Period 16 Activity Sheet Solutions: Motors

Transcription

1 Period 16 Activity Sheet Solutions: Motors 16.1 How Do Direct Current (DC) Electric Motors Work? a) Spinning rotors. In this activity, we see why the rotor of an electric motor spins. 1) Place a permanent magnet on a plastic spinner and make the magnet spin by holding another magnet nearby. The magnets simulate a motor. Could you make a practical motor using only permanent magnets? Explain why or why not. It would not be practical. A changing magnetic field is required to spin the rotor. To create a changing field using only permanent magnets, one magnet must continuously be moved to keep the second magnet spinning. 2) Place a solenoid near the magnet on the spinner. Make the magnet spin by alternately connecting and disconnecting the solenoid from a 3 battery tray. Could you make a practical motor using electromagnets (like the solenoid) and a continuous, unchanging current? Explain why or why not. No, a motor requires a changing magnetic field. A continuous current, such as the direct current from a battery, produces an unchanging magnetic field. 3) What type of current is required to make a motor run? _a changing current_ Activity 16.2: How Can You Make a Simple Motor? a) Building the motor. Refer to the model on your table. 1) Cut a 3 meter length of coated wire. 2) Wrap the wire into a circle 3 to 4 cm in diameter, leaving about 4 cm of wire protruding from each side of the circle. 3) Use sandpaper to carefully scrape the coating off of one side of one end of the protruding wire. Scrape all of the coating off of the other end of the wire. 4) Place the wire circle on a paper clip support stapled to a wooden block. 5) Use connecting wires to attach the positive end of a 3 battery tray to one side of the metal support and the negative end to the other side of the support. 6) Hold a strong magnet near the coil of wire and start the coil spinning with your finger. b) How does the motor work? Wire Coil of wire Coating End view of wire with coating scraped from one side of one end. 1) Why must you scrape the coating off of the ends of the wire? Why do you scrape it from only one side of one of the ends? 61

2 Scraping coating off of the wire is necessary to make a conducting pathway. If there is coating on only one side of one end of the wire, the conducting pathway is connected and disconnected as the wire spins, turning the current on and off. 2) What provides a changing magnetic field in this motor? Current flowing through the coil of wire during one half of the turn creates a changing magnetic field that is attracted the permanent magnet. During the second half of the turn, when the side of the wire with insulation is touching the support, the current is off and the coil of wire has no magnetic field. 3) What keeps the wire coil spinning? A strong magnet held near the coil of wire provides an unchanging magnetic field. When the coil of wire is spinning, a magnetic field is induced in the coil by the current flowing through it. The insulation scraped from one side of one wire causes the current, and thus the magnetic field, to turn on and off at the appropriate times to keep the coil spinning. Activity 16.3: How Does the St. Louis Motor Work? St. Louis motor: Connect the St. Louis motor to a 3-battery tray. 1) Does the St. Louis motor run better when the like poles (both north poles or both south poles) or the unlike poles (one north and one south pole) of its two magnets are oriented in the same direction? The motor works better when the unlike poles are oriented in the same direction, for example, the north end of one magnet across from the south end of the other magnet. 2) Remove one permanent magnet and adjust the remaining magnet until the motor runs. Does the rotor turn more rapidly using one or two permanent magnets? two magnets 3) What makes the St. Louis motor s rotor move initially? The magnetic force between the rotor coils and the permanent magnets causes the rotor to spin until it is in the position where it is most strongly attracted to the permanent magnets. 4) What makes the rotor continue to spin? Why doesn t the rotor turn until its poles are aligned with the opposite poles of the permanent magnets and then stop? The rotor spins toward the position where it is most strongly attracted to the permanent magnet. However, its spinning motion causes it to rotate a little past the point of greatest attraction. After it passes the first permanent magnet, the changing current in the rotor causes its magnetic field to reverse. Now the rotor is attracted 62

