13.6 Applications of the Motor Principle

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1 4. When you look at the apparatus used to demonstrate the motor principle using a straight conductor (Figure 5), you can imagine that the suspended bare copper wire might act like a swing. What would you do to get the wire to swing back and forth with a regular period of vibration? (With your teacher s approval, you might be able to try your design.) Reflecting 5. The explanation of the definition of the ampere is included in this section that discusses the motor principle. Explain why this is logical pplications of the Motor Principle The motor principle refers to a force acting on a conductor carrying a current in a magnetic field. It is the most important principle used in the development of electric motors. However, the development of electric motors is not the only application of the motor principle. The motor principle has also been applied in the development of devices such as loudspeakers for stereos and in ammeters and voltmeters. The Moving-Coil Loudspeaker loudspeaker reproduces sound waves by rapidly moving a paper or plastic sound cone back and forth in response to electrical signals from an amplifier. Figure 1 shows side and front views of a magnetically driven speaker. movable voice coil (attached to speaker cone) ring pole speaker cone electric current in voice coil Figure 1 In a moving-coil loudspeaker, a movable coil is attached to the sound cone and placed over the central shaft of a tubular permanent magnet. The external magnetic field lines run radially from the outer tubular magnet to the central shaft. s a result, when electric current runs through the voice coil, it is in a magnetic field that is always perpendicular to it. central pole side view field lines of magnet end view (Field lines of the permanent magnet are always perpendicular to the current in the coil.) 494 Chapter 13

2 13.6 ccording to the motor principle, the movable coil will experience a force that is parallel to the axis of the coil, causing the sound cone to move. The magnitude and frequency of the force on the coil will depend on the amount and frequency of the current flowing through the voice coil. This will in part determine the loudness and frequency of the sound produced. The suspension mechanism holding the vibrating coil returns it to its original position when there is no current flowing through it. The Moving-Coil Galvanometer galvanometer is a delicate device used to measure the magnitude and direction of small electric currents. s shown in Figure 2, a movable coil is wound around a light frame that surrounds a fixed iron core. The iron core increases the magnitude of the magnetic field, causing a larger force on the movable coil. The coil is free to rotate when a current runs through it. The amount of rotation depends directly on the amount of current running through the coil. The direction of rotation depends on the direction of the electric current flowing through the coil. The amount of rotation is limited by an attached spring. The amount of current (or some other quantity) is then indicated by the attached needle and the calibrated scale. ccording to the motor principle, there will be a force on the movable coil when an electric current is flowing through it (Figure 3). The -pole of the coil will be attracted to the -pole of the permanent magnet and repelled by the - pole. This will cause the coil and the attached needle to turn. Using the motor principle, we can see that the front and back of the coil will not experience any force since they are parallel to the magnetic field lines, and the sides will experience opposite forces since the currents are opposite and perpendicular to the magnetic field lines. These opposite forces on the sides will cause the coil to turn. If zero is marked at the centre of the scale, the galvanometer will be able to measure current flowing in either direction. n amount of current that causes the pointer to move completely across the scale is called the full-scale deflection current, and it is usually just a few milliamperes. galvanometer must be protected from any current greater than its full-scale deflection current. To protect the galvanometer, a device called a resistor is connected with it to limit the current passing through it. For a voltmeter (Figure 4(a)), the galvanometer is connected in series with a high resistance (multiplier resistance). (a) 1.0 V (b) 1.0 V galvanometer: device used to measure the magnitude and direction of small electric currents coil soft iron cylinder permanent magnet zero adjuster control spring pointer counterbalance Figure 2 In a moving-coil galvanometer, the round iron core and the curved ends of the permanent magnet ensure that the magnetic field lines radiate through the core and stay perpendicular to the sides of the movable coil. wires perpendicular to magnetic field wires parallel to magnetic field 1.0 Ω l R g G V g low resistance l Figure 3 Wires perpendicular to the magnetic field experience a force causing the coil to turn. Wires parallel to the magnetic field experience no force. G R g high resistance l g l Ω G high resistance V G low resistance Figure 4 Electromagnetism 495

