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Physics Chap 13

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Physics Chap 13 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 reg- ular 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. 13.6 Applications 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 A 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. speaker electric current cone in voice coil movable voice ring coil (attached to pole speaker cone) N N SNNS Figure 1 N N In a moving-coil loudspeaker, a movable coil field lines is attached to the sound cone and placed central of magnet over the central shaft of a tubular permanent pole magnet. The external magnetic field lines run radially from the outer tubular magnet to the end view (Field lines of the permanent central shaft. As a result, when electric cur- side view magnet are always perpendicular rent runs through the voice coil, it is in a to the current in the coil.) magnetic field that is always perpendicular to it. 494 Chapter 13 13.6 According to the motor principle, the movable coil will experience a force galvanometer: device used to measure that is parallel to the axis of the coil, causing the sound cone to move. The mag- the magnitude and direction of small electric nitude and frequency of the force on the coil will depend on the amount and currents 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. zero adjuster The Moving-Coil Galvanometer A galvanometer is a delicate device used to measure the magnitude and direc- control tion of small electric currents. As shown in Figure 2, a movable coil is wound spring around a light frame that surrounds a fixed iron core. The iron core increases the coil magnitude of the magnetic field, causing a larger force on the movable coil. soft iron The coil is free to rotate when a current runs through it. The amount of rota- cylinder tion depends directly on the amount of current running through the coil. The pointer direction of rotation depends on the direction of the electric current flowing counter- through the coil. The amount of rotation is limited by an attached spring. The permanent balance amount of current (or some other quantity) is then indicated by the attached magnet needle and the calibrated scale. According to the motor principle, there will be a force on the movable coil Figure 2 when an electric current is flowing through it (Figure 3). The N-pole of the coil In a moving-coil galvanometer, the round iron core and the curved ends of the permanent will be attracted to the S-pole of the permanent magnet and repelled by the N- magnet ensure that the magnetic field lines pole. This will cause the coil and the attached needle to turn. Using the motor radiate through the core and stay perpendicular principle, we can see that the front and back of the coil will not experience any to the sides of the movable coil. force since they are parallel to the magnetic field lines, and the sides will experi- ence 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. An 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. A galvanometer must be pro- tected from any current greater than its full-scale deflection current. S To protect the galvanometer, a device called a resistor is connected with it to N limit the current passing through it. For a voltmeter (Figure 4(a)), the gal- vanometer is connected in series with a high resistance (multiplier resistance). wires parallel wires perpendicular to magnetic field (a) (b) 1.0 V 1.0 V to magnetic field Figure 3 l Wires perpendicular to the magnetic field 1.0 Ω low experience a force causing the coil to turn. R G V l g g resistance Wires parallel to the magnetic field experi- ence no force. lg l 3 G high resistance R g 1.0 Ω G V high resistance low GA resistance Figure 4 Electromagnetism 495 Since 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 As we saw with the moving-coil galvanometer, a current-carrying coil pivoted in a uniform magnetic field will begin to rotate. A 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 Basic design of an electric motor upward force on YZ downward force on XW In an electric motor, the ends of the coil are according to motor X according to motor attached to a split copper ring, or commu- principle Y principle tator, that rotates with the coil. Continuous S contact with the commutator is made by two W commutator stationary graphite brushes that push gently field magnet Z against the rotating commutator. The brushes N are connected to a battery. Electric current enters the coil through one brush and leaves brush through the other. A rheostat is used to vary rheostat the current in the circuit. cell rheostat: device in an electric circuit that According to the right-hand rule for the motor principle, when electric can be adjusted to different resistances, current flows through the circuit, side YZ of the coil experiences an upward changing the current in the circuit force and side XW experiences a downward force, causing the coil to rotate in a clockwise direction, as illustrated. As the rotating coil reaches the vertical posi- tion, 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 13.6 (a) graphite (b) armaturebrush commutator field magnet NSN NSN A A B B S S graphite brush coil (c) (d) NSN NSN B B A A S S 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 B, and through the coil, eventually entering commutator segment A and leaving the motor through the top brush. End A of the arma- ture becomes an N-pole, using the right-hand rule, and is repelled by the N- 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 veri- fies that end A remains an N-pole and is, therefore, attracted toward the S-pole of the field magnet. In Figure 6(c), a significant change occurs. The top brush is now in contact with commutator segment B. Electric current continues to flow up through the coils, leaving by commutator segment B and the top brush. End A of the arma- ture now becomes an S-pole and is repelled by the S-pole of the field magnet, causing the clockwise motion to continue.
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