Physics
Electromagnetic Induction
Electromagnetic induction is the process of generating an electromotive force (emf) or voltage in a conductor by exposing it to a changing magnetic field. This phenomenon, discovered by Michael Faraday, is the underlying principle behind the operation of generators, transformers, and inductors. It forms the basis for many important technologies, including electric power generation and transmission.
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10 Key excerpts on "Electromagnetic Induction"
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
550 Chapter 22 | Electromagnetic Induction 22.1 | Induced Emf and Induced Current There are a number of ways a magnetic field can be used to generate an electric current, and Figure 22.1 illustrates one of them. This drawing shows a bar magnet and a helical coil of wire to which an ammeter is connected. When there is no relative motion between the magnet and the coil, as in part a of the drawing, the ammeter reads zero, indicating that no current exists. However, when the magnet moves toward the coil, as in part b, a current I appears. As the magnet approaches, the magnetic field B B that it creates at the location of the coil becomes stronger and stronger, and it is this changing field that produces the current. When the magnet moves away from the coil, as in part c, a current is also pro- duced, but with a reversed direction. Now the magnetic field at the coil becomes weaker as the magnet moves away. Once again it is the changing field that generates the current. A current would also be created in Figure 22.1 if the magnet were held stationary and the coil were moved, because the magnetic field at the coil would be changing as the coil approached or receded from the magnet. Only relative motion between the magnet and the coil is needed to generate a current; it does not matter which one moves. The current in the coil is called an induced current because it is brought about (or “in- duced”) by a changing magnetic field. Since a source of emf (electromotive force) is always needed to produce a current, the coil itself behaves as if it were a source of emf. This emf is known as an induced emf. Thus, a changing magnetic field induces an emf in the coil, and the emf leads to an induced current. 22 | Electromagnetic Induction © Jeff Greenberg/Age Fotostock Electric guitars are famous for their amplified and manipulatable sound. To produce this sound, virtually all of them have one or more electro- magnetic pickups located beneath the strings (see Section 22.6).
No longer available |Learn more- (Author)
- 2014(Publication Date)
- University Publications(Publisher)
____________________ WORLD TECHNOLOGIES ____________________ Chapter- 3 Faraday's Law of Induction Faraday's law of induction is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators. The law states that: The induced electromotive force (EMF) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit. Or alternatively: The EMF generated is proportional to the rate of change of the magnetic flux. History Electromagnetic Induction was discovered independently by Michael Faraday and Joseph Henry in 1831; however, Faraday was the first to publish the results of his experiments. ____________________ WORLD TECHNOLOGIES ____________________ Faraday's disk In Faraday's first experimental demonstration of Electromagnetic Induction (August 1831), he wrapped two wires around opposite sides of an iron torus (an arrangement similar to a modern transformer). Based on his assessment of recently-discovered properties of electromagnets, he expected that when current started to flow in one wire, a sort of wave would travel through the ring and cause some electrical effect on the opposite side. He plugged one wire into a galvanometer, and watched it as he connected the other wire to a battery. Indeed, he saw a transient current (which he called a wave of electricity) when he connected the wire to the battery, and another when he disconnected it. Within two months, Faraday had found several other manifestations of Electromagnetic Induction. For example, he saw transient currents when he quickly slid a bar magnet in and out of a coil of wires, and he generated a steady (DC) current by rotating a copper disk near a bar magnet with a sliding electrical lead (Faraday's disk). Faraday explained Electromagnetic Induction using a concept he called lines of force.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- The English Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 1 Faraday's Law of Induction Faraday's law of induction is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators. The law states that: The induced electromotive force (EMF) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit. Or alternatively: The EMF generated is proportional to the rate of change of the magnetic flux. History Electromagnetic Induction was discovered independently by Michael Faraday and Joseph Henry in 1831; however, Faraday was the first to publish the results of his experiments. ________________________ WORLD TECHNOLOGIES ________________________ Faraday's disk In Faraday's first experimental demonstration of Electromagnetic Induction (August 1831), he wrapped two wires around opposite sides of an iron torus (an arrangement similar to a modern transformer). Based on his assessment of recently-discovered properties of electromagnets, he expected