Activity 16, Electromagnetic Induction! from! Science & Global Issues: Global Energy & Power! This material is based upon work supported by the National Science Foundation under Grant No. ESI 0352453. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.!! 2014 The Regents of the University of California!!
16 Electromagnetic Induction INVESTIGATION 2 CLASS SESSIONS OVERVIEW Students investigate magnetic fields and their properties. They discover that current-carrying wires produce magnetic fields, and changing magnetic fields near a conductor induces a current in the conductor. Next Generation Science Standards (NGSS) DISCIPLINARY CORE IDEAS PS.3.A: Definitions of Energy PS.3.B: Conservation of Energy and Energy Transfer PS.3.C: Relationship Between Energy and Forces ETS.1.A: Defining and Delimiting Engineering problems ETS.1.B: Developing Possible Solutions SCIENCE AND ENGINEERING PRACTICES Analyzing and Interpreting Data Developing and Using Models Constructing Explanations and Designing Solutions Planning and Carrying Out Investigations CROSSCUTTING CONCEPTS Cause and Effect Energy and Matter System and System Models MATERIALS AND ADVANCE PREPARATION For the teacher Scoring Guide: DESIGNING INVESTIGATIONS (DI) Student Sheet 16.1, KWL: Magnetism (optional) Student Sheet 16.2, Sample Procedure: Electromagnetic Induction (optional) For each group of four students 3 neodymium magnets 2 wires with alligator clips D cell battery in holder 20 cm of copper wire coil of wire compass 2 Cir-Kit junctions Cir-Kit ammeter Cir-Kit switch computer with Internet connection* For each student 3 5 sticky notes Scoring Guide: DESIGNING INVESTIGATIONS (DI) (optional) Student Sheet 16.1, Sample Procedure: Electromagnetic Induction (optional) Literacy Transparency 3, Read, Think, and Take Note Guidelines (optional) Science Skills Student Sheet 4, Elements of Good Experimental Design (optional) *Not supplied in kit KEY CONTENT 1. Current-carrying wires produce magnetic fields. The magnetic field is proportional to the current in the wire. 2. Changing magnetic fields near a conductor induces a current in a conductor. 3. The force on a current-carrying wire in a magnetic field is proportional to the magnetic field, B, the current in the wire, I, the length of the wire, L, and the orientation of the wire with respect to the magnetic field, θ (F wire = BILsinθ). 4. Fleming s right-hand rule can be used to determine the direction of the induced current in a conductor that experiences a variation in magnetic flux. 5. Fleming s left-hand rules can be used to determine the direction of the force on a current-carrying conductor or moving charge in a magnetic field. Science Skills Student Sheets are in Teacher Resources II: Diverse Learners. Masters for Literacy transparencies are in Teacher Resources III: Literacy. Masters for Scoring Guides are in Teacher Resources IV: Assessment. Make sure the ends of the wires (the coil and the straight wire) in the first two inquiries conduct when connected to the batteries. If the ends are coated with an insulator, scrape off the coating with sandpaper. To maintain the accuracy of the compasses, store them apart from the magnets and require students to keep them apart. 161
SCIENCE & GLOBAL ISSUES/PHYSICS ELECTRICITY TEACHING SUMMARY Getting Started (LITERACY) Discuss the scenario in the introduction. Review characteristics of magnetic fields. Doing the Activity Students conduct inquiries. (DI ASSESSMENT) Students design and conduct an investigation while referring to the simulation. (LITERACY) Students read the Technology Connection. Follow-up (LITERACY) Discuss the results of the investigation, and relate them to the factors that affect the force on a conductor in a magnetic field. BACKGROUND INFORMATION Electric and Magnetic Fields Electric and magnetic fields have some similarities. For example, both act on other objects at a distance. There are differences as well. For example, electric fields can exist around a single charge but magnetic fields require two poles. While electric charge can be transferred between objects, giving an object with a net positive charge, negative charge, or neutral charge, there is no equivalent for magnetism there are no magnetic charges. An object that is magnetized will always exhibit a north pole and a south pole. An object can also be unmagnetized, but this is not due to any charge leaving the object. Electromagnetic Induction Changing electric fields create magnetic fields. This can be demonstrated by a wire with a current going through it. A current is a net movement of electric charges. These electric charges create an electric field, and when the charges move, the electric field moves too, changing the field in the vicinity of the wire. This changing electric field produces a magnetic field, which we can observe with a compass. In the reverse, changing magnetic