SCIENCE 1208 MAGNETISM

Transcription

1 Grade 12 Unit 8

2 SCIENCE 1208 MAGNETISM CONTENTS I. FIELDS AND FORCES FIELDS FORCES II. ELECTROMAGNETISM THE ELECTROMAGNETIC FIELD CURRENTS AND MAGNETISM ELECTROMAGNETIC INDUCTION III. ELECTRON EAMS MAGNETIC DEFLECTION THE CATHODE-RAY TUE GLOSSARY Author: Editor: Illustrations: James T. Coleman, Ph.D. Alan Christopherson, M.S. Alpha Omega Graphics 804 N. 2nd Ae. E., Rock Rapids, IA MM by Alpha Omega Publications, Inc. All rights resered. LIFEPAC is a registered trademark of Alpha Omega Publications, Inc. All trademarks and/or serice marks referenced in this material are the property of their respectie owners. Alpha Omega Publications, Inc. makes no claim of ownership to any trademarks and/or serice marks other than their own and their affiliates, and makes no claim of affiliation to any companies whose trademarks may be listed in this material, other than their own.

3 MAGNETISM Science LIFEPAC 1207 dealt with the nature of electric current and was the second unit on electrophysics: the relation of the electrical nature of matter to physical systems. Science LIFEPAC 1208 will deal with the force laws for natural and man-made magnets, and with the subject of electromagnetism. The flow of the electrical charge is a current; when the current flows, a magnetic field is created. You will see that understanding of electromagnetism is being used daily, often without our knowledge. Electromagnetism generates the power that comes to our homes, and electromagnetic transformers send it from the generators oer long distances to our homes. The laws of electromagnetic induction are so regular that an entire power distribution system may be designed for a city, including generators and transformers; and its performance would be predictable before a single wire is connected. Such is the regularity of these electromagnetic laws. This LIFEPAC will introduce you to the electromagnetic world that the Lord has proided. God has power-generating stations around the earth in the upper regions of the ionosphere, about 100 kilometers aboe the earth s surface. Aboe the equator the equatorial electrojet uses the tidal motion of the atmosphere to generate a 100,000 ampere current in that region. This electrojet is part of His inisible creation. Colossians 1:16 credits God with all Creation, which certainly includes electricity. We cannot see the flow of electrons, but we can the flow s effect, including magnetism and electromagnetism. OJECTIVES Read these objecties. The objecties tell you what you will be able to do when you hae successfully completed this LIFEPAC. When you hae finished this LIFEPAC, you should be able to: 1. Sketch arious magnetic fields. 2. Write the force law for magnetic pole interactions. 3. Describe forces on charges in magnetic fields. 4. Explain how electric and magnetic field effects interact. 5. Describe the application of electromagnetic induction. 6. Interpret electric and magnetic deflection of electron beams. 7. Explain the operating principles of a cathode ray tube (CRT). Surey the LIFEPAC. Ask yourself some questions about this study. Write your questions here. 1

4 I. FIELDS AND FORCES Magnetic fields are around us eerywhere. The earth s magnetic field is with us whereer we go, as is the graitational field. We are more aware of the force of graity, without which we could not walk across the street. The forces due to a magnetic field are more subtle, and we are exposed to these eery day. When we walk under electrical power lines, we are exposed to both alternating magnetic fields and the earth s magnetic field. The effects of these fields on the body are not well understood, but their effects on electrical conductors and on currents of electrical charge are well known. This section will introduce the laws for forces between magnetic poles (magnets), and the forces on moing electrical charges in magnetic fields. SECTION OJECTIVES Reiew these objecties. When you hae completed this section, you should be able to: 1. Sketch arious magnetic fields. 2. Write the force law for magnetic pole interactions. 3. Describe forces on charges in magnetic fields. VOCAULARY Study these words to enhance your learning success in this section. aurora borealis ionosphere magnetic pole electrojet left hand rule solenoid field line magnetic field FIELDS The region around a magnetic pole (magnet) in which a bit of iron experiences a force is called a magnetic field. It is the magnet s sphere of influence. The magnetic field has direction, as defined by the force on a test north pole at any point in the field. The path followed by the test north pole describes a magnetic line of force, or a magnetic field line. Magnetic fields, such as the earth s magnetic field, occur in nature and around such man-made objects as the bar magnet and the solenoid. N. Magnetic Pole N. Geographic Pole Natural fields. Magnets hae been known to man since the dawn of history. They were probably first regarded with an air of mystery; later they were used as naigational tools. The earth s magnetic field is used as a direction-finding aid. Suspended magnetic needles, acting as compasses, follow the direction of the earth s field lines. S. Geographic Pole S. Magnetic Pole The magnetic field of the earth extends from the center of the earth, up through the oceans, and 2