that when current started to flow in one wire, a sort of wave would travel through the ring and cause some electrical effect on the opposite side. He plugged one wire into a galvanometer, and watched it as he connected the other wire to a battery. Indeed, he saw a transient current (which he called a wave of electricity) when he connected the wire to the battery, and another when he disconnected it. Within two months, Faraday had found several other manifestations of Electromagnetic Induction. For example, he saw transient currents when he quickly slid a bar magnet in and out of a coil of wires, and he generated a steady (DC) current by rotating a copper disk near a bar magnet with a sliding electrical lead (Faraday's disk). Faraday explained Electromagnetic Induction using a concept he called lines of force.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Orange Apple(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter- 7 Faraday's Law of Induction Faraday's law of induction is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators. The law states that: The induced electromotive force (EMF) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit. Or alternatively: The EMF generated is proportional to the rate of change of the magnetic flux. History Electromagnetic Induction was discovered independently by Michael Faraday and Joseph Henry in 1831; however, Faraday was the first to publish the results of his experiments. ________________________ WORLD TECHNOLOGIES ________________________ Faraday's disk In Faraday's first experimental demonstration of Electromagnetic Induction (August 1831), he wrapped two wires around opposite sides of an iron torus (an arrangement similar to a modern transformer). Based on his assessment of recently-discovered properties of electromagnets, he expected that when current started to flow in one wire, a sort of wave would travel through the ring and cause some electrical effect on the opposite side. He plugged one wire into a galvanometer, and watched it as he connected the other wire to a battery. Indeed, he saw a transient current (which he called a wave of electricity) when he connected the wire to the battery, and another when he disconnected it. Within two months, Faraday had found several other manifestations of Electromagnetic Induction. For example, he saw transient currents when he quickly slid a bar magnet in and out of a coil of wires, and he generated a steady (DC) current by rotating a copper disk near a bar magnet with a sliding electrical lead (Faraday's disk). Faraday explained Electromagnetic Induction using a concept he called lines of force.
eBook - PDF- Paul Rosenberg, Robert Gordon Middleton(Authors)
- 2004(Publication Date)
- Audel(Publisher)
Chapter 5 Electromagnetic Induction Electricians work with Electromagnetic Induction in many practi-cal situations. Various electrical devices operate on the principle of Electromagnetic Induction, and circuits often have a response (cir-cuit action) based on this principle. For example, we may note the following typical devices: Transformers Generators Induction motors Watt-hour meters Ignition coils Magnetos Compensators Induction regulators Choke reactors Principle of Electromagnetic Induction When a permanent magnet is inserted into a coil, as shown in Fig-ure 5-1, the voltmeter deflects. In other words, the changing mag-netic field in the coil induces a voltage in the coil winding. When the permanent magnet is withdrawn from the coil, a voltage of opposite polarity is induced in the coiled winding. Note that if the perma-nent magnet is motionless in the coil, there is no induced voltage. Note also that when the magnet is moved faster, a greater voltage is induced. It is the relative motion of a magnetic field and a wire (or coil) that induces a voltage in the wire. For example, if a coil is moved through MOVING BAR MAGNET VOLTMETER COIL Figure 5-1 An example of Electromagnetic Induction. 107 108 Chapter 5 a magnetic field, a voltage is induced in the coil. This principle is used in an electrical instrument called a fluxmeter , or gaussmeter , to measure the strength of a magnetic field. The instrument consists of a flip coil connected to a galvanometer , as shown in Figure 5-2. A flip coil consists of a few turns of wire, usually less than an inch in diameter. A galvanometer is simply a sensitive current meter. To use a fluxmeter, the electrician places the flip coil in the magnetic field to be measured and then flips the coil out of the field. In turn, the galvanometer reads the strength of the magnetic field in lines of force per square inch. FLIP COIL GALVANOMETER Figure 5-2 Construction of a flip coil and galvanometer.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