fields can create electric fields. This may be demonstrated with a coil of wire and a magnet. If a magnet is quickly inserted into the center of a coil of wire, the space around the wire experiences a change in the magnetic field as the magnet gets closer and closer to the wires. This changing magnetic field creates an electric field, which then affects the electric charges in the wires. The charges in the wire are pushed (or pulled), which we can observe as a current. This effect is also the basis for an electromagnetic wave, which is a propagating electric and magnetic field, where the change in one field gives rise to the other. This is the basis for the production and propagation of electromagnetic energy, such as light and radio waves. Electromagnetic induction is also utilized in electric motors and generators. Lorentz Force When an electric current flows through a conductor that is placed in a magnetic field, a force may be produced, depending on the relative orientations of the field and current. The force on a length of current-carrying wire is derived from the relationship Hendrik Lorentz derived in 1892, F = ILB sinθ, where F is the magnetic force (in newtons), I is the current (in amperes), L is the length of wire (in meters), B is the magnetic field strength (in teslas), and θ is the angle between the wire and the magnetic field (in degrees). Maximum force is produced when the current and field are perpendicular to one another. However, the conductor will experience no force if the directions of the field and current are parallel. 162
ELECTROMAGNETIC INDUCTION ACTIVITY 16 Right-hand Rule When a current is induced in a moving conductor in a magnetic field, the direction of the current can be predicted by applying Fleming s right-hand rule for generators, which is a visual mnemonic originated by John Ambrose Fleming in the late 19th century. In the right-hand rule, the first finger, the thumb, and the second finger of the right hand are positioned so that each is held at 90 to the others. The thumb represents the direction of motion of the conductor, the first finger represents the direction of the magnetic field (N to S), and the second finger represents the direction of the induced current. Note the current referred to is motion conventional current, positive to negative, and not electron flow. magnetic field Left-hand Rule When a current moves through a magnetic field, the direction of the resulting force can be predicted with Fleming s left-hand rule for motors. This rule mirrors Fleming s righthand rule, which predicts the induced current. In the lefthand rule, the first finger, thumb, and second finger of the left hand are positioned so that each is held at 90 to the others. The thumb represents the direction of the force on the conductor, the first finger represents the direction of the magnetic field (N to S), and the second finger represents the direction of the electric current. If the force is strong enough, it will cause force the conductor to move, as is the case magnetic with an electric field motor. current current FLEMINGS RIGHT-HAND RULE FLEMINGS LEFT-HAND RULE 163
SCIENCE & GLOBAL ISSUES/PHYSICS ELECTRICITY GETTING STARTED 1 (LITERACY) Have students read the introduction to the activity. As a class, begin a KWL chart for the concept of magnetism. The letters KWL refer to the three sections of the reading strategy that ask, What do I Know? What do I Want to Know? What did I Learn? KWLs help students process and apply the information that they encounter in the reading. For more information see Teacher Resources III: Literacy. Ask the class to begin by listing what they know about magnetism in the left hand column of the chart and what they would like to know in the center column. It may help to refer to the comparison of various force fields in the reading from Activity 9, Electric Fields. The right hand column will be completed at the end of the activity. See the end of this activity for a sample student response to the KWL. 1 Ask students if they are aware of any commonalities between electricity and magnetism. Students may reply that they both have two poles, such as north and south and positive and negative. Students may also mention that they both can make things move. Use the KWL chart and this discussion as an opportunity to assess how much students know about magnetism before beginning the activity. The materials assume that students are familiar with magnetism. The SI unit tesla (T) is mentioned in the introduction as the unit of the magnetic field strength (B), also known as the magnetic-flux density, but it is not fully explained because of its complexity. While tesla is the SI unit, gauss is often used because it is smaller. One gauss is 1 10 4 tesla. Sample Magnetism KWL Know Want to know Learned Only certain metals are magnetic (iron, nickel, cobalt, etc) Are electric and magnetic fields the same thing? Can provide an attractive or repulsive force. magnetic fields work Do electric and against each other? Magnetism is a force at a distance How did the surgical instrument work? Similar to electricity 1 tesla = 1 N sec = 1 N/A meter. because it has opposite C meter poles (N/S and +/ ) Need two poles to have a magnetic field Is relatively stronger than gravity 164
