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Transcript
Chapter Introduction Lesson 1 Electric Charge and Electric Forces Lesson 2 Electric Current and Simple Circuits Lesson 3 Describing Circuits Chapter Wrap-Up How do electric circuits and devices transform energy? What do you think? Before you begin, decide if you agree or disagree with each of these statements. As you view this presentation, see if you change your mind about any of the statements. Do you agree or disagree? 1. Protons and electrons have opposite electric charges. 2. Objects must be touching to exert a force on each other. 3. When electric current flows in a wire, the number of electrons in the wire increases. Do you agree or disagree? 4. Electrons flow more easily in metals than in other materials. 5. In any electric circuit, current stops flowing in all parts of the circuit if a connecting wire is removed or cut. 6. The light energy given off by a flashlight comes from the flashlight’s batteries. Electric Charge and Electric Forces • How do electrically charged objects interact? • How can objects become electrically charged? • What is an electric discharge? Electric Charge and Electric Forces • static charge • polarized • electric insulator • electric discharge • electric conductor • grounding Electric Charges • Protons and neutrons make up the nucleus of an atom. • Protons in the nuclei of atoms have positive charge. • Electrons moving around a nucleus have negative charge. Electric Charges (cont.) • An atom is electrically neutral— it has equal numbers of protons and electrons. • Electrically neutral objects do not attract or repel each other. • An unbalanced negative or positive electric charge on an object is sometimes referred to as a static charge. Electric Forces • The region surrounding a charged object is called an electric field. • An electric field applies a force, called an electric force, to other charged objects, even if the objects are not touching. • Objects with opposite electric charges attract each other, and objects with similar electric charges repel each other. Charged objects can attract or repel each other. The arrows show the direction of the objects’ motion. Electric Forces (cont.) How do electrically charged objects interact? The strength of the electric force between two charged objects depends on the total amount of charge on both objects and the distance between the objects. Transferring Electrons • A material in which electrons cannot easily move is an electric insulator. • A material in which electrons can easily move is an electric conductor. Transferring Electrons (cont.) • Electrons transfer between objects by contact, induction, or conduction. • When objects made of different materials touch, electrons tend to collect on the object that holds electrons more tightly. This is called transferring charge by contact. • Both insulators and conductors can be charged by contact. Transferring Electrons (cont.) Transferring charge by induction is a process by which one object causes two other objects that are conductors to become charged without touching them. • A negatively charged balloon repels electrons in a metal soda can. • The can is not charged because it has not gained or lost any electrons. • When electrons concentrate at one end of an object, the object is polarized. • When a charged balloon is brought near two cans that are touching, the balloon polarizes the cans as if they are one object. • Electrons in both cans move toward the far end of the can on the right. Then, the two cans separate. • The cans that were originally polarized as a group are now individually charged. • The can on the right has an unbalanced negative electric charge, and the can on the left has an unbalanced positive electric charge. Transferring Electrons (cont.) • Another way that electrons transfer between two conductors is called transferring charge by conduction. • When conducting objects with unequal charges touch, electrons flow from the object with a greater concentration of negative charge to the object with a lower concentration of negative charge. Transferring Electrons (cont.) The flow of electrons continues until the concentration on charge of both objects is equal. Transfer by conduction is similar to water flowing from a container with a higher water level to a container with a lower water level. Transfer by conduction is similar to water flowing from a container with a higher water level to a container with a lower water level. Transferring Electrons (cont.) What are three ways by which objects can become electrically charged? Electric Discharge The process of an unbalanced electric charge becoming balanced is an electric discharge. electric from Greek ēlektron, means “beaming sun” Electric Discharge (cont.) • Lightning is a powerful electric discharge that occurs in an instant. • Providing a path for electric charges to flow safely into the ground is called grounding. • Lightning rods provide a path for the electrons released in a lightning strike to travel into the ground. Electric Discharge (cont.) What is an electric discharge? Within a storm cloud, warm air rises past falling cold air. The cold air is filled with hail, ice, and partially frozen water droplets that pick up electrons from the rising air. This causes the bottom of the cloud to become negatively charged. The negatively charged cloud polarizes Earth’s surface by repelling negative charges in the ground. Thus, the surface of the ground becomes positively charged. When the bottom of the cloud accumulates enough negative charge, the attraction of the positively charged ground causes electrons in the cloud to begin moving toward the ground. As electrons approach the ground, positive charges quickly flow upward, making an electric connection between the cloud and the ground. You see this electric discharge as lightning. • Any object can be positively charged, negatively charged, or neutral. • Charged objects exert forces on each other. • Lightning is one form of electric discharge. Which term refers to a material in which electrons can easily move? A. electric conductor B. electric field C. electric insulator D. lightning rod In which process does one object cause two other objects that are conductors to become charged without touching them? A. transferring charge by induction B. transferring charge by contact C. transferring charge by conduction D. electric discharge When electrons concentrate at one end of an object, that object can be described as which of these? A. balanced B. electric C. grounded D. polarized Do you agree or disagree? 