3 ince a voltmeter is connected in parallel, this will limit the amount of current flowing through the galvanometer. For an ammeter, a resistor of low resistance (a shunt resistance) is connected to the galvanometer in parallel (Figure 4(b)). The ammeter is connected in series, allowing it to measure large currents while allowing only a small current through the galvanometer. The Electric Motor s we saw with the moving-coil galvanometer, a current-carrying coil pivoted in a uniform magnetic field will begin to rotate. closer examination would reveal that the coil will rotate only until it is at right angles to the field, and then it will stop. For the coil to continue to rotate, the direction of the force on it would have to change every half rotation. This could happen only by changing the direction of either the external magnetic field or the current flowing through the coil. Figure 5 shows how it is possible to switch the direction of the current every half rotation. Y X Z W Figure 5 asic design of an electric motor In an electric motor, the ends of the coil are attached to a split copper ring, or commutator, that rotates with the coil. Continuous contact with the commutator is made by two stationary graphite brushes that push gently against the rotating commutator. The brushes are connected to a battery. Electric current enters the coil through one brush and leaves through the other. rheostat is used to vary the current in the circuit. upward force on YZ according to motor principle field magnet brush Y Z X W cell downward force on XW according to motor principle commutator rheostat rheostat: device in an electric circuit that can be adjusted to different resistances, changing the current in the circuit ccording to the right-hand rule for the motor principle, when electric current flows through the circuit, side YZ of the coil experiences an upward force and side XW experiences a downward force, causing the coil to rotate in a clockwise direction, as illustrated. s the rotating coil reaches the vertical position, both brushes come opposite the gap between the commutator segments and no charge flows. However, the inertia of the coil keeps it rotating until the brushes make contact again, this time each with the other half of the ring. This causes the direction of the electric current through the coil to be reversed, so there is now a downward force on YZ, causing it to continue rotating in a clockwise direction. This switching procedure is repeated every half cycle as long as there is electric current in the brushes. Reversing the polarity of either the magnet or the battery will cause the coil to rotate in the opposite direction. Figure 6 shows the relative positions of the armature, coil, graphite brushes, and commutator at four positions during one cycle of a DC motor, with an iron armature and an external source connected. 496 Chapter 13

4 13.6 (a) armature graphite brush commutator (b) field magnet graphite brush coil (c) (d) PHY11_U5_F13.7.5a Figure 6 Design and operation of an electric motor In Figure 6(a), electric current flows in through the bottom brush, into commutator segment, and through the coil, eventually entering commutator segment and leaving the motor through the top brush. End of the armature becomes an -pole, using the right-hand rule, and is repelled by the - pole of the field magnet, causing it to move away and rotate clockwise. In Figure 6(b), tracing the path of electric current through the motor verifies that end remains an -pole and is, therefore, attracted toward the -pole of the field magnet. In Figure 6(c), a significant change occurs. The top brush is now in contact with commutator segment. Electric current continues to flow up through the coils, leaving by commutator segment and the top brush. End of the armature now becomes an -pole and is repelled by the -pole of the field magnet, causing the clockwise motion to continue. gain, tracing the flow of electric current through the motor in Figure 6(d) confirms that end of the armature remains an -pole and is attracted toward the -pole of the field magnet, completing one full rotation of the motor. This simple electric motor is not very powerful or efficient. To increase its power an armature is used. The high relative magnetic permeability of the iron core and the large number of windings increases the magnetic field strength of the armature. These factors combine to produce a large force on the coil, causing it to rotate rapidly. strong electromagnet is often used as the field magnet. Practical electric motors have more coils connected to a multi-segmented commutator. Each coil is connected to two oppositely located commutator segments that allow current to flow through when the coil is perpendicular to the magnetic field. This will help to maximize the force on the armature, making a more powerful motor that doesn t require an initial push to get it going. Electromagnetism 497

5 (a) C The rate of rotation of such a motor is easily controlled by varying the current in the coils using a rheostat. mall motors controlled in this way are often used in battery-operated toys. ubway trains, street cars, and diesel electric locomotives use large-scale motors based on these principles. Practice (b) D D Understanding Concepts 1. tate the function of each of the following parts of a DC motor: (a) commutator (b) armature (c) brushes (d) field magnet 2. Figure 7 represents a single loop in a DC electric motor. Determine the direction of the forces on each part shown and the rotation of the loop. Explain two ways to reverse the rotation of the loop. C Figure 7 Figure 8 For question 3 3. For each of the diagrams in Figure 8, (a) name the parts of the motor that are labelled (b) determine which end of the coil is (c) state in which direction the coil will spin 4. Examine Figure 6 of this section, which shows the armature moving through one revolution. Explain in detail how the armature completes one revolution if the electric current is reversed. tape ctivity Constructing a imple DC Motor How can a simple DC motor be constructed? Figure 9 Wrap tape around this portion. These bare ends become the commutator. Figure 10 Materials insulated wire tape pencil pin 2 common iron nails (about 7 cm long) 6-V battery, or DC power supply block of wood cardboard Procedure 1. Using something round and about 3 or 4 cm in diameter, make a coil consisting of about 20 turns of wire. 2. Remove the coil and tape the loops together on one side. 3. plit the coil windings in half, and position the coil straddling a sharp pencil as shown in Figure 9. When the coil is positioned so that the pencil will rotate like a well-balanced wheel on an axle, tape the coils on the side opposite that in step 2 (Figure 10). 498 Chapter 13