ELECTROMAGNETIC INDUCTION ACTIVITY 16 2 For Part A, students should work in pairs and switch materials with the other pair that makes up their group of four. DOING THE ACTIVITY 3 The term electromagnetic induction is not defined until Procedure Step 5. Allow students to conduct this inquiry without that introduction. 2 Safety When students connect the wire to see if the compass deflects, they are essentially short-circuiting the battery. Review the Safety Note in the student book, and discuss safety issues related to working with batteries. Warn students that they should open the switch after just a second so as not to ruin the battery or generate too much heat in the wire. The compass should deflect even with the shortest of connection times. 4 Procedure Step 3 is an opportunity for you to describe the orienta- 3 tion of the current and the 4 corresponding magnetic field via the corkscrew rule. The corkscrew rule states that the relationship between the direction of current and the magnetic field is the same as the orientation of a corkscrew. If the direction the screw moves is the current, the magnetic field wraps itself around in concentric circles. The positions of the thumb and fingers of the right hand represent the orientation, as shown below. Reviewing the corkscrew rule is important because students will need this information to answer Analysis Question 3. current magnetic field magnetic field current 165
SCIENCE & GLOBAL ISSUES/PHYSICS ELECTRICITY 5 Emphasize that magnets and wires do not have to touch for induction to occur. The electric and magnetic fields provide a force at a distance, as opposed to a contact force as described in Activity 8, Electric Fields. Reintroduce Michael Faraday as one of the main contributors to the field of electromagnetic field induction. He was the first to discover the relationship between electric and magnetic fields and is most famous for experimentally determining it. He described the entire relationship in words only because he was not mathematically proficient. Later, James Clerk Maxwell described the relationship in a famous set of mathematical equations, but it is generally known as Faraday s law. 6 Sample Student Response: Procedure Step 8 5 6 7 8 Amount of current in wire ( controlled by voltage and resistance) Direction of current in wire Length of wire Strength of magnetic field Angle between magnetic field and direction of current. 7 (DI ASSESSMENT) This is an opportunity for you to use the DESIGNING INVESTIGATIONS (DI) Scoring Guide to evaluate students work. Explain to the class how you will apply the DI Scoring Guide to provide feedback on their investigations. You might also review the information on Science Skills Sheet 4, Elements of Good Experimental Design. A sample procedure is shown in Student Sheet 16.1, Sample Procedure: Electromagnetic Induction, which will guide the inquiry for those groups that may have difficulty in designing their own experimental procedures. In combination with sample student results below, it may also serve as a sample Level-3 response. A sample Level-3 response is shown at the end of this Teacher s Guide. Introduce Fleming s left-hand rule (See Background Information) in the simulation. This rule guides students in predicting the orientation of the current, the direction of the magnetic field, and the force on the conductor. The simulation shows the lines of force between the poles of the magnet, which is assumed to be uniform in a sufficiently large magnet. Students may notice that the Magnetic Orientation range is 90 90 instead of 0 180. These quantities make the students experience consistent with the quantitative calculations for Lorentz formula. You may want to point out to students where the lines of force surround the magnet and connect this to the electric field lines shown in Activity 9, Electric Potential. Applying what they did in that activity, they should be able 166
ELECTROMAGNETIC INDUCTION ACTIVITY 16 to predict that the lines are more completely drawn like this: N S 8 (LITERACY) Give students time to read the Technology Connection, Maglev Trains, while following the Read, Think, and Take Note literacy strategy. After they have finished reading, have them work in pairs, groups, or as a class to compare sticky notes, discuss what they wrote, and answer each other s questions. 167
SCIENCE & GLOBAL ISSUES/PHYSICS ELECTRICITY 168
ELECTROMAGNETIC INDUCTION ACTIVITY 16 169