1. Protons and electrons have opposite electric charges. 2. Objects must be touching to exert a force on each other. Electric Current and Simple Circuits • What is the relationship between electric charge and electric current? • What are voltage, current, and resistance? How do they affect each other? Electric Current and Simple Circuits • electric current • voltage • electric circuit • Ohm’s law • electric resistance Electric Current and Electric Circuits The movement of electrically charged particles is an electric current. current Science Use the flow of electric charge Common Use occurring at the present time Electric Current and Electric Circuits (cont.) A closed, or complete, path which an electric current travels is an electric circuit. circuit from Latin circuire, means “to go around” Electric Current and Electric Circuits (cont.) • All electric circuits have one thing in common—they transform electric energy to other forms of energy. • The number of electrons flowing into a wire from a power source equals the number of electrons flowing out of the wire back into the source. Electric Current and Electric Circuits (cont.) • An electric current flows in a circuit if the circuit is complete, or closed. • Current will not flow if the circuit is broken, or open. Electric Current and Electric Circuits (cont.) • Electric current is approximately measured as the number of electrons that flow past a point every second. • Scientists count electrons by a quantity called the coulomb. Electric Current and Electric Circuits (cont.) • The SI unit for electric current is the ampere, commonly called an amp (A). • One ampere of current equals about one coulomb of electrons flowing past a point in a circuit every second. Electric Current and Electric Circuits (cont.) What is the relationship between electric charge and electric current? What is electric resistance? • Electric resistance is a measure of how difficult it is for an electric current to flow in a material. • The unit of electric resistance is the ohm (Ω). What is electric resistance? (cont.) • A good electric insulator has high electric resistance. Atoms of an insulator hold electrons tightly, which prevents electric charges from easily moving through material. • A material’s electric resistance depends on the material’s length and thickness. • Increasing the length of a conductor also increases its electric resistance. What is voltage? • Voltage is the amount of energy the source uses to move one coulomb of electrons through a circuit. • A circuit with a high voltage source transforms more electric energy to other energy forms than a circuit with a low voltage source. What is voltage? (cont.) • Electric energy transforms to other forms of energy in all parts of a circuit. • The voltage measured across a portion of a circuit indicates how much energy transforms in that portion of the circuit. Voltage can be different in different parts of a circuit. The voltmeter shows where most of the battery’s energy is used. Ohm’s Law • Named after the German physicist Georg Ohm, Ohm’s law is a mathematical equation that describes the relationship among voltage, current, and resistance. • Ohm’s law states that as the voltage of a circuit’s electric energy source increases, the current in the circuit increases, too. Ohm’s Law (cont.) As the resistance of a circuit increases, the current decreases. Ohm’s law can be used to calculate unknown quantities in a circuit. Ohm’s Law (cont.) How do voltage, current, and resistance affect each other? Ohm’s law states that, with equal current, the voltage is greater across the device with the greater resistance. • An electric current is the flow of negative electric charges through a conductor. • Electrical devices function only when they are connected in a closed circuit. • Ohm’s law shows the relationship among voltage, current, and resistance. Which is the unit of electric resistance? A. amp B. coulomb C. ohm D. voltage Which term refers to the amount of energy the source uses to move one coulomb of electrons through a circuit? A. amp B. coulomb C. ohm D. voltage What property of a wire increases when it is made longer? A. charge B. current C. resistance D. voltage Do you agree or disagree? 3. When electric current flows in a wire, the number of electrons in the wire increases. 4. Electrons flow more easily in metals than in other materials. Describing Circuits • What are the basic parts of an electric circuit? • How do the two types of electric circuits differ? Describing Circuits • series circuit • parallel circuit Parts of an Electric Circuit • Three common parts of most electric circuits are a source of electric energy, electrical devices that transform the electric energy, and conductors that connect the components. • An energy source, such as a battery, produces an electric current in a circuit. Parts of an Electric Circuit (cont.) What are the three basic parts of an electric circuit? Parts of an Electric Circuit (cont.) • Some electrical devices are designed to transform the electric energy of the current to kinetic energy. • As the chemicals in a battery react, electrons in the battery concentrate at the battery’s negative terminal. • When a closed circuit connects the terminals, electrons flow in the circuit from the battery’s negative terminal to the positive terminal. Parts of an Electric Circuit (cont.) • Electrical devices in a circuit transform most of the electric energy to other