6 trip the insulation from the wires at the ends of the coil and position them along and on opposite sides of the pencil, as shown in Figure 10. Tape them in position with the bare wires exposed. This completes the armature for the motor. 5. Tightly wrap a double layer of coils around each nail, leaving the bottom 1 cm free of windings, and ensuring that the ends of the wires are at the pointed end of the nails. 6. Obtain a small wooden block about the same length as the pencil used for the armature. Prepare two armature support brackets using pieces of cardboard cut to 10 cm 5 cm. 7. Mount all of the items as shown in Figure 11. The tops of the nails should be level with the pencil. pin may be inserted in the eraser end of the pencil to act as a pivot. The windings on the nails have not been shown; they must be connected so the nails have the opposite polarity. 8. Complete the electrical connections as shown in Figure 12. When the battery is connected and the armature is given a small push, the motor should rotate. Minor adjustments to the alignment of the commutator or the balance of the armature may be necessary. nalysis cardboard armature support 10 cm 5 cm Figure 11 battery pencil pin wooden block (a) Explain in detail how the motor works. Make reference to the electric current in the motor and the magnetic fields involved. Use diagrams to help with your explanations. (b) What improvements could be made to your motor to make it work better? Case tudy: Magnetic Resonance Imaging (MRI) Magnetic Resonance Imaging (MRI) is a diagnostic technique that produces high-quality images of the inside of the human body. In 1977, Dr. Raymond V. Damadian, an merican physician and scientist, developed the first MRI scanner as an improvement over X ray machines and other methods of diagnosing illnesses. Today, MRI is one of the most powerful tools doctors have for diagnosing illnesses; this life-saving technology enables them to see and study soft tissues of the body without the need for invasive exploratory surgery. The asics of Magnetic Resonance Imaging MRI machines are usually cube-shaped (Figure 13). patient is set inside the machine and around her is a large electromagnet that generates an extremely powerful magnetic field. The magnetic field can be used to build a three-dimensional image of part of the patient s body or to visually slice through a portion, making a two-dimensional image. The magnet is the most important part of the MRI machine. The magnet is so powerful that anything metallic will be drawn toward it. Objects with magnetic encoding, such as bank cards, must also be left outside the room or run the risk of being erased. People with certain medical conditions or equipment cannot go near an MRI machine. Different MRI machines use three different types of magnets. The first are resistive magnets, which consist of a coil through which electric current is passed. The second are permanent magnets, which always maintain their magnetic field, and the third are powerful superconducting magnets. These superconducting Figure 12 Magnetic Resonance Imaging: (MRI) diagnostic technique that produces highquality images of soft tissue inside the body Figure 13 n MRI machine pencil end view Electromagnetism 499

7 radio frequency coils Figure 14 Figure 15 MRI image of a normal human brain magnetic field coils magnets are similar in design to resistive magnets, but use a material that when cooled down far enough, will allow electricity to pass through without resistance. s a result they are very efficient and far less expensive to use. The superconductor is in the shape of a large coil. The large current in the superconductor makes a uniform magnetic field inside the coil. The patient is then moved inside the coil so that the area to be scanned is completely surrounded by the magnetic field (Figure 14). How the MRI Looks Into You The nuclei in atoms in the human body are just like the nuclei in anything else, and function under the same set of rules and properties. One of these nuclear properties is called spin, and this is the basis behind the idea of MRI. The human body has a large number of hydrogen atoms, each of which has a single proton in its nucleus and a large magnetic moment, meaning that the spins of the protons tend to line up with any strong magnetic field. Once the magnetic field of the MRI machine has caused the hydrogen nuclei in the body to line up, a radio frequency pulse is directed at the person and causes the affected nuclei to spin in a different direction than before. When the pulse is stopped, the nuclei slowly return to their normal direction of spin. This gives off energy that can be detected, and the information is directed to a computer. The computer converts this data into images (Figure 15). If the location of the problem is known, the radio frequency pulse can be localized to that area. Doctors can identify different types of problems by examining MRI images and comparing them with normal MRI images. ometimes a patient is injected with a certain kind of dye to help the doctor isolate different types of tissue under examination. This helps to identify abnormalities such as scar tissue and cancer. dvantages and Disadvantages MRI has some definite advantages over other types of imaging systems. It is good at detecting torn ligaments and muscles, abnormal growths, and infections, and is ideal for helping to diagnose tumours, cysts, multiple sclerosis, and conditions leading to strokes. It can image in any plane, allowing the doctor to see only what is needed. Unlike other imaging systems, the dyes do not use radiation. However, MRI has some drawbacks as well. ome people cannot be scanned, such as those with pacemakers or those who experience claustrophobia. The machine is very noisy, which can cause headaches and discomfort. n MRI machine is expensive, and maintaining it is costly. Practice Understanding Concepts 5. Explain the function of the following in producing an MRI image: (a) the superconducting coils (b) the magnetic field (c) the radio pulse (d) the hydrogen atoms in the body 500 Chapter 13

