CHAPTER 11ELECTRICITY

Electricity

• 11.1 – Electric Charges• 11.2 – Static Electricity• 11.3 – Electric Current• 11.4 - Electric circuits• 11.5 – Electric Power & Safety

11.1 Electric ChargesObjectives• Identify two forces that result from electric charges.• Explain why objects attract and repel each other• Communicate how a positively charged object can be

used to determine the charge on another object.• Infer how electric charges behave in everyday situations.

Charge and Force• All matter is made of atoms that contain electrons, neutrons, and

protons.

• Protons and electrons in atoms have an electric charge.

• Electrons have a negative charge, and protons have a positive charge. Neutrons have no electric charge.

• When the atoms in an object have more electrons than protons, the total charge of the object is negative.

• When the atoms in an object have more protons than electrons, the total charge of the object is positive.

• When an object has an equal number of protons and electrons, the object has no charge. It is neutral.

• Objects are forced together, or attracted, when their charges are different.

• An object with a positive charge is attracted to an object with a negative charge.

• When objects have the same electric charge, they are pushed apart, or repelled.

• SO…• Unlike charges attract • Like charges repel.

Force of Attraction• A cloth and the balloon are both

neutral objects. • When two neutral objects are rubbed

together, electrons will leave one object and move onto the other.

• The total, or net, charge of each object will become either positive or negative.

Force of Attraction• When the cloth rubs against the balloon some electrons in

the cloth move to the balloon.• Since the balloon now has more electrons, its net charge is

negative. Since the cloth loses electrons, its net charge is now positive.

• Because unlike charges attract, the positively-charged cloth attracts the negatively-charged balloon.

Force of Repulsion• Imagine a second balloon is rubbed with the cloth.• How will this balloon react to the negatively-charged

balloon when they are brought near each other?• The second balloon now has more electrons than it has

protons.• Like the first balloon, its net charge is negative.• This force of repulsion occurs because the objects have

like charges.• Why do you think the hairs on your head stand away

from each other when you comb your hair?• What is the charge of the comb?

Electric Field• How close do you think two charged objects must be in

order to be affected by each other?• The forces of attraction and repulsion depend on the

electric field around each object.• An electric field is the region surrounding a charged

object. • The strongest part of the field is the area closest to the

charged object. The weakest part of the field is the area farthest from the charged object.

• An object with a large amount of charge has a bigger electric field than an object with a small amount of charge.

Electric Field

Check & Explain

1. Name the two types of forces that can result from electric charges.

2. You bring two balloons together and they repel. Explain what happens in terms of charges .

3. How can a negatively charged comb be used to determine the charge of an object?

4. Why does the television screen attract dust after the screen has been wiped clean?

11.2 Static Electricity

Objectives1. Identify three ways static charge can build up.2. Explain what causes lightning.3. Compare electric conductors and insulators4. Infer why lightning can be dangerous.

Static Electricity• Lightning and the shock that you might get from a metal

doorknob are alike.• Both are caused by static electricity.• All electricity is energy that results from the movement of

electrons. • Static electricity, however, consists of electric charges at

rest. • The electrons that cause static electricity can move from

one object to another, and then remain at rest. • When electrons gather in one location, a build-up of static

electricity results.

Static Build Up• A neutral object builds up an electric

charge by gaining or losing electrons. • Movement of static electricity depends on

the forces of attraction and repulsion and the way charges act in different materials.

• The build-up of static charge can be caused by friction, conduction, or induction.

• An electroscope is a device that detects static electricity by using thin metal-like leaves, which separate when charged.

• An object with a suspected static electric charge is brought near the metal ball of the electroscope.

• Electrical charges move to the metal and down to the metal leaves, which then repel each other. A simple electroscope detects

static charges

Friction• Charging by friction occurs when

objects rub together and electrons move from the surface of one object to the surface of the other.

• Friction separates electrons from the surface of an object whose atoms hold electrons loosely.

• Once separated, the electrons move onto another object.

Conduction• Charging by conduction occurs when electrons are

transferred from one material to another by direct contact.

• Conduction causes the small electric shock you feel when you rub your shoes on a carpet and touch metal.

• The shock you feel occurs when electrons move from you to the metal.

Induction• When charges on an object are rearranged without

physical contact, charging by induction occurs.

Conductors• A material through which electric charges move easily is

called a conductor.• Most metals are good conductors.• Good conductors have a large number of freely-moving• electrons.

• Gold, silver, copper, aluminum, and mercury are the best conductors of electricity.

• Metals that are good conductors can be useful.

• Copper and aluminum are used in electric wiring and telephone lines.

• Besides electrons, heat also moves easily through conductors.

• Other materials are also good conductors.• For example, the acid used in a car battery

conducts electric charges.• Water is also a good conductor.

Insulators• A material through which electric charges can't

move easily is called an insulator.• Electrons move extremely slowly through good

insulators.• Electrons are tightly bound to the atoms in good

insulating materials and the electrons are not free to move around.

• Materials such as wood, ceramic, rubber, glass, and plastic are electric insulators.

