HIGH SCHOOL SCIENCE. Physical Science 7: Electricity & Magnetism

Transcription

1 HIGH SCHOOL SCIENCE Physical Science 7: Electricity & Magnetism WILLMAR PUBLIC SCHOOL EDITION

2 CHAPTER 7 Electricity & Magnatism In this chapter you will: 1. Analyze factors that affect the strength and direction of electric forces and fields. 2. Describe how electric charges are transferred. 3. Describe and identify electric current, conduction, and resistance. 4. Explain how voltage produces electric current. 5. Describe the effects of magnetic forces and magnetic fields and explain how magnetic poles determine the direction of magnetic force. 6. Describe how a moving electric charge creates a magnetic field and determine the direction of the magnetic field based on the types of charge and the direction of its motion. 7. Describe how electric current is generated by electromagnetic induction. 8. Summarize how electrical energy is produced, transmitted, and converted.

3 SECTION 7.1 Electricity OBJECTIVE: 1. Analyze factors that affect the strength and direction of electric forces and fields. 2. Describe how electric charges are transferred. 3. Describe and identify electric current, conduction, and resistance. 4. Explain how voltage produces electric current. Vocabulary: A lightning bolt is like the spark that gives you a shock when you touch a metal doorknob. Of course, the lightning bolt is on a much larger scale. But both the lightning bolt and spark are a sudden transfer of electric charge. Electric charge is a property that causes subatomic particles such as protons and electrons to attract or repel each other. There are two types of electric charge, positive and negative. Protons have a positive charge and electrons have a negative charge. The unit of electric charge is the coulomb (C). It takes about electrons to produce a single coulomb. The atom is neutral because it has an equal number of positive and negative charges. If an atom gains one or more electrons, it becomes a negatively charged ion. If an atom loses electrons, it becomes a positively charged ion. An excess or shortage of electrons produces a net electric charge. electric charge electric field conduction static discharge ampere alternating current voltage electrical insulator electric force static electricity induction electric current direct current electric potential energy electrical conductor resistance Like charges repel, and opposite charges attract. The force of attraction or repulsion between electrically charged objects is electric force. The electric force between two objects is directly proportional to the net charge on each object and inversely proportional to the square of the distance between them. 2

4 The effect an electric charge has on other charges in the space around it is the charge's electric field. The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge. Because of their force fields, charged particles can exert force on each other without actually touching. Electric fields are generally represented by arrows. The arrows show the direction of electric force around a positive particle and a negative particle. When charged particles are close enough to exert force on each other, their electric fields interact. An electric field exerts forces on any charged object placed in the field. The force depends on the net charge in the object and on the strength and direction of the field at the object's position. The more net charge an object has, the greater is the force on it. The direction of each field line shows the direction of the force on a positive charge. Whenever electrons are transferred between objects, neutral matter becomes charged. This occurs even with individual atoms. Atoms are neutral in electric charge because they have the same number of negative electrons as positive protons. However, if atoms lose or gain electrons, they become charged particles called ions. Static electricity is a buildup of electric charges on objects. Charges build up when negative electrons are transferred from one object to another. The object that gives up electrons becomes positively charged, and the object that accepts the 3

5 electrons becomes negatively charged. There are several ways that a net charge can build up on an object or move from one object to another. Charge can be transferred by friction, by contact, and by induction. Did you ever rub an inflated balloon against your hair? Friction between the balloon and hair cause electrons from the hair to rub off on the balloon. That s because a balloon attracts electrons more strongly than hair does. After the transfer of electrons, the balloon becomes negatively charged and the hair becomes positively charged. The individual hairs push away from each other and stand on end because like charges repel each other. The balloon and the hair attract each other because opposite charges attract. Conduction occurs when there is direct contact between materials that differ in their ability to give up or accept electrons. A van de Graff generator produces a negative charge on its dome, so it tends to give up electrons. Human hands are positively charged, so they tend to accept electrons. Therefore, electrons flow from the dome to the man s hand when they are in contact. Induction is a transfer of charge without contact between materials. Keep in mind that whenever there is a charge transfer, the total charge is the same before and after the transfer occurs. This is the law of conservation of charge the total charge in an isolated system is constant. Static discharge occurs when a pathway through which charges can move forms suddenly. The continuous flow of electric charge is an electric current. The unit of electric current is the ampere (A), or amp, which equals 1 coulomb per second. The two types of current are direct current and alternating current. Charge flows only in one direction in direct current (DC). A flashlight and most other battery-operated devices use direct current. Electric current in your home and school is mostly alternating current. Alternating current (AC) is a flow of electric charge that regularly reverses its direction. Resistance is opposition to the flow of electric charges in an electric current as it travels through matter. The SI unit for resistance is the ohm. Resistance occurs because moving electrons in current bump into atoms of matter. Resistance reduces the amount of electrical energy that is transferred through matter. That s because some of the electrical energy is absorbed by the atoms and changed to other forms of energy, such as heat. All materials have resistance. How much resistance a material has depends on the type of material, its width, its length, and its temperature. Resistance is greater in a longer wire because the charges travel farther. As temperature increases, a metal's resistance increases because electrons collide more often. Resistance is a hindrance when a material is being used to transmit electric current. Resistance is helpful when a material is being used to produce heat or light. Ohm found a mathematical relationship between voltage, current, and resistance. This relationship became known as Ohm's law. According to Ohm's law, the voltage (V) in a circuit equals the product of the current (I) and the resistance (R). Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current. Electric potential energy comes from the position of a charged particle in an electric field. For example, when two 4