5 into outer space. It influences the outer atmosphere in particular (the ionosphere) and has a strong influence on the flow of currents in that region. At great distances from the earth, the earth s magnetic field controls the flow of electrical currents that come from the surface of the sun. The current systems closer to the earth are called electrojets. One exists in the region near the North Pole and is called the auroral electrojet, which helps to produce the northern lights of high latitudes. Another high current system is near the equator and is called the equatorial electrojet. oth current systems are controlled and, to a degree, are generated by the earth s magnetic field. An estimated 1 million amperes of current flow eery night in the auroral electrojet. The Inuit depend upon the northern lights to enable them to see at night. The northern lights (aurora borealis) are a marelous display of multicolored and beautifully formed sheets of light in the sky. Currents from outer space funnel down the earth s magnetic field lines to form these beautiful displays. Thus the natural magnetic field of the earth has a profound influence on nature and the outer atmosphere. You will study this field and will discoer some of its properties and attributes. Man-made fields. A simple example of a man-made magnetic field is the bar magnet. The magnet has two poles. The pole that points toward the earth s north pole is termed the north-seeking pole, or simply the north pole, of the magnet. If the magnet is floated in water on a piece of cork, it will again align itself with the earth s field, with its north pole pointing toward the earth s north pole. The poles behae somewhat like electric charges: Like poles repel and unlike poles attract. The form of the magnetic field around a bar magnet will be found by experiments you will perform. The direction of the field lines can be traced and will generally be considered to flow from the north pole outside the magnet to the south pole. In other words, the presence of a magnetic field is sensed by a test north pole. The similarity to the case of two opposite electric charges ends at this point, because when a bar magnet is cut in half, two new smaller magnets are formed, each complete with a north pole and a south pole. A magnetic field is formed when a current flows in a wire. A compass brought near a current-carrying wire will align itself as shown in Figure 1. A compass FIGURE 1 The magnetic field will form as indicated by the compass needle s direction. The needle aligns with the magnetic field. When the wire is wound into a coil, the fields around the wires add to form a solenoid. A solenoid is a coil of wire designed to produce a magnetic field by means of a current passed through its windings. The field around the solenoid will be similar to the one around a bar magnet, with north and south poles as shown. This field is termed an electromagnet. Increasing the electric current will produce a stronger magnetic field. I I e S a. ar Magnet Field b. ar Magnet Cut in Half AR MAGNET FIELDS N 3

6 Choose the correct answer. 1.1 The earth s magnetic field extends. a. from the earth s surface to the upper atmosphere b. from the center of the earth to the sun c. from the earth s core to outer space 1.2 The earth s magnetic field captures the flow of current from. a. the moon c. the sun b. Saturn d. the center of the earth 1.3 High natural currents flow aboe the earth s atmosphere in. a. the aurora c. outer space b. upper clouds d. the asthenosphere 1.4 Two north poles will. a. attract each other c. experience neither attraction nor repulsion b. repel each other d. cancel each other 1.5 A north compass needle points to the. a. north magnetic pole c. north geographic pole b. south magnetic pole d. south geographic pole 1.6 The field around a solenoid is. a. similar to a bar magnet c. nonsymmetical b. unipolar Try this actiity to inestigate the nature of magnetic fields. These supplies are needed: 2 bar magnets 3 sheets of stiff cardboard iron filings Follow these directions and complete the actiities. Place a check in the box after each step is completed. 1. Place the sheet of stiff cardboard oer one of the bar magnets so that the cardboard is leel. 2. Sprinkle the iron filings on the cardboard. 3. Tap the cardboard to help the filings align with the magnetic field lines. 1.7 Sketch the magnetic field lines or take a Polaroid photograph. 1.8 Indicate where the magnetic field lines appear to be concentrated. 4. Place the north pole of one magnet 1 inch from the south pole of the other magnet. 5. Place the second cardboard oer the two magnets. 6. Sprinkle iron filings on the cardboard. 7. Tap the cardboard. 1.9 Sketch the field lines or take a Polaroid photograph Indicate where the magnetic field lines appear to be concentrated. 4