• Charges don't flow easily through insulators.

• But charges do collect on their surfaces. The electrons that build up on the surface of an insulator tend to remain there as a static charge.

Lightning• Electrons that build up as static charge

eventually leave the object.• The electrons may move to the surface of

another object, or they may move onto molecules in the air.

• The loss of static electricity by an object that occurs when electrons move is called a static discharge.

• A static discharge may occur slowly and quietly, or it may occur rapidly, converting electric energy into sound, light, and heat energy.

• The lightning you see during a thunderstorm is caused by the discharge of static electricity .

• During a storm, water molecules and particles of matter in clouds are rubbed together by the wind.

• As they move, the water molecules and particles become charged by friction. As a result, areas of positive and negative charges form in the clouds.

Check & Explain1. Name three different ways static charges can buildup on an object.

2. Explain how lightning and a shock from static electricityare related to each other.

3. Compare and Contrast - How do conductors and insulatorsof electricity differ? Identify three materials that are good conductors and three materials that are good insulators.

11.3 Electrical CurrentsObjectives1. Identify three sources of electric current.2. Distinguish between current, voltage, and

resistance.3. Calculate resistance using Ohm's law4. Predict how the resistance of a wire changes with

wire temperature, length, thickness, and type of material.

Sources of Electrical Current• You know that electrons at rest produce static

electricity.• However, electrons in a wire move, or flow.• These moving electrons are called electric

current.• Electric current flows through a closed,

continuous path, called a circuit. • A source of electrons is needed to produce

electric current in the wire of the circuit.

• To keep electric current moving, the electron source must also provide a difference in the charges present at each end of the wire.

• One end of the wire must be negatively charged. The other end must be positively charged.

• The difference in the charges at each end of the wire is called potential difference, or voltage.

• The potential difference gives electrons energy to move from the negative end of the wire toward the positive end.

• A device that produces the potential difference needed to move electrons through a circuit is an energy source.

• Electrochemical cells and thermocouples are sources of electric current.

• The two kinds of electrochemical cells are wet cells and dry cells, often referred to as batteries.

• An electrochemical cell changes chemical energy into electric energy.

• Differences in temperature are also used to generate electric current with a device called a thermocouple.

Types of Currents• When an electrochemical cell is connected to a

circuit, it causes a steady flow of electric current.• Current that flows from an electrochemical cell

source moves in one direction.• Electrons that flow in the same direction in a wire

produce direct current, or DC.• Electrons that flow in different directions in a wire

produce an alternating current, or AC. The electricity in your home is alternating current.

Flow Rate• The number of electrons that pass a specific point in

a circuit in one second indicates the flow rate of electric current.

• The flow rate increases as the number of electrons passing a point each second increases. If fewer electrons pass a specific point each second, the rate of current flow decreases.

• In the SI system, the unit measure for current is amperes.

• Amperes are written as "amps," or as the letter "A."

• An instrument called an ammeter can be connected to an electric circuit to measure the flow rate of electric current.

Voltage• Electrons need an energy source to force them through

a wire.• The energy to move electrons depends on the potential

difference between the positive and negative terminals of a dry cell.

• The positive terminal of a dry cell has high potential. The negative terminal has low potential. The difference between high and low potential is the voltage of the dry cell.

• Voltage is measured in the SI system in volts. The symbol for volt is the capital letter V.

• Voltage is measured with an instrument called a voltmeter.

• Notice how the voltmeter is connected to the circuit.

Resistance• Electrons flow easily through a copper wire to the

filament in a light bulb.• However, when electrons reach the filament, its

resistance is so great that the electric energy is converted into heat and light energy.

• The force opposing the flow of electrons through• the filament is called resistance. • Good conductors have low resistance. • Poor conductors, called resistors, have a high• resistance.

• A wire's resistance depends on the material from which it is made and its length, thickness, and temperature.

• Metals, such as copper and aluminum, have relatively low resistance.

• Long wires have more resistance than do short wires.

• Thin wires have greater resistance than do thick wires.

• An increase in temperature also increases the resistance of a material.

• A long, thin, hot, wire would have more resistance than a short, thick, cold one.

• The symbol for resistance is the capital letter R.

• The SI unit of resistance is the ohm (OHM), and its symbol is n, the Greek capital letter ‘omega’.

• Resistance force can be measured by connecting an ohm meter to a circuit, as shown in the Figure .

Ohm’s Law• A German schoolteacher named Georg Ohm related

electric current to voltage and resistance.• He discovered that when he divided the voltage (V) of the

circuit by the current, he always got the same number.• Ohm identified this number as resistance.• The relationship among current, voltage, and resistance is

known as Ohm's law. • Ohm's law states that the current in a circuit is equal to

the voltage divided by the resistance.• Ohm's law is written: I = V/R

Sample Problem’s Ohm’s Law• The current in a closed circuit measures 6.0 amps. The

resistance through the circuit is 20 ohms. What is the voltage in the circuit?