6 negative charges are close together, they have potential energy because they repel each other and have the potential to push apart. For an electric charge to move from one position to another, there must be a difference in electric potential energy between the two positions. A difference in electric potential energy is called voltage. Electrical potential difference is measured in joules per coulomb, or volts. An electrical conductor is a material through which charge can flow easily. Metals such as copper and silver are good electrical conductors. A metal is made up of ions in a lattice. The ions are not free to move. But each ion has one or more electrons that are not tightly bound to it. These free electrons can conduct charge. A material through which charge cannot flow easily is called an electrical insulator. Wood, plastic, rubber, and air are good electrical insulators. Most materials do not easily conduct charge because they don't have free electrons. Section Review: 1. What are the two types of electric charge? 2. What makes up positive charge? Negative Charge? 3. What happens to an atom if it loses an electron? 4. What produces a net electric charge? 5. What is the unit for electric charge? 6. How do like charges behave? 7. How do opposite charges behave? 8. What is the relationship of electric forces between two objects? 9. What does the strength of the electric field depend on? 10.What does an electric field exert forces on? 11.What does the force depend on? 12.What causes a greater force? 13.How can charger be transferred? 14.What is the law of conservation of charge? 15.When does static discharge occur 16.What are the two types of electric current? 17.Where do you find direct current? Alternating current? 18.What is Ohm s Law? 5

7 Section Review Continued: 19.What affects resistance? 20.What is necessary for charge to flow? 21.What materials make good conductors? Insulators? 6

8 SECTION 7.2 Magnetism OBJECTION: 1. Describe the effects of magnetic forces and magnetic fields and explain how magnetic poles determine the direction of magnetic force. 2. Describe how a moving electric charge creates a magnetic field and determine the direction of the magnetic field based on the types of charge and the direction of its motion. Magnetism is the ability of a material to be attracted by a magnet and to act as a magnet. Magnetism is due to the movement of electrons within atoms of matter. When electrons spin around the nucleus of an atom, it causes the atom to become a tiny magnet, with north and south poles and a magnetic field. In most materials, the north and south poles of atoms point in all different directions, so overall the material is not magnetic. Examples of nonmagnetic materials include wood, glass, plastic, paper, copper, and aluminum. These materials are not attracted to magnets and cannot become magnets. A magnet is an object that attracts certain materials such as iron. All magnets have two magnetic poles: north and south magnetic poles. The poles are regions where the magnet is strongest. The poles are called north and south because they always line up with Earth s north-south axis if the magnet is allowed to move freely. (Earth s axis is the imaginary line around which the planet rotates.) The direction of magnetic force between two magnets depends on how the poles face. Like magnetic poles repel one another, and opposite magnetic poles attract one another. Vocabulary: magnetism poles magnetic field magnet magnetic force The force that a magnet exerts on certain materials, including other magnets. Magnetic force is the force a magnet exerts on another magnet, on iron or a similar metal, or on moving charges. The force is exerted over a distance and includes forces of attraction and repulsion. North and south poles of two magnets attract each other, while two north poles or two south poles repel each other. A magnet can exert force over a 7

9 distance because the magnet is surrounded by a magnetic field. A magnetic field surrounds a magnet and can exert magnetic forces. A magnetic field, which is strongest near a magnet's poles, will either attract or repel another magnet that enters the field. When two magnets are brought close together, their magnetic fields interact. The lines of force of north and south poles attract each other whereas those of two north poles repel each other. Earth is like a giant magnet surrounded by a magnetic field. Earth acts as a giant magnet with magnetic poles and a magnetic field over which it exerts magnetic force. Earth has north and south magnetic poles like a bar magnet. Earth s magnetic poles are not the same as the geographic poles. Earth s magnetic field is called the magnetosphere. It is strongest at the poles. In the 1600s, William Gilbert demonstrated that Earth is basically a spherical magnet, with north and south poles and a magnetic field. In the 1900s, scientists used earthquake data to determine that Earth has a solid inner core and molten outer core. Scientists think that Earth is a magnet because of charged particles moving through the molten outer core as Earth spins on its axis. The field lines begin near the magnet's north pole and extend toward its south pole. The arrows on the field lines indicate what direction a compass needle would point at each point in space. Where lines are close together, the field is strong. Where lines are more spread out, the field is weak. 8