7 8. Place the north pole of one magnet 2 inches away from the north pole of the second magnet. 9. Place the third cardboard oer the two magnets. 10. Sprinkle iron filings on the cardboard. 11. Tap the cardboard Sketch the field lines or take a Polaroid photograph Indicate the concentration of field lines and where the field lines seem to be opposing rather than adding. Answer these questions Where are the magnetic field lines concentrated in Actiity 1.7? 1.14 Are the magnetic field lines continuous between the poles in Actiity 1.9? 1.15 Are the lines of force continuous between the poles in Actiity 1.11? Without the magnet, the filings are without any order. When the magnet is brought near, the filings themseles become little magnets, haing magnetism induced into them. The filings hae north poles and south poles, similar to the big bar magnet. FORCES Forces are pushes and pulls. Magnets both attract and repel each other through empty space. Magnetism is one of the three forces that operate without an interening medium, the others being graity and electrostatics. Magnets also produce currents in conductors and exert forces on current-carrying conductors. Forces of attraction or repulsion. The laws of attraction and repulsion for magnetic poles are similar to Coulomb s law for electrical charges. The force will be repulsie if the poles are alike, and will be attractie for dissimilar poles. An example is the splitting of a bar magnet. When a bar magnet is cut in half, two new northsouth magnet pairs are formed. Each will be similar to the unbroken magnet. A force of attraction exists between the two magnet hales. The unlike poles (north and south) of the two magnet ends will attract each other. Indiidual magnetic poles cannot exist. A north pole cannot exist without a corresponding south pole. In other words, a separate north pole cannot be moed completely away from its south pole. The two opposite poles always exist in pairs. The force of attraction between two magnetic poles is expressed in a form similar to Coulomb s law: FM = K M1M2 r 2 M1 is the strength of magnetic pole M1, M2 is the strength of magnetic pole M2, r is the separation between M1 and M2, and K is a constant that is dependent upon the units used in the formula (usually the metric system). The sign of the pole is used in the formula, as in Coulomb s law. Like poles repel and unlike poles attract. The product of M1 and M2 will be negatie for attraction and positie for repulsion, similar to Coulomb s law. 5

8 If the distance between the two poles is cut in half the force FM will be increased four times, the result of the inerse square relationship in the formula. Similarly, if we cause the distance between two magnetic poles to be tripled, the force between them will be reduced to one-ninth of its preious alue. The iot-saart force law. Consider an electric charge in motion in a magnetic field. The moing charge will experience a magnetic force FM. The magnitude of the force will depend on the magnitude of the charge, its elocity, and the direction in which the charge moes relatie to the field direction. Experiments determine the relationship between these quantities. (positie) charge flow. The force, or thrust, is then in the direction of the thumb. (Note: If electron flow is considered instead, is reersed, q is then negatie, which gies the same direction for F.) F q FM = O LEFT HAND RULE ZERO FORCE CONDITION Another way to remember the directions is the motion for turning a right-hand screw. A charge q moing parallel to the field experiences no force because no field lines are crossed. If the opposite condition is chosen and a charge is moed normal to the field lines, the force on the charge is the maximum force Fmax. screw a. b. q MAXIMUM FORCE CONDITION The force FM will be perpendicular to both the directions of the elocity and the field. The magnitude of the force FM is gien by Fmax = q FM = Fmax The direction of the force can be found by using the left-hand rule. To use the left-hand rule, align the first finger of the left hand with the field and the second finger with the speed of a conentional FIGURE 2 In Figure 2a the elocity of the positie conentional charge flow is in the direction of, and the magnetic field is in the direction. If we arrange the elocity of ector (translate it to just meet ) as shown in Figure 2b, then a screw turned the clockwise direction formed by and would moe into the paper, the force being into the paper in agreement with the left-hand rule. The ectors and in the figure define a plane. Vectors in a plane can always be translated in position (but not in direction) so that they will meet at some point in the plane. The direction of the force will then be perpendicular to this x-y plane, as indicated aboe. If the ectors and are not perpendicular to each other but pass through the same point, the rule can still be applied. 6