8 Discuss practical ways of increasing the strength of the magnetic field produced by an MRI machine. Why is putting a material inside the core of high relative magnetic permeability not an option? 7. construction worker had an accident in the past and a small piece of metal was lodged in his eye and was never removed. Why would a doctor not allow him near an MRI machine? Explore an Issue Medical Funding Today there is a great deal of interest in medical research and the treatment of illness. Much of the funding available goes to research. Research involves finding new and more effective ways to treat illness and detect it early, with the ultimate goal being to find a cure. ome of the funding goes to the treatment of the illness. These funds are used to buy new equipment (such as MRI machines), maintain this equipment, and pay doctors and other experts who make use of the equipment. One of the challenges faced by society today is deciding what percentage of the funds should be directed toward research and what percentage should go to treatment. DECIIO MKIG KILL Define the Issue Identify lternatives Research nalyze the Issue Defend the Proposition Evaluate Understanding the Issue 1. Why would it be beneficial to put a high percentage of the funding available into medical research? 2. Why would it be beneficial to put a high percentage of the funding available into medical treatment of illness? 3. List three possible impacts on society if funding was very low for (a) medical research (b) treatment of illness Use the Internet or any other resource to investigate the funding models for medical research and treatment. Concentrate on a particular illness and investigate the current reseach and treatment techniques for this illness. Follow the links for elson Physics 11, GO TO Take a tand Working in small groups, write a short report on the research and treatment of a particular illness. Discuss how you would distribute the funds between each of these areas, giving reasons for the distribution. UMMRY pplications of the Motor Principle pplications of the motor principle include the moving-coil loudspeaker, the moving-coil galvanometer, and the electric motor. Electromagnetism 501

9 Figure 16 For question 3 Figure 17 For question 4 ection 13.6 Questions Understanding Concepts 1. Go back to Figure 1 of this section, which shows a loudspeaker, and apply the right-hand rule for the motor principle to determine the direction of the force on the coil at the instant shown. 2. oth a voltmeter and an ammeter use a galvanometer as a basis of construction. How do these meters differ in their design and their connection in an electric circuit? 3. The conductors shown in Figure 16 represent a loop in a magnetic field. Determine whether the force on the loop is clockwise or counterclockwise. Use a diagram to explain your answer. 4. For the instant shown in Figure 17, is the force on the loop clockwise or counterclockwise? Explain your reasoning. 5. Describe two possible ways of forcing the loop in Figure 6 of this section to rotate counterclockwise. 6. (a) When would it be an advantage to use an electromagnet as a field magnet rather than a permanent field magnet? Under what conditions would you use a permanent magnet? (b) Why does the armature pass very close to the field magnet in a practical electric motor? (c) Why is it an advantage to have many coils and a multisegmented commutator in a practical motor? pplying Inquiry kills 7. Use a demonstration electric motor to learn firsthand how it operates. Look at the armature windings carefully to determine the direction of the electric current when the motor is connected to the power supply. Predict the direction of the armature rotation, and then check your prediction. Write a short report explaining how the motor works. 8. One type of galvanometer or milliammeter has the zero at the centre of the scale, while another type has the zero at the extreme left side of the scale. (a) If you were using these galvanometers in the lab, how would the precautions differ for the two different types? (b) What other precautions would be wise to follow? Making Connections 9. The motors described in this section are DC motors. However, C motors are also commonly available. Find out, through research or looking at a model, how the construction and operation of an C motor differs from that of a DC motor. With the aid of a diagram, describe the C motor s operation. Reflecting 10. If you know of anybody who has experienced an MRI scan, find out about the experience, and describe ways in which a patient could become mentally prepared for the scan. 502 Chapter 13