Transcript Ch 17 ppt: Introduction to Electricity

Chapter 17
Introduction to Electricity
Preview
Section 1 Electric Charge and Static Electricity
Section 2 Electric Current and Electrical Energy
Section 3 Electrical Calculations
Section 4 Electric Circuits
Concept Mapping
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Bellringer
Write a definition for electric charge in your own words
in your science journal. When do you experience
electric charges most, in winter or in summer? Explain
your reasoning.
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Objectives
• Describe how charged objects interact by using the
law of electric charges.
• Describe three ways in which an object can become
charged.
• Compare conductors with insulators.
• Give two examples of static electricity and electric
discharge.
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Electric Charge
• Charges Exert Force Atoms are composed of
particles with electric charge. The law of electric
charges states that like charges repel and opposite
charges attract.
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Chapter 17
Section 1 Electric Charge and
Static Electricity
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Electric Charge, continued
• The Force Between Protons and Electrons
Because protons and electrons have opposite
charges, they are attracted to each other.
• The Electric Force and the Electric Field The
force between charged objects is an electric force.
An electric field is the region around a charged
object in which an electric force is exerted on another
charged object.
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Chapter 17
Section 1 Electric Charge and
Static Electricity
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Charge It!
• Friction Charging by friction happens when
electrons are “wiped” from one object onto another.
• Conduction Charging by conduction happens when
electrons move from one object to another by direct
contact.
• Induction Charging by induction happens when
charges in an uncharged metal object are rearranged
without direct contact with a charged object.
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Charge It!, continued
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Charge It!, continued
• Conservation of Charge
When you charge something
by any method, no charges
are created or destroyed.
The numbers of electrons
and protons stay the same.
• Detecting Charge You
can use a device called an
electroscope to see if
something is charged.
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Charging by Contact
Click below to watch the Visual Concept.
Visual Concept
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Moving Charges
• Conductors An electrical conductor is a material
in which charges can move easily.
• Insulators An electrical insulator is a material in
which charges cannot move easily.
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Static Electricity
• Static electricity is the electric charge at rest on an
object.
• Electric Discharge The loss of static electricity as
charges move off an object is called electric
discharge. One of the most dramatic examples of
electric discharge is lightning. The next slide shows
how lightning is formed.
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Chapter 17
Section 1 Electric Charge and
Static Electricity
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Chapter 17
Section 1 Electric Charge and
Static Electricity
Static Electricity, continued
• Lightning Dangers It is particularly dangerous to
be at the beach or on a golf course during a lightning
storm. Even standing under a tree during a storm is
dangerous.
• Lightning Rods A lightning rod is a pointed rod
connected to the ground by a wire. Objects that are
joined to Earth by a conductor, such as a wire, are
grounded.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Bellringer
What is the difference between something that is direct
and something that is alternating? How does the
difference relate to electric current?
Record your responses in your science journal.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Objectives
• Describe electric current.
• Describe voltage and its relationship to electric
current.
• Describe resistance and its relationship to electric
current.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Objectives, continued
• Explain how a cell generates electrical energy.
• Describe how thermocouples and photocells
generate electrical energy.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Electric Current
• Electric current is the rate at which charges pass
through a given point. Electric current is expressed in
units called amperes, or amps.
• Making Charges Move When you flip a switch, an
electric field is set up in the wire at the speed of light.
The electric field causes the free electrons in the wire
to move.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Electric Current, continued
• Commanding Electrons to Move This electric field
is created so quickly that all electrons start moving
through the wire at the same instant. Think of the
electric field as a command to the electrons to charge
ahead.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Electric Current, continued
• AC and DC There are two kinds of electric current—
direct current (DC) and alternating current (AC).
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Voltage
• Voltage is the potential difference between two points
in a circuit. Voltage is expressed in volts (V).
• Voltage and Energy Voltage is a measure of how
much work is needed to move a charge between two
points.You can think of voltage as the amount of
energy released as a charge moves between two
points in the path of a current.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Voltage, continued
• Voltage and Electric
Current As long as there is a
voltage between two points on
a wire, charges will flow in the
wire. The size of the current
depends on the voltage.
• Varying Nature of Voltage
Different devices need
different levels of voltage.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Resistance
• Resistance is the opposition to the flow of electric
charge.
• Resistance and Material Good conductors, such as
copper, have low resistance. Poor conductors, such as
iron, have higher resistance.
• Resistance, Thickness, and Length Thick, short
wires have less resistance than thin, long wires.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Resistance, continued
• Resistance and Temperature In general, the
resistance of metals increases as temperature rises.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Factors That Affect Resistance
Click below to watch the Visual Concept.
Visual Concept
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Generating Electrical Energy
• Parts of a Cell A cell contains a mixture of
chemicals called an electrolyte. Every cell also has a
pair of electrodes made from conducting materials.
• Kinds of Cells Two kinds of cells are wet cells and
dry cells. The electrolytes in dry cells are solid or
pastelike.
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Chapter 17
Section 2 Electric Current and
Electrical Energy
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Chapter 17
Section 2 Electric Current and
Electrical Energy
Generating Electrical Energy, continued
• Thermocouples
Thermal energy can be
converted into electrical
energy by a
thermocouple.
• Photocells A photocell
converts light energy into
electrical energy.