10 Section Review: 1. What are the two types of magnetic poles? 2. How do magnetic poles interact with each other? 3. Where is a magnetic field strongest? 4. How are the lines in the magnetic field drawn? 5. Why is the Earth a magnet? 9

11 SECTION 7.3 Electromagnetism OBJECTIVES: 1. Describe how electric current is generated by electromagnetic induction. 2. Summarize how electrical energy is produced, transmitted, and converted. Electromagnetism is magnetism produced by an electric current. When electric current flows through a wire, it creates a magnetic field that surrounds the wire in circles. The direction of the magnetic field created when current flows through a wire depends on the direction of the current. A simple rule, called the right hand rule, makes it easy to find the direction of the magnetic field if the direction of the current is known. When the thumb of the right hand is pointing in the same direction as the current, the fingers of the right hand curl around the wire in the direction of the magnetic field. Vocabulary: electromagnetism electromagnetic force electromagnetic wave solenoid electromagnet electromagnet induction Faraday s law generator Electricity and magnetism are different aspects of a single force known as the electromagnetic force. The electric force results from charged particles. The magnetic force usually results from the movement of electrons in an atom. Both aspects of the electromagnetic force are caused by electric charges. transformer 10

12 Moving electric charges create a magnetic field. These moving charges may be the vibrating charges that produce an electromagnetic wave. The magnetic field lines form circles around a straight wire carrying a current. A coil of current-carrying wire that produces a magnetic field is called a solenoid. Any wire with current flowing through it has a magnetic field. However, the magnetic field around a coiled wire is stronger than the magnetic field around a straight wire. That s because each turn of the wire in the coil has its own magnetic field. Adding more turns to the coil of wire increases the strength of the field. Increasing the amount of current flowing through the coil also increases the strength of the magnetic field. A solenoid is generally used to convert electromagnetic energy into motion. Solenoids are often used in devices that need a sudden burst of power to move a specific part. An electromagnet is a solenoid with a ferromagnetic core. Changing the current in an electromagnet controls the strength and direction of its magnetic field. You can also use the current to turn the magnetic field on and off.the combined magnetic force of the magnetized wire coil and iron bar makes an electromagnet very strong. In fact, electromagnets are the strongest magnets made. Electromagnetic devices such as galvanometers, electric motors, and loudspeakers change electrical energy into mechanical energy. Electromagnetic induction is the process of generating a current by moving an electrical conductor relative to a magnetic field. It occurs whenever a magnetic field and an electric conductor, such as a coil of wire, move relative to one another. As long as the conductor is part of a closed circuit, current will flow through it whenever it crosses lines of force in the magnetic field. According to Faraday's law, a voltage is induced in a conductor by a changing magnetic field. Moving the magnet in and out of the coil causes an electric current first in one direction and then in the other. The same alternating current occurs if you move the coil and keep the magnet still. As long as the magnet and coil are moving relative to one another, the galvanometer will record a current. Two important devices depend on electromagnetic induction: electric generators and electric transformers. Both devices play critical roles in producing and regulating the electric current we depend on in our daily lives. Electric generators use electromagnetic induction to change kinetic energy to electrical energy. They produce electricity in power plants. Electric transformers use electromagnetic induction to change the voltage of electric current. Some transformers increase voltage and other decrease voltage. A generator is a device that converts mechanical energy into electrical energy by rotating a coil of wire in a magnetic field. Electric current is generated by the relative motion of a conducting coil in a magnetic field. The two types of generators are AC generators and DC generators. 11

13 The electrical energy produced by power plants is transmitted through power lines at very high voltages. These voltages are too high to be used safely in homes. The voltage must first be changed, or transformed. A transformer is a device that increases or decreases the voltage and current of two linked AC circuits. A series of transformers changes high-voltage current in power lines into 240-volt current that can be used safely in your home. A transformer works only with alternating current because only alternating current induces a constantly changing magnetic field. A transformer changes voltage and current by inducing a changing magnetic field in one coil. This changing field then induces an alternating current in a nearby coil with a different number of turns. Section Review: 1. What causes the electromagnetic force? 2. What creates a magnetic field? 3. What happens when current is changed? 4. What happens when a magnet is moved in and out of a coil? 5. What are the two types of magnetic poles? 6. Why must we transform electrical energy so homes can use it? 7. Why do transformers only use alternating current? 8. How do transformers work? 12