# CHAPTER 11 ELECTRICITY.

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CHAPTER 11 ELECTRICITY

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

11.1 Electric Charges Objectives
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. The atoms in each object have an equal number of protons and electrons.

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. The objects are no longer neutral. 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? As the second balloon is rubbed with the cloth, more electrons leave the cloth and move onto the balloon. When the balloons are brought near each other, they will repel.

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 Objectives
Identify three ways static charge can build up. Explain what causes lightning. Compare electric conductors and insulators 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. The electricity used by the lights in your classroom consists of charges that continuously flow through a wire. The word static means not moving, or stationary.

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.

A simple electroscope detects static charges
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 A simple experiment to demonstrate this method of detecting the presence of static electricity is to pick up some pieces of tissue with a comb you have run through your hair or a balloon that was rubbed on wool.

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. This induction charging process can be used to charge a pair of pop cans. It is a simple enough experiment to be repeated at home. Two pop cans are mounted on Styrofoam cups using scotch tape. The cans are placed side-by-side and a negatively charged rubber balloon (having been rubbed with animal fur) is brought near to one of the cans. The presence of the negative charge near a can induces electron movement from Can A to Can B (see diagram). Once the cans are separated, the cans are charged. The type of charge on the cans can be tested by seeing if they attract the negatively charged balloon or repel the negatively charged balloon. Of course, we would expect that Can A (being positively charged) would attract the negatively charged balloon and Can B (being negatively charged) should repel the negatively charged balloon. During the process of induction charging, the role of the balloon is to simply induce a movement of electrons from one can to the other can. It is used to polarize the two-can system. The balloon never does supply electrons to can A (unless your hear a spark, indicating a lightning discharge from the balloon to the can).

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. Recall that metals have a crystalline structure. Electrons in metal crystals are free to move through the crystalline structure. Good conductors have a large number of freely-moving electrons.

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. To prevent an electric shock, you should handle electrical appliances carefully around water.

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. You are probably aware that wires used in electric wiring, such as lamp cords, are covered with rubber or plastic. Rubber and plastic are insulators and prevent electric charges inside the wire [Tom flowing to the outside of th e wire.

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. Lightning occurs within the thundercloud when a large charge difference exists between the positive charge at the top of cloud and the negative charge at the bottom of the cloud. The negative charge in the bottom of the cloud will induce a positive charge in the objects on the ground below it. Lightning rods protect buildings by conducting an electric charge from the cloud directly to the ground.

Check & Explain 1. Name three different ways static charges can build up on an object. 2. Explain how lightning and a shock from static electricity are related to each other. 3. Compare and Contrast - How do conductors and insulators of electricity differ? Identify three materials that are good conductors and three materials that are good insulators.

11.3 Electrical Currents Objectives
Identify three sources of electric current. Distinguish between current, voltage, and resistance. Calculate resistance using Ohm's law 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. Metals plus heat equals electricity A thermocouple contains two pieces of different metal, welded together and attached to electrical leads, which get placed in the pilot light flame. A reaction between the heated metals generates a small amount of electricity that travels to the appliance's main gas valve and allows gas to flow. No heat, no electricity, no gas If the pilot flame goes out, the valve cuts off the gas supply to both the pilot light and the main burner.

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. For alternating current in the U.S., electrons change direction about 120 times each second.

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." The rate at which electric current flows through a wire can be compared to water flowing through a hose.

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 . In a light bulb, the electricity flowing through the filament, or the tiny wires inside the bulb, cause them to glow white hot. If all the oxygen were not removed from inside the bulb, the wires would burn up. An important point to mention here is that the resistance is higher in smaller wires. Therefore, if the voltage or EMF is high, too much current will follow through small wires and make them hot. In some cases hot enough to cause a fire or even explode. Therefore, it is sometimes useful to add components called resistors into an electrical circuit to restrict the flow of electricity and protect the components in the circuit.

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

Check & Explain pg. 273 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 Circuits Objectives Identify the parts of a circuit.
Trace the path of electrons through two types of circuits. Compare series and parallel circuits. 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. A resistor, does just that - it RESISTS electricity, the same way an element in a toaster gets red hot as it RESISTS electricity. While a toaster element just gets red or orange hot, the 'filament' or resistor in a lightbulb gets white hot, and would burn itself up in a poof of smoke instantly, if it were not in a sealed glass bulb with all the oxygen removed. The heat from the wire makes light as a by- product .

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. What happens to the current if one of the bulbs bums out? If current can't pass through one bulb in a series, the path of the current is broken and the current stops. If one light goes out, all the lights go out. If you had a long string of lights connected in series, how would you find out which bulb was burned out?

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? One way to prevent all the lights in a circuit from going out at the same time is to connect them in a parallel circuit. In a parallel circuit, there are multiple pathways between the circuit’s beginning and end. Parallel circuits in your home allow each light or appliance to use the amount of current it needs to work. A parallel circuit also prevents all the lights or appliances from shutting off when one of them stops working. Wall switches make it possible to use only one light or appliance at a time. Electric devices that require different amounts of currents can be used at the same time. Each device uses only the amount of current it needs.

Check & Explain pg. 278 Name and describe three parts of a circuit.
Draw a simple circuit. Label your drawing with arrows to trace the path of electrons through the circuit. 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 & Safety
Objectives Calculate electric power and energy. Apply safety guidelines for household use of electricity. Calculate the amount of electricity used by an appliance. 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 Power is the change in work with respect to time work is a measure of energy

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. The rounded third prong of a three-way electric plug is attached to the ground wire. work is a measure of energy

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.

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. A number on the [use indicates the maximum current that will flow through it.

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. 282 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?