Transcript Chapter 17 - apphysicswarren

Lecture Outline
Chapter 17
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
Units of Chapter 17
Batteries and Direct Current
Current and Drift Velocity
Resistance and Ohm’s Law
Electric Power
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Big Questions
• What is electric current and how does it
travel?
• What causes current to move when
switched on?
17.1 Batteries and Direct Current
• What is required to produce a flow of
charge?
– Voltage
• Energy is required to move electrons.
– 2 examples
– Any device that can produce and maintain a
potential difference (voltage) is called a power
supply.
17.1 Batteries and Direct Current
A battery is a source
of electric energy—it
converts chemical
energy into electric
energy.
Let’s discuss how the
battery works.
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17.1 Batteries and Direct Current
In a complete circuit, electrons flow from the
negative electrode to the positive one.
The positive electrode is called the anode; the
negative electrode is the cathode.
A battery provides a constant source of
voltage—it maintains a constant potential
difference between its terminals.
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17.1 Batteries and Direct Current
• A battery can do work on, and transfer
energy to, the electrons in the wire, which
then delivers that energy to other circuit
elements.
• Energy can be converted into different
forms:
– Heat
– Light
– Mechanical
17.1 Batteries and Direct Current
The potential difference
between the battery terminals
when the battery is not
connected to anything is called
the electromotive force, emf.
Name is misleading is because
not a force.
Represents work done per
charge. J/C
Represents maximum potential
difference across terminals.
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17.1 Batteries and Direct Current
The actual voltage of the battery is always
less than the emf, due to internal resistance.
This “operating voltage,” is called terminal
voltage. Usually the difference is very small.
Let’s think
about an
example.
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17.1 Batteries and Direct Current
When batteries are
connected in
series, the total
voltage is the sum
of the individual
voltages.
(Positive ends are
connected to
negative ends)
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17.1 Batteries and Direct Current
When batteries of
equal voltage are
connected in parallel,
the total voltage does
not change; each
battery supplies only
a fraction of the total
current.
The voltage is the
same for all batteries.
(+’s connected to +’s)
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17.1 Batteries and Direct Current
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17.1 Batteries and Direct Current
• The figure to the right shows a
circuit diagram that represents 2
identical batteries connect in
parallel to a lightbulb. It is assumed
that the wires have no resistance.
What happens to the voltage when
S1 is opened?
– a.) The voltage remains the
same as that before the switch
was opened.
– b.) The voltage drops to half
because only one battery is
now connected.
– c.) The voltage drops to zero.
17.2 Current and Drift Velocity
• To sustain an electric current requires a _________
source and a ________ circuit.
• A complete circuit is…
– Open switch
– Closed switch
17.2 Current and Drift Velocity
Current is the time rate of flow of charge.
SI unit of current: the ampere, A
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17.2 Current and Drift Velocity
Electrons move away
from…
This is opposite of the
current’s direction.
Current is one in which
positive charges flow.
A circuit draws current
from a battery. A battery
can only deliver current
in 1 direction.
1 directional current is
called…
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17.2 Current and Drift Velocity
• Suppose there is a steady current of 0.50A in
a flashlight bulb lasting for 2.0 min. How
much charge passes through the bulb during
this time? How many electrons does this
represent?
17.3 Resistance and Ohm’s Law
The greater the voltage, the greater the current.
However, resistance influences current as well.
Resistors
The ratio between the voltage and the current is
called the resistance.
SI unit of resistance: the ohm, Ω
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17.3 Resistance and Ohm’s Law
17.3 Resistance and Ohm’s Law
Ohm’s law states that when resistance is
constant over ranges of voltages a material
is said to be ohmic. [Key here is that
resistance is constant!]
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17.3 Resistance and Ohm’s Law
• Suppose a person steps out of a shower and
inadvertently touches an exposed 120-V wire
with a finger this creating a complete circuit.
The human body when wet can have an
electrical resistance as low as 300 ohms.
Using this value, estimate the current in that
person’s body.
17.3 Resistance and Ohm’s Law
• Resistance arises when
electrons collide with
the atoms that make up
the material. There are
4 factors that influence
resistance:
–
–
–
–
Type of Material
Length
Cross-Sectional Area
Temperature
17.3 Resistance and Ohm’s Law
As expected, the resistance is proportional
to the length and inversely proportional to
the cross-sectional area (why?):
The constant ρ is called the resistivity,
and is characteristic of the material.
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17.3 Resistance and Ohm’s Law
• Suppose an electric eel touches the head
and tail of a long approximately cylindrically
shaped fish, and applies a voltage of 600 V
across it. If a current of 0.80 A results,
estimate the average resistivity of the fish’s
flesh, assuming it is 20 cm long and 2.0 cm
in diameter.
17.3 Resistance and Ohm’s Law
Some materials exhibit a curious
phenomenon: at a very low temperature called
the critical temperature, their resistivity drops
abruptly to zero.
These are called superconductors; they have
a number of unique properties.
Many more studies and research are being
done right now to implement more
superconductors into everyday appliances to
make items more efficient.
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17.4 Electric Power
Power, as usual, is the rate at which work is
done. For work done by electricity:
Rewriting,
For ohmic materials, we can write: (3 ways)
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17.4 Electric Power
Using the figure below, find the current and
the power.
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17.4 Electric Power
Electric appliances
are rated in watts,
assuming standard
household voltage.
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17.4 Electric Power
The electric company
typically bills us for
kilowatt-hours (kWh),
a unit of energy.
1 kWh
We can reduce our
energy usage by buying
efficient appliances.
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17.4 Electric Power
• Consider 2 appliances that operate at the same voltage.
Appliance A has a higher power rating than Appliance B.
How does the resistance of A compare to B?
– Larger
– Smaller
– The Same
• A computer system includes a color monitor with a
power requirement of 200W, whereas a countertop
toaster oven is rated at 1500W. What is the resistance of
both if each run at 120 V?