Transcript CHAPTER 1 SAFETY

Essentials of
Electronics
Second Edition
Frank D. Petruzella
Glencoe/McGraw-Hill
CHAPTER 4
SOURCES AND
CHARACTERISTICS
OF ELECTRICITY
Most materials belong to one of three
Any device that develops and maintains
a voltage can be considered a voltage
source. To create electricity, the voltage
source causes electrons to move from
one point to another point.
CHAPTER 4
SOURCES AND
CHARACTERISTICS
OF ELECTRICITY
In this chapter, we will explore the
different methods used to produce
voltage.
OBJECTIVES
• Define static and current electricity.
• Explain how static positive and negative charges
are produced.
• State the law of electric charges.
• Explain the difference between direct current (dc)
and alternating current (ac) electricity.
• List the basic sources of electricity and electrical
devices used to convert the various energy forms.
What is Electricity?
• The flow of free electrons through a
conductor.
• What else is electricity???
– http://www.amasci.com/miscon/whatis.html
Producing Static Electricity by Friction
Charge accumulates
at end of rod only
Fur and rubber
rod rubbed together
Negative charge
produced on the rod
Electrons move from
the fur to the rod
Law of Electric Charges
Like charges repel
Unlike charges attract
Charging by Contact
Electroscope
Negative
charge
Positive
charge
produced
produced
Neutral
Charging by Induction
Metal
sphere
Insulated
stand
Placing negative charge
near sphere causes
redistribution of charges
Grounding sphere allows
electrons to leave
When ground is removed
sphere remains positively charged
Materials which are called electrical "INSULATORS" have electrons strongly attached to the nucleus of the atoms in
the material so that no flow of electrons (or current) can take place in the material.
Materials which are called electrical "CONDUCTORS" have electrons which can leave the atoms and migrate freely
through the material.
Most metals are good electrical conductors because they only have a few electrons in the outer shell of the atom which
can be easily detached and moved through the material,becoming an electric current.
When a negatively charged rod is placed close to the neutral metal sphere the (negatively) charged electrons in the
sphere are repelled to the far side of the sphere, leaving the atoms on the near side positively charged owing to their
missing electrons.
If we then connect a copper wire to the negative side of the sphere and an electrical ground some of the free electrons
will flow into the ground.
When we then remove the copper wire and the negatively charged rod what remains is a metal sphere with a uniform
distributed positively charge.
Current Electricity
Electrical Charge in Motion
Electromotive Force
• Emf, Voltage, V, E all the same…
• Pressure that causes electrons to flow.
• From the primary sources of energy…
Direct Current (dc) Electricity
Battery
Symbol
Waveform
Alternating Current (ac) Electricity
Symbol
Outlet
Waveform
Producing Electricity from Light
Video on how a solar cell works
http://www.youtube.com/watch?v=x2zjdtxrisc
Solar
Cell
Practical
Application
Producing Electricity by Means of a
Chemical Reaction
Electrolyte
Solution
Positive Electrode
Negative Electrode
Electrons move from the positive electrode to
the negative electrode of the battery.
http://www.rayovac.com/wizard/battery_howwork.htm
Producing Electricity Using Heat
Millivoltmeter:
calibrated in degrees
Iron:
losses electrons
Copper:
Industrial
Thermocouple
gains electrons
http://en.wikipedia.org/wiki/Thermocouple
Thermocouple
heated at junction
Producing Electricity Using Pressure
Voltage produced at plates
Pressure exerted
on crystal
http://www.explainthatstuff.com/piezoel
ectricity.html
Practical use:
microphone
Producing Electricity Using
Mechanical-Magnetic Action
Coiled
conductor
Application:
generator
Moving
magnetic
field
Voltage
generated