This emf is known as an induced emf. Thus, a changing magnetic field induces an emf in the coil, and the emf leads to an induced current. Electric guitars are famous for their amplified and manipulatable sound. To produce this sound, virtually all of them have one or more electro- magnetic pickups located beneath the strings (see Section 22.6). These pickups work because of Electromagnetic Induction, which is the process by which a magnet is used to create or induce an emf in a coil of wire. In this photograph the pickup is indicated by the shiny rectangle in the white area. 22 | Electromagnetic Induction Chapter | 22 615 LEARNING OBJECTIVES After reading this module, you should be able to... 22.1 | Predict when an induced current will flow. 22.2 | Solve motional emf problems. 22.3 | Calculate magnetic flux. 22.4 | Solve problems using Faraday’s law of induction. 22.5 | Predict the direction of an induced current using Lenz’s law. 22.6 | Describe how sound is reproduced via induction. 22.7 | Solve problems involving generators. 22.8 | Define mutual induction and self-in- ductance. 22.9 | Solve problems involving transformers. © Jeff Greenberg/Age Fotostock S S N N N S S N N N S S N N N Ammeter Current upward Bar magnet Coil I I B (a) (b) I I (c) Current downward B Figure 22.1 (a) When there is no relative motion between the coil of wire and the bar magnet, there is no current in the coil. (b) A current is created in the coil when the magnet moves toward the coil. (c) A current also exists when the magnet moves away from the coil, but the direction of the current is opposite to that in (b). 616 Chapter 22 | Electromagnetic Induction The physics of an automobile cruise control. Induced emf and induced current are fre- quently used in the cruise controls found in many cars. Figure 22.2 illustrates how a cruise control operates. Usually two magnets are mounted on opposite sides of the vehicle’s drive shaft, with a stationary sensing coil positioned nearby.
eBook - PDF- Michael Tammaro(Author)
- 2019(Publication Date)
- Wiley(Publisher)
| 603 On the left is a U.S. quarter-dollar coin (a quarter). On the right is another quarter that has undergone electromagnetic forming, in which the coin was subjected to a strong and rapidly changing magnetic field. The changing magnetic field creates circulating electric currents in the coin—a process called Electromagnetic Induction. The electric current, in turn, creates a magnetic field that repels the original field, resulting in the strong forces that deformed the coin. The entire process lasts only a few microseconds and has numerous industrial applications. Ted Kinsman/Photo Researchers** 603 Electromagnetic Induction 22 604 | Chapter 22 22.1 Solve problems dealing with motional emf. In Chapter 21, we learned that an electric current produces a magnetic field. The reverse is true also—namely, a magnetic field can produce electric current in a process called elec- tromagnetic induction. Electromagnetic Induction Electromagnetic Induction is illustrated in Animated Figure 22.1.1, where a rectangular loop of wire is positioned between a magnetic north and south pole. The magnetic field is confined to the region between the poles and points from the north pole to the south pole, as shown. The loop is connected to an ammeter (a device that measures electric current) and the plane of the loop perpendicular to the magnetic field. The ammeter needle will deflect in the direction of the electric current passing through it. In animation (a), the loop is moved out from between the poles, held in place for a moment, then moved back to its original position. This is repeated a second time, but now the loop is moved faster. Watch the animation several times and pay careful attention to the ammeter reading. Note the following: ∙ There is no current when the loop is stationary. ∙ The current flows one way when the loop is moved out and the other way when it is moved back in. ∙ The faster the loop is moved, the greater the current.
eBook - PDF- Pierluigi Zotto, Sergio Lo Russo, Paolo Sartori(Authors)
- 2023(Publication Date)
- Società Editrice Esculapio(Publisher)
Electromagnetic Induction 12.1 Introduction Consider a circuit, which features a coil, and a permanent magnet. It is experimentally observed that, moving the magnet either close to the coil or away from it, a current flows in the circuit even if no voltage generator is present. The current flows in a certain direction if the magnet moves towards the coil; it flows in the opposite direction if the magnet moves away from the coil; it behaves symmetrically if the position of the magnetic poles is in- verted. A current also flows if the magnet is kept at rest and the circuit is moved towards or away from the magnet. A N S v A N S A S N A S N v v v A N S A N S v v 187 12 On the other hand, no current flows if no relative motion exists between the circuit and the magnet. Moreover, an electric current flows if the intensity of the magnetic field generated by the coil is varied by inserting in it a block of soft iron, even if both the circuit and the mag- net are at rest, or it flows during the transients when opening or closing an RL circuit, put close to the circuit without a voltage generator, until the current in the RL circuit becomes steady. A S N A S N v v A N S A N S v v v v A N S Fe v A Electromagnetic Induction Chapter 12 188 12.2 Faraday’s Law The number of turns of the coil amplifies the effect which must anyway also occur in the case of a single turn. Therefore, considering a one-turn coil, it can be remarked that the only feature common to all the different cases is a variation of the magnetic flux linked to the coil. In fact, in all the presented cases, the number of field lines linked to the coil changes in time, because the magnetic field magnitude in the coil position changes. The appearance of a current is not, however, the main effect: if the circuit is opened in a whatever point, a voltage, independent of the opening position, is measured between the circuit extremities. The just described phenomenon is called Electromagnetic Induction.
