Transcript Chapter 16 - Ms

Chapter 16
Section 2 Current
Objectives
• Describe how batteries are sources of voltage.
• Explain how a potential difference produces a
current in a conductor.
• Define resistance.
• Calculate the resistance, current, or voltage, given
the other two quantities.
• Distinguish between conductors, superconductors,
semiconductors, and insulators.
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Chapter 16
Section 2 Current
Bellringer
1. Dry cell batteries are a source of mobile electrical
power. Name five devices that use dry cell batteries.
2. Give reasons why copper is normally used to wire a
home for electricity.
3. Why is it important to unplug a device by pulling the
plug instead of by yanking it out of the socket by
pulling on the electrical cord?
4. Why are electrical appliances, such as razors, hair
dryers, and curling irons, not to be used in the
bathtub or shower?
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Chapter 16
Section 2 Current
Voltage and Current
• Electrical potential energy is the ability to move an
electric charge from one point to another.
• The electrical potential energy of the moving
charge decreases because the electric field does
work on the charge.
• The electrical potential energy depends on the
distance between two charged objects for both an
attractive and a repulsive electric force.
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Chapter 16
Section 2 Current
Electrical Potential Energy
The electrical potential energy between two negative charges decreases as the
distance between them increases.
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Chapter 16
Section 2 Current
Electrical Potential Energy and Relative
Position
The electrical potential energy of a charge depends on its position in an electric field.
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Chapter 16
Section 2 Current
Electrical Potential Energy
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Chapter 16
Section 2 Current
Voltage and Current, continued
• Potential difference is measured in volts.
• The potential difference between any two points,
is the work that must be done against electric
forces to move a unit charge from one point to the
other.
• The volt, V, is equivalent to one joule per coulomb
(1 J/C).
• Potential difference is often called voltage.
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Chapter 16
Section 2 Current
Potential Difference
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Chapter 16
Section 2 Current
Voltage
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Chapter 16
Section 2 Current
Voltage and Current, continued
• There is a voltage across the terminals of a battery.
• A cell is a device that is a source of electric
current because of a potential difference, or
voltage, between the terminals.
• One terminal is positive, and the other is
negative.
• Batteries convert chemical energy into electrical
energy.
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Chapter 16
Section 2 Current
Battery
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Chapter 16
Section 2 Current
Electric Cell
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Chapter 16
Section 2 Current
Voltage and Current, continued
• A voltage sets charges in motion.
• Current is the rate that electric charges move
through a conductor.
• The SI unit of current is the ampere, A.
• 1 amp = 1 C/s
• A battery is a direct current source because the charges
always move from one terminal to the other in the same
direction.
• Conventional current is defined as movement of
positive charge.
• The direction of current in a wire is opposite the direction
that electrons move in that wire.
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Chapter 16
Section 2 Current
Comparing Direct and Alternating Current
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Chapter 16
Section 2 Current
Conventional Current
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Chapter 16
Section 2 Current
Electrical Resistance
• Resistance is the opposition posed by a material or a
device to the flow of current.
• Resistance is caused by internal friction, which slows
the movement of charges through a conducting
material.
• Resistance can be calculated from current and voltage.
voltage
resistance =
current
V
R=
I
• The SI unit of resistance is the ohm, Ω. 1 Ω = 1
V/A
• A resistor is a special type of conductor used to
control current.
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Chapter 16
Section 2 Current
Math Skills
•
Resistance The headlights of a typical car are
powered by a 12 V battery. What is the resistance of
the headlights if they draw 3.0 A of current when
turned on?
1. List the given and unknown values.
Given:
current, I = 3.0 A
voltage, V = 12 V
Unknown: resistance, R = ? Ω
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Chapter 16
Section 2 Current
Math Skills, continued
2. Write the equation for resistance.
voltage
resistance =
current
V
R=
I
3. Insert the known values into the equation, and
solve.
V
12 V
R=
=
I
3.0 A
R = 4.0 
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Chapter 16
Section 2 Current
Electrical Resistance, continued
• Conductors have low resistances.
• Some materials become superconductors below a
certain temperature.
• Certain metals and compounds have zero resistance when
their temperature falls below a certain temperature called
the critical temperature.
• Semiconductors are intermediate to conductors and
insulators.
• The controlled addition of specific atoms of other materials
as impurities dramatically increases a semiconductor’s
ability to conduct electric charge.
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