useful forms of energy. • Metal wires often are used to complete circuits by connecting a circuit’s energy source and a device. As electrons flow through a wire, they move in random directions, colliding with the atoms that make up the wire. Series and Parallel Circuits • There are two types of electric circuits— series circuits and parallel circuits. • A series circuit is an electric circuit with only one path for an electric current to follow. • According to Ohm’s law, when resistance increases and voltage remains the same, there is less current in the circuit. Adding devices to a series circuit adds resistance to the total circuit. Series and Parallel Circuits (cont.) • A parallel circuit is an electric circuit with more than one path, or branch, for an electric current to follow. • A device in a parallel circuit connects to the source with its own branch. • Adding branches to a parallel circuit does increase the total current through the source. Parallel circuits have more than one path through which electric current can flow. Series and Parallel Circuits (cont.) How do the two types of electric circuits differ? Electric Circuits in the Home • Electric energy used in most homes and businesses is generated at large power plants. • A complex system of transmission wires carries the electric energy to all parts of the country. Electric energy enters your home through a main wire and travels through an electric meter which measures your energy use. Electric Circuits in the Home (cont.) • Fuses and circuit breakers automatically open a circuit when the current becomes dangerously high. • A fuse is a piece of metal that breaks a circuit by melting from the thermal energy produced by a high current. • A circuit breaker is a switch that automatically opens a circuit when the current is too great. Electric Safety • An electric shock occurs when an electric current passes through the human body. • Each year, more than 500 people die by accidental electric shock in the United States. Electric Safety (cont.) • There are many ways to help protect yourself from electric shock. • Never use electrical devices with damaged power cords. • Stay away from water when using electrical devices plugged into an outlet. Electric Safety (cont.) • Avoid using extension cords and never plug more than two home appliances into an outlet at once. • Never climb utility poles or play on fences surrounding electricity substations. • A battery can be the source of electric energy in a circuit. • A series circuit has only one path for all devices in the circuit. • A parallel circuit has a separate path for each device in the circuit. Which is an example of an energy source? A. wire B. device C. circuit D. battery Which term refers to an electric circuit with more than one path, or branch, for electric current to follow? A. electrical device B. fuse C. parallel circuit D. series circuit Electric energy enters your home through which of these? A. circuit breaker B. electric meter C. fuse D. main wire Do you agree or disagree? 5. In any electric circuit, current stops flowing in all parts of the circuit if a connecting wire is removed or cut. 6. The light energy given off by a flashlight comes from the flashlight’s batteries. Key Concept Summary Interactive Concept Map Chapter Review Standardized Test Practice An electric energy source changes the motion of the electrons in a circuit. Energy is conserved as a circuit transforms the energy of moving electrons to light, sound, and thermal energy. Lesson 1: Electric Charge and Electric Forces • Particles that have the same type of electric charge repel each other. Particles that have different types of electric charge attract each other. • Objects become negatively charged when they gain electrons, and become positively charged when they lose electrons. • An electric discharge is the loss of static charge. Lesson 2: Electric Current and Simple Circuits • An electric current is the flow of electrically charged particles through a conductor. • According to Ohm’s law, across any portion of an electric circuit, voltage (V), current (I), and electric resistance (R) are related by the equation V = IR. Lesson 3: Describing Circuits • Electric circuits have a source of electric energy to produce an electric current, one or more electric devices to transform electric energy to useful forms of energy, and wires to connect the circuit’s device(s) to the energy source. • A series circuit has one path in which current flows. A parallel circuit has more than one path in which current flows. Which term describes an unbalanced negative or positive electric charge on an object? A. electric conductor B. electric insulator C. polarized D. static charge Which term refers to when objects made of different materials touch, and electrons tend to collect on the object that holds electrons more tightly? A. charge by conduction B. charge by contact C. charge by induction D. electric discharge Which term describes a closed, or complete, path which an electric current travels? A. voltage B. electric resistance C. electric circuit D. coulomb What is the name that scientists use to count electrons by a quantity? A. amp B. circuit C. coulomb D. voltage Which term describes an electric circuit with only one path for an electric current to follow? A. battery B. circuit breaker C. parallel circuit D. series circuit Which is a material in which electrons cannot easily move? A. electric conductor B. electric discharge C. electric field D. electric insulator Which term refers to an unbalanced electric charge becoming balanced? A. electric discharge B. grounded C. polarized D. static charge Which is the SI unit for electric current? A. amp B. coulomb C. ohm D. voltage Which term refers to the movement of electrically charged particles? A. electric circuit B. electric current C. electric resistance D. voltage In an electric circuit, which transforms most of the electric energy to other useful forms of energy? A. battery B. circuit breaker C. electrical device D. fuse