Transcript Chapter 8

Vern J. Ostdiek
Donald J. Bord
Chapter 7
Electricity
(Section 4)
7.4 Electric Circuits and Ohm’s Law
• An electric current will flow in a lightbulb, a radio,
or other such device only if an electric field is
present to exert a force on the charges.
• A flashlight works because the batteries produce
an electric field that forces electrons to flow
through the lightbulb.
7.4 Electric Circuits and Ohm’s Law
• An electric circuit is any such system consisting of
a battery or other electrical power supply, some
electrical device such as a lightbulb, and wires or
other conductors to carry the current to and from
the device.
7.4 Electric Circuits and Ohm’s Law
•
The power supply acts like a “charge pump”:
•
•
it forces charges to flow out of one terminal, go
through the rest of the circuit, and flow into the
other terminal.
Electrons typically move through a circuit quite
slowly, about 1 millimeter per second.
• In this respect, an electric circuit is much like the
cooling system in a car in which the water pump
forces coolant to flow through the engine, radiator,
and the hoses connecting them.
7.4 Electric Circuits and Ohm’s Law
• The concepts of energy and work are used to
quantify the effect of a power supply in a circuit. In
a flashlight, for instance, the batteries cause
electrons to flow through the bulb’s filament.
• Because a force acts on the electrons and causes
them to move through a distance, work is done on
the electrons by the batteries.
•
•
In other words, the batteries give the electrons
energy.
This energy is converted into internal energy and
light as the electrons go through the lightbulb.
7.4 Electric Circuits and Ohm’s Law
• This leads to the concept of electric voltage.
Voltage The work that a charged particle can do
divided by the size of the charge.
work
v=
q
•
•
•
E
V=
q
The energy per unit charge given to charged
particles by a power supply.
The SI unit of voltage is the volt (V), which is equal
to 1 joule per coulomb.
Voltage is measured with a device called a
voltmeter.
7.4 Electric Circuits and Ohm’s Law
• A 12-volt battery gives 12 joules of energy to each
coulomb of electric charge that it moves through a
circuit.
•
Each coulomb does 12 joules of work as it flows
through the circuit.
7.4 Electric Circuits and Ohm’s Law
7.4 Electric Circuits and Ohm’s Law
• If we return to the analogy of a battery as a charge
pump, the voltage plays the role of pressure.
•
A high voltage causing charges to flow in a circuit is
similar to a high pressure causing a fluid to flow.
7.4 Electric Circuits and Ohm’s Law
• Even when the circuit is disconnected from the
power supply and there is no charge flow, the
power supply still has a voltage.
•
In this case, the electric charges have potential
energy.
• Voltage is also referred to as electric potential.
7.4 Electric Circuits and Ohm’s Law
• The size of the current that flows through a
conductor depends on its resistance and on the
voltage causing the current.
•
Ohm’s law, named after its discoverer, Georg Simon
Ohm, expresses the exact relationship.
• Ohm’s Law: The current in a conductor is equal to
the voltage applied to it divided by its resistance:
V
I=
or V = IR
R
•
•
The units of measure are consistent in the two
equations:
if I is in amperes and R is in ohms, then V will be in
volts.
7.4 Electric Circuits and Ohm’s Law
• By Ohm’s law, the higher the voltage for a given
resistance, the larger the current.
• The larger the resistance for a given voltage, the
smaller the current.
•
By applying different sized voltages to a given
conductor, one can produce different-sized
currents.
7.4 Electric Circuits and Ohm’s Law
• A graph of the voltage versus the current will be a
straight line with a slope that is equal to the
conductor’s resistance.
•
Reversing the polarity of the voltage (switching the
“–” and “+” terminals) will cause the current to flow
in the opposite direction.
7.4 Electric Circuits and Ohm’s Law
Example 7.1
• A lightbulb used in a 3-volt flashlight has a
resistance equal to 6 ohms.
•
What is the current in the bulb when it is switched
on?
• By Ohm’s law,
V 3V
I= =
= 0.5 A
R 6W
7.4 Electric Circuits and Ohm’s Law
Example 7.2
• A small electric heater has a resistance of 15
ohms when the current in it is 2 amperes.
•
What voltage is required to produce this current?
V = IR = 2 A´15 W = 30 V
7.4 Electric Circuits and Ohm’s Law
• Not all devices remain “ohmic”—that is, obey
Ohm’s law—as the voltage applied to them
changes.
•
Often, instead of remaining constant, the resistance
of a conductor changes when the voltage changes.
• At higher voltages, a larger current flows through
the filament of a lightbulb, so its temperature is
also higher.
