Download Chapter 13: Electric Circuits

Chapter 13: Electric
Circuits
Genius is one percent inspiration, ninety-nine
percent perspiration.
Thomas Alva Edison (1847 – 1931)
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13.1 Electric Circuits
• Explain how electrical energy is supplied to
devices in a circuit.
• Use electrical symbols to draw simple circuit
diagrams.
• Distinguish between open and closed circuits.
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Electricity
• When we think of
“electricity” we usually
think of electric current
in wires, motors, light
bulbs, and other
devices.
• Electric current flows
and does work similar
to water flowing and
doing work.
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How to develop a current
– A water Analogy
• Water flows from high
pressure to a lower pressure.
• Electric current flows from a
high voltage to a lower
voltage.
• A “pump” or “battery” keeps
the current flowing!
• A “resistance” in the pipe will
restrict the flow of water.
• An electrical resistance will do
the same in an electrical
circuit.
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Electric Circuits
• An electric circuit is a complete path through which
electricity travels.
• Wires carry electricity in electric circuits in a way
similar to the way pipes carry water.
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Components of Electrical Circuits
• Circuits are made up of wires
and electrical devices such as
batteries, light bulbs, motors,
and switches.
• A resistor is an electrical device
that uses the energy carried by
electric current. Many electrical
devices have a “resistance” such
as light bulbs, heating elements,
speakers, and motors.
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A Schematic (or Circuit Diagram)
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Open and Closed Circuits
• You need a continuous path
for electric current to flow.
• All electric circuits must have
a source of energy.
• In the examples to the right it
is the battery that provides
the energy.
• An unintended break can
cause the current to stop.
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13.2 Current and Voltage
• List the units used to measure current and
voltage.
• Describe how to measure current and voltage
in a circuit.
• Explain the function of a battery in a circuit.
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Current
• Electric current is measured in Amperes (A), or
amps, for short.
• An Ammeter is used to measure electrical
current.
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Voltage
• Voltage is a measure of
electric potential energy, just
like height is a measure of
gravitational potential energy.
• Voltage is measured in Volts
(V).
• A Voltmeter is used to
measure voltage.
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How to develop a current
– A water Analogy
• Water flows from high
pressure to a lower pressure.
• Electric current flows from a
high voltage to a lower
voltage.
• A “pump” or “battery” keeps
the current flowing!
• A “resistance” in the pipe will
restrict the flow of water.
• An electrical resistance will do
the same in an electrical
circuit.
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A Couple Diagrams from the Textbook
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Batteries Use Stored Chemical Energy
A battery uses chemical energy to create a voltage difference between its
two terminals.
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Multimeters
• A Multimeter can be used to
measure both current and
voltage.
• Hook up a multimeter in
series to measure current,
and in parallel to measure
voltage!
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Multimeters
In parallel to measure voltage
In series to measure current
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Fuses & Circuit Breakers
•
•
•
•
Fuses and circuit breakers are two different ways
of protecting against suddenly large overloads of
electrical current.
Large power overloads are dangerous, potentially
destroying electrical equipment or causing a fire.
During normal flow of electricity, the fuse permits
the current to pass freely. But during an unsafe
overload, the small piece of metal melts, stopping
the flow of electricity. When a fuse blows out, it
must be thrown away and replaced with a new
fuse.
Circuit breakers are switches that are tripped
when the electrical current passes a safe limit.
Once tripped, the switch is simply turned off. That
stops the flow of electricity, which will remain off
until the switch is reset.
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13.3 Resistance and Ohm’s Law
• Explain the relationships between current,
voltage, and resistance.
• Use Ohm’s law to calculate current, resistance,
or voltage.
• Distinguish between conductors and
insulators.
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Resistance and Ohm’s Law
• A high resistance leads
to a smaller “current” of
water!
• A low resistance leads
to a larger “current” of
water!
• The same holds true for
electrical currents.
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Resistance and Ohm’s Law
• Electrical resistance is
measured in Ohms (W).
• Ohm’s law states that
the current, i, is directly
proportional to the
voltage, V, and inversely
proportional to the
resistance, R.
V  iR
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Resistance and Ohm’s Law
V  iR
Copper wire has a very low resistance, the resistance in circuits
comes from the light bulbs, motors, toasters, hair dryers, or
other “loads” connected to the circuit.
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Class Problems
1. Section 13.3: A typical light bulb has a resistance of 240
W. What is the current going through the light bulb
when hooked up to the household voltage of 120 Volts?
2. Section 13.3: On average your skin can feel a current of
about 0.0005 amps. If your skin has a resistance of
100,000 W, what voltage is needed to just feel a tingle?
3. Section 13.3: In lab, we will hook up a Christmas tree
light to a 5 volt battery. If the current is 0.03 amps,
what is the resistance of the light bulb?
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Conductors and Insulators
• Current passes easily through some
materials, such as copper, which are called
conductors. A conductor can conduct, or
carry, electric current. The electrical
resistance of wires made from conductors
is low. Most metals are good conductors.
• Other materials, such as rubber, glass, and
wood, do not allow current to easily pass
through them. These materials are called
insulators, because they insulate against,
or block, the flow of current.
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Resistors
• Resistors are designed
to regulate the current
flow in a circuit.
• They are found in many
common electronic
devices such as
computers, televisions,
telephones, and
stereos.
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Resistor Chart
11 W
68 W
330 W
47,000 W
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Potentiometers
• Simple resistors have a
predetermined resistance
over a wide range of
currents.
• Variable resistors can have
their resistance adjusted.
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Class Problems
4. Section 13.3: Using the formula i = V/R, describe two
ways of increasing a circuits current.
5. Section 13.3: The resistance of the human body when
the skin is dry is about 1 x 105 ohms. Find the current
through the body from a 120 V household outlet. A
person will feel a tingle at about 2 mA (milliamps). If
the skin becomes wet the resistance falls to about 1500
ohms, what would the current be now? Respiratory
paralysis occurs between 20 and 100 milliamps! It is
dangerous to work with electricity around water!
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