is used to cause heat
Conductors and Insulators
• Conductor: material that has free
electrons available
– Can transmit electricity or electrons
• Example: most metals
• Insulator: material that does not easily
give up or take on electrons
– Retards flow of electrons
• Example: glass, rubber, and asbestos
Electric Potential
• Important terms:
– Voltage
• Volt, ampere, and ohm
– Potential difference
– Electromotive force
– Field of force
– Electric pressure
Electric Potential (cont’d.)
Figure 2.6 A dry cell battery
supplying electric potential
(voltage) to an electric circuit.
(Delmar/Cengage Learning)
Figure 2.7 A generator supplying electric
potential (voltage) to an electric circuit.
(Delmar/Cengage Learning)
Current Flow
• Electron flow in an electric circuit
– Obtained by a bolt of lighting, static
electricity, or electron flow from a generator
– Types:
• Direct current: flows in one direction
• Alternating current: reverses direction at regular
intervals
– Measured in amperes (A)
Resistance
• Opposition to flow of free electrons
– Measured in ohms
Figure 2.9
Two electric systems with
different resistances.
(Delmar/Cengage Learning)
Electric Power and Energy
• Electric power:
– Rate at which the electrons do work
• Rate at which electricity is being used
– Measured in watts (W)
• Electric energy:
– Rate at which electric power is being used
at a specific time
• Measured in watthours (Wh)
Ohm’s Law
• Relationship among current,
electromotive force, and resistance
Figure 2.13 Using Ohm’s law (not
applicable on AC inductive circuits).
(Delmar/Cengage Learning)
Calculating Electric Power
• Formula to calculate electric power:
P = IE
• Two other formulas:
P = E2/R
P = I2 R