• V = IR• Voltage = current x resistance• = 6.0 A x 20 ohms• = 120 V

• Do Practice Problems page 271. 1-4

Check & Explain pg. 273

1. Name three common sources of electric current. Name a use for each source.

2. Explain how current, voltage, and resistance relate to one another. Identify the symbol for each term.

3. Calculate Use Ohm's law to find the resistance of a radio that uses a 9-V battery and carries a current of 3 amps.

11.4 Electrical CircuitsObjectives1. Identify the parts of a circuit.2. Trace the path of electrons through two types of

circuits.3. Compare series and parallel circuits.4. Predict how circuits are wired in your home.

Electrical Circuits• An electric circuit

requires a source of electrons and voltage to force the electrons along the circuit. If there is a break in an electric circuit, the electrons stop moving.

Parts of a Circuit• An electric circuit is a complete,

closed path through which electrons travel.

• In a circuit that uses a dry cell as its electron source, the path of electrons begins in a wire attached to the negative terminal.

Parts of a Circuit• For work to be done by the flowing electrons, there must

be a resistor connected in the circuit.• A resistor is a device that uses electric energy to do work.• Machines, computers, lights, appliances, and motors are

examples of resistors.

Parts of a Circuit• A wire connected from the resistor

to the positive terminal completes the circuit.

• A circuit may have a switch to open and close the circuit.

• Electrical engineers draw circuit diagrams.

Path of a Circuit

Types of Circuits• Most electric circuits contain more

than one resistor that uses electric energy.

• The resistors in a circuit can be connected in two ways.

• They can be connected in series or in parallel.

Series Circuits• A circuit in which the

current must pass through all the resistors is a series circuit.

• All the resistors in a series circuit lie along a single path.

Parallel Circuits• The electrons in a parallel circuit can travel

through more than one path.• Each path is separate.• If there's a break in one path in the circuit,

electrons can still flow through the other paths and maintain a complete circuit.

• The circuits in your home are connected in parallel. What is the advantage of parallel wiring?

Check & Explain pg. 2781. Name and describe three parts of a circuit.

2. Draw a simple circuit. Label your drawing with arrows to trace the path of electrons through the circuit.

3. Compare and Contrast How does the path of electrons differ between a circuit connected in series and one connected in parallel?

11.5 Electric Power & SafetyObjectives1. Calculate electric power and energy.2. Apply safety guidelines for household use of

electricity.3. Calculate the amount of electricity used by an

appliance.4. Infer about the proper use of safety devices

Electric Power• Electricity is useful because it changes

easily into other forms of energy.• For example, a toaster changes electricity

into heat energy.• An electric mixer changes electric energy

into mechanical energy.• Each of these devices makes work easier.

Electric Power & Energy• The rate at which electricity does work or

provides energy is called electric power.• The amount of electric power a device uses

to do work is determined by its resistance.• You can calculate power (P) if you know the

voltage (V) an appliance uses and the current (I) in the circuit.

• Use the formula: P = (V)(I)

• The amount of power used by different appliances is shown in the table to the left.

• Recall the unit for power is watts (W).• Power is measured in watts (W). Time is in

hours (h).• The SI unit for energy is the joule. However,

energy is usually measured with a unit called a watt-hour.

• Because a watt-hour is a very small unit of energy, electric companies measure electric energy in 1,000 watt hours, or kilowatt-hours (kWh). Kilowatt-hour meters measure the electricity used in your home.

• Do Skills Builder Activity page 280

Electric Safety• Electricity is useful, but it can also be

dangerous.• Each year, many people are seriously

injured or killed from shocks or fire caused by electricity.

• Most appliances and buildings have safety features built into them.

• Many appliances are equipped with a "ground" wire on the plug.

• The ground wire prevents electric shocks from the outside of the appliance.

• The ground wire constantly moves static electricity from the appliance to the ground.

• Broken wires or water can cause electric appliances to short-circuit.

• A short circuit occurs when electricity takes a short path and bypasses the resistors in the circuit.

• As a result, the resistance in the circuit is less, and the current in the wire increases.

• The increased current can produce enough heat to melt wires and start a fire, or cause a serious electric shock.

• Overloading a circuit also may cause a fire.

• A circuit can become overloaded if too many electric devices are plugged into it. Each added electric device increases the electric current flowing through the wire.

• When the electric current load is greater than the capacity of the wire, the wire is overloaded.

Circuit Protectors

• Fuses and circuit breakers protect against overloaded circuits.

• When the current passes through the fuse that goes above its maximum, the metal in the fuse melts. The fuse must be replaced.

Circuit Protectors• Circuit breakers are often used instead of

fuses.• A circuit breaker is a switch that opens

automatically when electric current in a circuit reaches its maximum.

• When the switch opens, it breaks the flow of electrons in the circuit.

• Circuit breakers can be reset when the switch is closed again.

Check and Explain pg. 2821. A radio has a current of 1.0 amp from a

120-V source. What is the power of the radio?

2. What can you do to minimize the risk of an electric shock or fire in your home caused by electricity?