SCIENCE & GLOBAL ISSUES/PHYSICS ELECTRICITY FOLLOW-UP 170 9 Analysis Question 6 reinforces students awareness of the SI units for measuring magnetism, particularly the tesla, as introduced in this activity. The relationship discovered in the simulation is that the force on the wire is directly proportional to the magnetic-field strength, the magnitude of the current, and the orientation of the wire in the field. Students are not expected to understand the algebraic relationship unless they are familiar with the trigonometry. However, they can identify the individual quantities that affect the force on the wire. Present the equation, if appropriate, and ask students if it is consistent with their data. The governing equation, formulated by Heinrich Lenz in 1833, is given as F = BILsinθ, where F is the force on the current (in newtons), B is the magnetic field strength (in teslas), L is the length of wire (in meters), and θ is the angle between the magnetic field and the direction of the current in the wire (in degrees). If students are not ready to work with sinθ, simplify the situation by using the maximum and minimum values of sin(90 ) = 1 (strongest) where the wire and magnetic field are perpendicular, and sin(0 ) = 0 (weakest) 9 Sample Magnetism KWL Know Want to know Learned Only certain metals are magnetic (iron, nickel, cobalt, etc) Can provide an attractive or repulsive force. Magnetism is a force at a distance Similar to electricity because it has opposite poles (N/S and +/ ) Need two poles to have a magnetic field Are electric and magnetic fields the same thing? Do electric and magnetic fields work against each other? How did the surgical instrument work? Is relatively stronger where the wire and magnetic field are parallel. than gravity Students data should show that perpendicular has the maximum force on the wire, and parallel the minimum. (LITERACY) Conclude the activity by revisiting (which the KWL chart. Fill in the third column and then consider additions to the second column. An electric current can induce a magnetic field. A magnetic field can induce an electric current. A current in a wire that passes through a magnetic field experiences a force perpendicular to the wire. The force depends on the length of wire, the strength of the magnetic field, and the angle between the direction of the current and the magnetic field. The surgical tool was likely an electromagnet with the current controlling the magnetic tip that removed the steel shard has iron in it).
ELECTROMAGNETIC INDUCTION ACTIVITY 16 SAMPLE RESPONSES 1. The instrument used in the surgery had a magnetic field that was induced by an electric current running through the instrument. The magnetic field attracted the piece of steel. 2. a. The force on the wire increased as the amount of current increased, and the force decreased when the amount of current decreased. b. Changing the direction of the current reversed the direction of the magnetic field and the direction of the force. c. The force on the wire increased as the length of wire increased, and the force decreased when the length of wire decreased. d. Increasing the magnetic field increased the force on the wire, and decreasing the magnetic field decreased the force on the wire. e. There is no force when the angle in between the direction of the magnetic field and the direction of current is 0, or when the direction of the magnetic field and the direction of current are parallel. As the angle increases, the force increases until the direction of the magnetic field and the direction of current are perpendicular. There is maximum force when the angle between them is 90. 3. By the corkscrew rule, the needle of the compass would be deflected east. 4. The lines of force drawn in the in introduction look similar to the field lines drawn in previous activities for electric fields. Also the lines of force shown by the compasses placed in a magnetic field in the inquiries and in the simulation show lines of force. 5. The corresponding magnetic field changed when the electric field was changed. Benjamin Franklin chose the + and arbitrarily. At that time, electrons had not been discovered so he had no basis for knowing which way the electrons flowed. He was only able to see attraction and repulsion because it was not possible for him to distinguish or see the charges moving. 