•
The resistance of the hotter filament is
consequently greater.
7.4 Electric Circuits and Ohm’s Law
7.4 Electric Circuits and Ohm’s Law
•
Some semiconductor devices, called diodes, are
designed to have very low resistance when current
flows through them in one direction but very high
resistance when a voltage tries to produce a
current in the other direction.
• Water with salt dissolved in it generally has lower
resistance when higher voltages are applied to it:
•
doubling the voltage will more than double the
current. A graph of V versus I for ordinary tap water
is less steep at higher voltages.
7.4 Electric Circuits and Ohm’s Law
• Many electrical devices are controlled by changing
a resistance.
• The volume control on a radio or a television
simply varies the resistance in a circuit.
•
Turning up the volume reduces the resistance, so
more current flows in the circuit, resulting in louder
sound.
• A dimmer control used to change the brightness of
the lights in a room works the same way.
7.4 Electric Circuits and Ohm’s Law
Series and Parallel Circuits
• In many situations, several electrical devices are
connected to the same electrical power supply.
•
•
A house may have a hundred different lights and
appliances all connected to one cable entering the
house.
An automobile has dozens of devices connected to
its battery.
• There are two basic ways in which more than one
device can be connected to a single electrical
power supply—
•
by a series circuit and by a parallel circuit
7.4 Electric Circuits and Ohm’s Law
• In a series circuit, there is only one path for the
charges to follow, so the same current flows in
each device.
7.4 Electric Circuits and Ohm’s Law
• In such a circuit, the voltage is divided among the
devices:
•
the voltage on the first device plus the voltage on
the second device, and so on, equals the voltage of
the power supply.
• For example, if three lightbulbs with the same
resistance are connected in series to a 12-volt
battery, the voltage on each bulb is 4 volts.
•
If the bulbs had different resistances, each one’s
“share” of the voltage would be proportional to its
resistance.
7.4 Electric Circuits and Ohm’s Law
• Notice that the current in a series circuit is stopped
if any of the devices breaks the circuit.
7.4 Electric Circuits and Ohm’s Law
• A series circuit is not normally used with, say, a
number of lightbulbs because if one of them burns
out, the current stops and all of the bulbs go out.
•
A string of Christmas lights that flash at the same
time uses a series circuit so that all the bulbs go on
and off together.
7.4 Electric Circuits and Ohm’s Law
• In a parallel circuit, the current through the power
supply is “shared” among the devices while each
has the same voltage.
7.4 Electric Circuits and Ohm’s Law
• The current flowing in the first device plus the
current in the second device, and so on, equals
the current output by the power supply.
•
There is more than one path for the charges to
follow—in this case, three.
• If one of the devices burns out or is removed, the
others still function.
•
The lightbulbs in multiple-bulb light fixtures are in
parallel so that if one bulb burns out, the others
remain lit.
• Often, the two types of circuits are combined:
• one switch may be in series with several lightbulbs
that are in parallel.
7.4 Electric Circuits and Ohm’s Law
Example 7.3
• Three lightbulbs are connected in a parallel circuit
with a 12-volt battery. The resistance of each bulb
is 24 ohms.
•
What is the current produced by the battery?
• The voltage on each bulb is 12 volts. Therefore,
the current in each bulb is
V 12 V
I= =
= 0.5 A
R 24 W
• The total current supplied by the battery equals
the sum of the currents in the three bulbs.
I = 0.5 A +0.5 A +0.5 A =1.5A
7.4 Electric Circuits and Ohm’s Law
• The concept of voltage is quite general and is not
restricted to electrical power supplies and electric
circuits.
• Whenever there is an electric field in a region of
space, a voltage exists because the field has the
potential to do work on electric charges.
•
The strength of an electric field can be expressed in
terms of the voltage change per unit distance along
the electric field lines.
7.4 Electric Circuits and Ohm’s Law
• For example, air conducts electricity when the
electric field is strong enough to ionize atoms in
the air.
• The minimum electric field strength required for
this to happen is between 10,000 and 30,000 volts
per centimeter, depending on the conditions.
•
This means that if there is a spark one-fourth of an
inch long between your finger and a doorknob, the
voltage that causes the spark is at least 7,500 volts.
7.4 Electric Circuits and Ohm’s Law
• As transistors and other components on integrated
circuit chips (ICs) are made smaller, even the low
voltages that are used to make them operate
(typically around 1 volt) produce very strong
electric fields.
•
Inside modern ICs, electric field strengths can
reach 400,000 V/cm.
• Designers of ICs must keep this in mind because
electric fields only about 25 percent stronger than
this can disrupt circuit processes.