eBook - PDF- William Moebs, Samuel J. Ling, Jeff Sanny(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
In motors, this is called back emf because it opposes the emf input to the motor. 13.7 Applications of Electromagnetic Induction • Hard drives utilize magnetic induction to read/write information. • Other applications of magnetic induction can be found in graphics tablets, electric and hybrid vehicles, and in transcranial magnetic stimulation. CONCEPTUAL QUESTIONS 13.1 Faraday’s Law 1. A stationary coil is in a magnetic field that is changing with time. Does the emf induced in the coil depend on the actual values of the magnetic field? 2. In Faraday’s experiments, what would be the advantage of using coils with many turns? 3. A copper ring and a wooden ring of the same dimensions are placed in magnetic fields so that there is the same change in magnetic flux through them. Compare the induced electric fields and currents in the rings. 4. Discuss the factors determining the induced emf in a closed loop of wire. 5. (a) Does the induced emf in a circuit depend on the resistance of the circuit? (b) Does the induced current depend on the resistance of the circuit? 6. How would changing the radius of loop D shown below affect its emf, assuming C and D are much closer together compared to their radii? 7. Can there be an induced emf in a circuit at an instant when the magnetic flux through the circuit is zero? 8. Does the induced emf always act to decrease the 616 Chapter 13 | Electromagnetic Induction This OpenStax book is available for free at http://cnx.org/content/col12074/1.3 magnetic flux through a circuit? 9. How would you position a flat loop of wire in a changing magnetic field so that there is no induced emf in the loop? 10. The normal to the plane of a single-turn conducting loop is directed at an angle θ to a spatially uniform magnetic field B → . It has a fixed area and orientation relative to the magnetic field. Show that the emf induced in the loop is given by ε = (dB/dt)( A cos θ), where A is the area of the loop.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
This drawing shows a bar magnet and a helical coil of wire to which an ammeter is connected. When there is no relative motion between the magnet and the coil, as in part a of the drawing, the ammeter reads zero, indicating that no current exists. However, when the magnet moves toward the coil, as in part b, a current I appears. As the magnet approaches, the magnetic field → B that it creates at the location of the coil becomes stronger and stronger, and it is this changing field that produces the current. When the magnet moves away from the coil, as in part c, a 694 CHAPTER 22 Electromagnetic Induction current is also produced, but with a reversed direction. Now the magnetic field at the coil becomes weaker as the magnet moves away. Once again it is the changing field that generates the current. A current would also be created in Interactive Figure 22.1 if the magnet were held stationary and the coil were moved, because the magnetic field at the coil would be changing as the coil approached or receded from the magnet. Only relative motion between the magnet and the coil is needed to generate a current; it does not matter which one moves. The current in the coil is called an induced current because it is brought about (or “induced”) by a changing magnetic field. Since a source of emf (electromotive force) is always needed to produce a current, the coil itself behaves as if it were a source of emf. This emf is known as an induced emf. Thus, a changing magnetic field induces an emf in the coil, and the emf leads to an induced current. THE PHYSICS OF . . . an automobile cruise control. Induced emf and induced current are frequently used in the cruise controls found in many cars. Interactive Figure 22.2 illustrates how a cruise control operates. Usually two magnets are mounted on opposite sides of the vehicle’s drive shaft, with a stationary sensing coil positioned nearby.
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