6. Answers will vary. For most communities, the main factor that has prevented widespread adoption is the cost of the infrastructure. To implement maglev requires a huge investment in building the guideways, stations, and trains. The amounts of money involved would likely be so large that it would be difficult to generate from government and/or the private sector. To get a return on this investment would require heavy ridership, which limits the potential locations. Without policies and incentives to support the establishment of maglev, it is unlikely that there will be an investment in the technology in most communities. ENRICHMENT 7. F = ILBsinθ B = ILsinθ F F = 4.8 10 4 N I = 7A L = 1.5 m sin(90 ) = 1 10 4 N B = 4.8 (7A) (1.5m) (1) B = 4.6 10 5 T REVISIT THE CHALLENGE Students should be able to identify that magnetic fields are generated two ways: first, magnets produce magnetic fields, and second, moving electric charges produce magnetic fields by electromagnetic induction. In electromagnetic induction, the source of the electric current is the changing magnetic field, such as the one in the coil experiment when the magnet is moved rapidly near the coil. From the simulation, students should conclude that the electromagnetic force (F) due to the magnetic field is directly proportional to the strength of the magnetic field (B), the amount of current (I), the length of the current (l), and the orientation of the wire (90 strongest, 0 is weakest). 171
NAME Sample Student Response DATE Sample Procedure: Electromagnetic Induction 1. Investigate the effect of changing the strength of the magnetic field, while all other variables remain constant. Relative strength of magnetic field (0 3) Resistance setting (Minimum, medium, maximum) Number of D cells (1 3) Battery direction (Positive to left, positive to right) Orientation of magnet to conductor ( ) Length of conductor (cm) Magnetic field intensity (T) Current (A) Force (N) 0 Medium 1 Left 90 20 0 0.231 0 1 Medium 1 Left 90 20 0.025 0.231 0.00115 2 Medium 1 Left 90 20 0.05 0.231 0.00231 3 Medium 1 Left 90 20 0.1 0.231 0.00462 2. Investigate the effect of changing the resistance, while all other variables remain constant. Relative strength of magnetic field (0 3) 2014 The Regents of the University of California Resistance setting (Minimum, medium, maximum) Number of D cells (1 3) Battery direction (Positive to left, positive to right) Orientation of magnet to conductor ( ) Length of conductor (cm) Magnetic field intensity (T) Current (A) Force (N) 3 Minimum 1 Left 90 20 0.1 0.333 0.00667 3 Medium 1 Left 90 20 0.1 0.231 0.00462 3 Maximum 1 Left 90 20 0.1 0.176 0.00353 SCIENCE AND GLOBAL ISSUES/PHYSICS STUDENT SHEET 16.1 (continued on next page)
NAME Sample Student Response DATE Sample Procedure: Electromagnetic Induction (Continued) 3. Investigate the effect of the changing the voltage (D-cells), while all other variables remain constant. Relative strength of magnetic field (0 3) Resistance setting (Minimum, medium, maximum) Number of D cells (1 3) Battery direction (Positive to left, positive to right) Orientation of magnet to conductor ( ) Length of conductor (cm) Magnetic field intensity (T) Current (A) Force (N) 3 Minimum 1 Left 90 20 0.1 0.333 0.00667 3 Minimum 2 Left 90 20 0.1 0.667 0.01333 3 Minimum 3 Left 90 20 0.1 1.0 0.02 4. Investigate the effect of the direction of the current by changing only the orientation of the batteries, while all other variables remain constant. Relative strength of magnetic field (0 3) Resistance setting (Minimum, medium, maximum) Number of D cells (1 3) Battery direction (Positive to left, positive to right) Orientation of magnet to conductor ( ) Length of conductor (cm) 2014 The Regents of the University of California Magnetic field intensity (T) Current (A) Force (N) 3 Minimum 1 Left 90 20 0.1 0.333 0.00667 3 Medium 1 Left 90 20 0.1 0.231 0.00462 SCIENCE AND GLOBAL ISSUES/PHYSICS STUDENT SHEET 16.1 (continued on next page)
NAME Sample Student Response DATE Sample Procedure: Electromagnetic Induction (Continued) 5. Investigate the effect of the direction of the magnetic field by changing the orientation of the magnet to the conductor, while all other variables remain constant. Relative strength of magnetic field (0 3) Resistance setting (Minimum, medium, maximum) Number of D cells (1 3) Battery direction (Positive to left, positive to right) Orientation of magnet to conductor ( ) Length of conductor (cm) Magnetic field intensity (T) Current (A) Force (N) 3 Minimum 1 Left 90 20 0.1 0.333 0.00667 3 Minimum 1 Left 60 20 0.1 0.333 0.00577 3 Minimum 1 Left 30 20 0.1 0.333 0.00333 3 Minimum 1 Left 0 20 0.1 0.333 0 3 Minimum 1 Left 30 20 0.1 0.333 0.00333 3 Minimum 1 Left 60 20 0.1 0.333 0.00577 3 Minimum 1 Left 90 20 0.1 0.333 0.00667 6. Investigate the effect of changing the length of the bar while all other variables remain constant. 2014 The Regents of the University of California Relative strength of magnetic field (0 3) Resistance setting (Minimum, medium, maximum) Number of D cells (1 3) Battery direction (Positive to left, positive to right) Orientation of magnet to conductor ( ) Length of conductor (cm) Magnetic field intensity (T) 3 Minimum 1 Left 90 2.5 0.1 0.333 0.00083 3 Minimum 1 Left 90 5 0.1 0.333 0.00167 3 Minimum 1 Left 90 10 0.1 0.333 0.00333 3 Minimum 1 Left 90 15 0.1 0.333 0.005 3 Minimum 1 Left 90 20 0.1 0.333 0.00667 Current (A) Force (N) SCIENCE AND GLOBAL ISSUES/PHYSICS STUDENT SHEET 16.1