9 b. a. FIGURE 3: NONPERPENDICULAR CASE In Figure 3a elocity is not perpendicular to the field. In Figure 3b the ectors can be made to intersect at a point, but the angle θ between them is not 90. To find the equialent of mutually perpendicular and ectors, multiply by the sine of the angle θ. The force will then be indicated: FM = q sin θ This more general form of the iot-saart force law will apply to all elocity and field directions. If the elocity is in the same direction as the field, then the force FM is zero, since θ would be zero and the sine of θ is zero. The formula Fmax gien preiously assumes θ is 90, a condition giing the maximum force. The preceding expression for FM is the more general case. The force on a long, straight, current-carrying wire in a magnetic field is gien by FM = I L. 0 FM Fmax is the magnetic field, L is the length of wire, and I is the current in wire. The force F will be in a direction found from using the left-hand rule: It results from the flow of electrical charge perpendicular to the magnetic field. The geometry for the force on the wire is gien in Figure 4. FIGURE 4 Apply the left-hand rule to this figure, and the force FM will be in the direction shown. If the magnetic field is not perpendicular to the current in the wire, the force will be gien by FM = L I sin θ where θ is the angle between the ectors I and, as before. F I Complete these sentences The laws of attraction and repulsion for magnetic poles are similar in form to The force between two magnetic poles is repulsie if the poles are A bar magnet cut in half will form a total of poles The equation that relates two poles to the force between them and their separation is the square law The iot-saart force law predicts the force on a charge moing perpendicular to a field The left-hand rule uses the thumb to predict the direction of a on a positie charge moing in a magnetic field If the elocity of a charge is parallel to the magnetic field, the force will be The force on a straight current-carrying wire in a magnetic field is proportional to the current in the wire, the strength of the field, and the of the wire. 7

10 Choose the correct answer A magnetic field applies forces on. a. static charges c. water flow b. moing charges 1.25 Magnetic poles cannot be. a. isolated c. relocated b. canceled Complete this actiity The formulas F = q and F = IL both deal with moing charges in a wire. Start with F = q and derie F = IL. [Hint: = d /t] Reiew the material in this section in preparation for the Self Test. This Self Test will check your mastery of this particular section. The items missed on this Self Test will indicate specific areas where restudy is needed for mastery. 8

11 Answer true or false (each answer, 1 point). SELF TEST Man first regarded magnets with mystery, but later used them as naigation tools The earth s magnetic field extends from the center of the earth to outer space Like magnetic poles attract A bar magnet cut in half will yield two complete bar magnets A current-carrying wire is surrounded by a magnetic field The field around a solenoid is similar to the field around a bar magnet The force between two magnetic poles aries directly with the distance of separation A charged particle moing parallel to a magnetic field line will experience no force The left-hand rule predicts the direction of force on a positiely charged object moing in a magnetic field The iot-saart force law does not apply if the elocity is parallel to the field direction. Choose the correct answer (each answer, 2 points) A positie charge q moes with elocity in the z direction through a magnetic field whose ector is in the x direction. y -z -x x z The force FM will be in the. a. positie-y direction c. positie-z direction b. negatie-z direction d. positie-x direction If the charge is negatie and it moes in the negatie-z direction, the force on the charge will be. a. reersed c. zero b. unchanged d. increased In the formula -y r is the distance. FM = K M1M2 r 2 a. from the north pole c. from M1 to the nearest magnetic body b. between poles M1 and M2 d. between the magnetic lines of force 9

12 1.014 A solenoid field is. a. neer zero c. increased with an increased current flow b. located outside the coil only d. entirely within the coil A bar magnet s field is maintained by. a. microscope currents c. a difference in temperature between north and south poles b. orientation with the earth s field d. the lines of force Complete these sketches (each answer, 5 points) Sketch the magnetic field lines for the two magnets. S N S N Sketch the magnetic field lines around the two magnets. N S S N Score Adult check Initial Date 10