chapter 11 electricity. electricity 11.1 – electric charges 11.2 – static electricity 11.3 –...
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CHAPTER 11 ELECTRICITY Slide 2 Electricity 11.1 Electric Charges 11.2 Static Electricity 11.3 Electric Current 11.4 - Electric circuits 11.5 Electric Power & Safety Slide 3 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. Slide 4 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. Slide 5 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. Slide 6 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. Slide 7 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. Slide 8 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? Slide 9 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. Slide 10 Electric Field Slide 11 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? Slide 12 11.2 Static Electricity Objectives 1.Identify three ways static charge can build up. 2.Explain what causes lightning. 3.Compare electric conductors and insulators 4.Infer why lightning can be dangerous. Slide 13 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. Slide 14 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. Slide 15 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 Slide 16 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. Slide 17 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. Slide 18 Induction When charges on an object are rearranged without physical contact, charging by induction occurs. Slide 19 Slide 20 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. Slide 21 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. Slide 22 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. Slide 23 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. Slide 24 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. Slide 25 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. Slide 26 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. Slide 27 11.3 Electrical Currents Objectives 1.Identify three sources of electric current. 2.Distinguish between current, voltage, and resistance. 3.Calculate resistance using Ohm's law 4.Predict how the resistance of a wire changes with wire temperature, length, thickness, and type of material. Slide 28 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. Slide 29 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. Slide 30 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. Slide 31 Differences in temperature are also used to generate electric current with a device called a thermocouple. Slide 32 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. Slide 33 Flow Rate The number of electrons that pass a specific point in a circuit in one second indicates the flow rate of electric current.