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Chapter 17
Section 3 Electrical Calculations
Bellringer
How fast is a nanosecond? A nanosecond (ns) is onebillionth of a second. Electrical signals travel at
30 cm/ns. Calculate how far electrical signals travel in
1 second.
Record your answers in your science journal.
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Chapter 17
Section 3 Electrical Calculations
Objectives
• Use Ohm’s law to calculate voltage, current, and
resistance.
• Calculate electric power.
• Determine the electrical energy used by a device.
• Compare the power ratings of different electrical
appliances.
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Chapter 17
Section 3 Electrical Calculations
Connecting Current, Voltage, and Resistance
• Georg Ohm (1789 –1854) studied the resistances of
materials. He measured the current that resulted from
different voltages applied to a piece of metal wire.
• Ohm’s Law Ohm found that the ratio of voltage to
current is a constant for each material. This ratio is the
resistance of the material.
R=
V
, or V = I  R
I
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Chapter 17
Section 3 Electrical Calculations
Connecting Current, Voltage, and
Resistance, continued
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Chapter 17
Section 3 Electrical Calculations
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Chapter 17
Section 3 Electrical Calculations
Electric Power
• What Is Electric Power? The rate at which electrical
energy is changed into other forms of energy is electric
power. Electric power is calculated using the following
equation:
power = voltage  current, or P = V  I
• Watt: The Unit of Power The unit for power is the
watt (W). Another common unit of power is the
kilowatt (kW). One kilowatt is equal to 1,000 W.
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Chapter 17
Section 3 Electrical Calculations
Electric Power, continued
• Power Ratings When you
read the wattage label on a light
bulb, you are reading the bulb’s
power rating. The power rating
describes the rate at which an
electric device uses electrical
energy.
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Chapter 17
Section 3 Electrical Calculations
Measuring Electrical Energy
• The amount of electrical energy used in a home
depends the power of the electrical devices in the
house and the length of the time that those devices
are on. The equation for electrical energy is as
follows:
electrical energy = power  time, or E = P  t
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Chapter 17
Section 3 Electrical Calculations
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Chapter 17
Section 3 Electrical Calculations
Measuring Electrical Energy, continued
• Measuring Household Energy Use Different
amounts of electrical energy are used each day in a
home. Electric companies usually calculate electrical
energy by multiplying the power in kilowatts by the
time in hours.
• How to Save Energy Every appliance uses
energy. Replacing items that have high power ratings
with items that have lower ratings is a way to save
energy.
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Chapter 17
Section 3 Electrical Calculations
Measuring Electrical Energy, continued
• It’s All About the Bulb All of the lighting in a home
could make up as much as 25% of a home’s energy
consumption. So, buying and using energy–efficient
light bulbs can make a big difference in the amount of
energy used by a household.
• Energy–Saving Programs Many governments and
organizations around the world have started programs
to help save energy and natural resources.
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Chapter 17
Section 4 Electric Circuits
Bellringer
What happens when you turn the lights on? What
allows lights to be turned on and off? Write your
answer or draw an explanatory picture for later
reference in your science journal. What happens
beyond the confines of the room when you flip the
light switch?
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Chapter 17
Section 4 Electric Circuits
Objectives
• Name the three essential parts of a circuit.
• Compare series circuits with parallel circuits.
• Explain how fuses and circuit breakers protect your
home against short circuits and circuit overloads.
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Chapter 17
Section 4 Electric Circuits
Parts of an Electric Circuit
• Forming a Loop An electric circuit is a complete,
closed path through which electric charges flow.
• All circuits need three basic parts: an energy source,
wires, and a load.
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Chapter 17
Section 4 Electric Circuits
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Chapter 17
Section 4 Electric Circuits
Parts of an Electric Circuit, continued
• A Switch to Control a Circuit Sometimes, a circuit
also contains a switch. A switch is used to open and
close a circuit.
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Chapter 17
Section 4 Electric Circuits
Types of Circuits
• Series Circuits A series circuit is a circuit in which all
parts are connected in a single loop.
• Uses for Series Circuits Series circuits are useful in
wiring burglar alarms. If any part of the circuit fails, there
will be no current in the system and the alarm will sound.
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Chapter 17
Section 4 Electric Circuits
Types of Circuits, continued
• Parallel Circuits A parallel circuit is a circuit in
which loads are connected side by side.
• Uses for Parallel Circuits Almost all appliances are
built with parallel circuits so that they will keep working
if part of the system fails.
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Chapter 17
Section 4 Electric Circuits
Household Circuit Safety
• Circuit Failure Broken wires or water can cause a
short circuit. In a short circuit, charges do not go
through one or more loads in the circuit.
• Fuses A fuse has a thin strip of metal. Fuses keep
charges from flowing if the current is too high.
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Chapter 17
Section 4 Electric Circuits
Household Circuit Safety, continued
• Circuit Breakers A circuit
breaker is a switch that
automatically opens if the current
is too high. Charges stop flowing.
• Electrical Safety Tips Do not
overload circuits by plugging in too
many electrical devices. Do not
use electrical devices near water.
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Chapter 17
Introduction to Electricity
Concept Mapping
Use the terms below to complete the Concept
Mapping on the next slide.
battery
electrical energy
thermocouple
light energy
thermal energy
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Chapter 17
Introduction to Electricity
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Chapter 17
Introduction to Electricity
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