chapter7-Section6

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Transcript chapter7-Section6

Vern J. Ostdiek
Donald J. Bord
Chapter 7
Electricity
(Section 6)
7.6 AC and DC
• The current flows out of the positive (+) terminal of
the power supply, moves through the circuit, and
flows into the negative (–) terminal of the power
supply.
•
•
If the total resistance in the circuit doesn’t change,
the size of the current remains constant (as long as
the battery doesn’t run down).
A graph of the current I
versus time t is simply a
horizontal line.
7.6 AC and DC
• In an AC power supply, the polarity of the two
output terminals switches back and forth—the
voltage alternates.
• This causes the current in any circuit connected to
the power supply to alternate as well.
•
It flows counterclockwise, then clockwise, then back
to counterclockwise, and so on.
7.6 AC and DC
• All the while, the size of the current is increasing,
then decreasing, and so forth.
•
•
A graph of the current in an AC circuit shows this
variation in the size and direction of the current.
When I goes below zero, it means that the direction
has reversed.
7.6 AC and DC
• One can even think of AC as a kind of “wave”
causing the charges in a conductor to oscillate
back and forth.
• Almost all public electric utilities in the United
States supply 60-hertz AC.
•
•
The voltage between the two slots in a wall outlet
oscillates back and forth 60 times per second.
In Europe, the standard frequency of AC is 50
hertz.
7.6 AC and DC
• Some electronic devices (such as lightbulbs) can
operate on AC or DC, whereas others require one
or the other.
•
Electric motors and generators must be designed to
operate on or produce either AC or DC.
• There are devices that can convert an AC voltage
to a DC voltage and vice versa.
•
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Batteries can produce direct current only.
For this reason, automobiles have DC electrical
systems.
The alternator in an automobile generates AC,
which is then converted into DC to be compatible
with the battery.
7.6 AC and DC
• Alternating current has one distinct advantage
over DC:
•
simple, highly efficient devices called transformers
can “step up” or “step down” AC voltages.
• This makes it possible to generate AC at a power
plant at some intermediate voltage, step it up to a
very high voltage (typically more than 300,000
volts) for economical transmission, and then step it
down again to lower voltages for use in homes
and industries.
7.6 AC and DC
• There is no counterpart of the transformer for DC.
Another important use of AC is in electronic sound
equipment.
•
One example: if a 440-hertz tone is recorded on
tape and then played back, the “signal” going to
the speaker will be an alternating current with a
frequency of 440 hertz.
Summary
• Electrons, protons, and certain other subatomic
particles possess a physical property called
electric charge that is the basic source of
electrical and magnetic phenomena.
• Forces act between any objects that possess a net
electric charge, positive or negative.
•
•
Objects with like charges experience repulsive
forces;
objects with unlike charges experience attractive
forces.
Summary
• The electrostatic force, expressed by Coulomb’s
law, is responsible for binding electrons to the
nucleus in atoms, for the amber effect (such as
static cling), and for a number of other natural or
technological phenomena.
• Electric charges produce electric fields in the
space around them.
•
This field is the agent for the electrostatic force, just
as the gravitational field is the agent of the
gravitational attraction between objects.
Summary
• Most useful applications of electricity involve
electric currents.
•
They most often involve the flow of electrons
through metal wires driven by an electrical power
supply, such as a battery.
• The flow of charge is analyzed using the physical
concepts voltage, current, and resistance.
• Ohm’s law states that the current in a circuit
equals the voltage divided by the resistance.
Summary
• The power consumption in a circuit depends on
the voltage and the current.
• The electrical energy needed to cause a current to
flow through a resistive element is converted into
internal energy.
•
This ohmic heating is usefully exploited directly by
space heaters and toasters, as well as by
incandescent lightbulbs to produce light.
Summary
• Fuses and circuit breakers are used to
automatically disconnect a circuit if the current is
large enough to cause excessive ohmic heating.
• At extremely low temperatures, many materials
become superconductors—they have zero
resistance.
•
•
Consequently, no energy is lost to heating when
electric currents flow through them.
Superconductors now in use are still primarily
limited to special-purpose scientific and medical
instruments.
Summary
• There are two types of electric current: alternating
(AC) and direct (DC).
•
Because batteries produce DC, battery-powered
devices generally employ DC.
• Transformers can be built to “step up” or “step
down” an AC voltage from one value to another.
•
This makes AC particularly convenient for electrical
supply networks such as electric utilities.