3 to the other permanent magnet, and it rotates toward it. After the rotor has spun a little past the second magnet, the magnetic field reverses once again. The rotor continues to spin toward the first permanent magnet. 5) We have found that a changing current is necessary to make a rotor spin. The St. Louis motor is connected to an unchanging direct current source. What causes a changing current in a direct current motor? a commutator_ 16.4 Do Alternating Current Motors Need Commutators? a) Synchronous motor: Connect a solenoid to the screws on the base of a stepdown transformer. Plug the transformer in the power strip. Hold a magnaprobe (a permanent magnet that spins and is attached to a handle) vertically at one end of the solenoid s metal core. Use your finger to start the mangnaprobe spinning. Explain why the magnaprobe continues to spin. The alternating current (AC) from the power strip reverses direction 120 times per second. This changing current flows through the transformer where its voltage is stepped down to a safe level and then flows into the solenoid. The changing current induces a changing magnetic field in the solenoid. The changing magnetic field alternately attracts and repels the ends of the permanent magnet in the magnaprobe. Because the magnaprobe spins in synch with the reversals in direction of the AC current, this is called a synchronous motor. b) Universal motor are AC motors with commutators: Your instructor will demonstrate a motor that operates on alternating current, but uses a commutator. Explain how this motor differs from a DC motor, such as the St. Louis motor, and from a synchronous AC motor. The synchronous motor in part a) provided only one rotational speed. Some AC motors, such as the mixer, use a commutator to provide varying rotational speeds. The mixer motor uses an electromagnet to produce the magnetic field in which the rotor turns. An AC motor constructed in this way will work on DC current as well. Such a motor is similar to the St. Louis motor with electromagnets instead of permanent magnets. Activity 16.5: Building a Buzzer a) Directions for Building the Buzzer Step 1 1) Cut off approximately 3 meters of coated copper wire from the wire spool. With a piece of sandpaper, remove the insulating coating from about 1 inch of one end of the wire. Be sure to remove all the insulation so that electrical contact can be made. 63

4 2) Let the end of the wire extend several inches beyond the edge of the wood strip that is farther from the nail. With the wire next to the wood, begin wrapping the wire around the nail, distributing the wire evenly along the length of the nail. 3) Wrap the wire until all but 4 inches of the wire have been used. Finish winding with the last turn of wire at the bottom of the nail. 4) Using sandpaper, remove the insulation from about 1 inch of the other end of the wire so that both ends have been stripped of insulation. 5) Which component of your buzzer did you just make? the wire wrapped around the nail is an electromagnet The non-magnetic wire lies above the paper clip. The magnetic material (paper clip) lies under the non-magnetic wire. Wire stripped of insulation and wrapped around the paper clip. Step 2 1) Bend the paper clip so that it looks like the buzzer model on your table. 2) Position the base of the paper clip on the board. Staple the base of the paper clip to the board so that the staple straddles the two smaller parts of the paper clip. You may need more than one staple. 3) Bend the nonmagnetic wire to look like the model. Position the wire on the board so that the loop is over the top of the nail. Make sure that the top end of the clip extends just slightly above the end of the paper clip. 4) Staple the wire to the board. Step 3 1) Take one end of the copper wire wrapped around the nail and wrap it several times around the base of the paper clip. Make sure the cleaned portion of the copper wire is in contact with the paper clip. After wrapping the wire, you may want to secure it to the board with a staple. 2) Attach one clip lead from a 3-battery tray to the far end of the nonmagnetic wire and the other clip lead to the free end of the coil of wire wrapped around the nail. 3) If adjusted properly, the buzzer should begin to buzz when the clip leads are connected. (You may need to adjust the position of the nonmagnetic wire relative to the nail head and the paper clip.) 64

5 b) How does the buzzer work? 1) What type of circuit results when the paper clip and wire are not in contact? An open circuit results due to the break in the conducting pathway. 2) Why is it necessary for the paper clip and the nonmagnetic wire to break contact? This turns the current on and off, creating a changing current that induces a changing magnetic field. 3) What causes a changing current in the buzzer? Current from the power supply flows through the wire wrapped around the nail and creates an electromagnet. The electromagnet s magnetic field attracts the paperclip down toward the nail head. As the clip moves toward the nail, it loses contact with the nonmagnetic wire to the left of the nail. When the paperclip and the wire are no longer touching, the circuit is broken and current stops flowing. 4) What causes the paper clip to move up and down? The paperclip makes a buzzing sound as vibrates up and down. When the circuit is broken and no current is flowing, the wire around the nail has no magnetic field and does not attract the paper clip. The paper clip springs back up until it touches the nonmagnetic wire again. Now the circuit is complete and current flows through the wire wrapped around the nail, making it into an electromagnet. The electromagnet pulls the paper clip down toward the nail and away from the nonmagnetic wire, breaking the circuit. This process repeats. 5) On the diagram below, identify the components of the buzzer. The non-magnetic wire is a conducting pathway. The motion of the paperclip opens and closes the circuit. The magnetic material (paper clip) is attracted to the nail when the nail becomes an electromagnet. Wire stripped of insulation provides an electrical connection to the paper clip. When current flows through the wire, the nail becomes an electromagnet 65