Period 16 Activity Sheet: Motors and Generators

Transcription

1 Name Section Period 16 Activity Sheet: Motors and Generators Activity 16.1: How Are Electric Motors and Generators Related? a) Generators. 1) Attach a hand-cranked generator to a small motor and turn the crank. Explain what happens inside the generator when the crank turns to create an electric current. The crank spins a coil of wire relative to a magnet located inside the generator. This movement causes a changing magnetic field, which induces current in the wires. 2) List the energy conversions that take place when you crank the generator and make the motor s shaft turn. The chemical energy (of your body) kinetic energy of the moving coil electrical energy kinetic energy of the moving motor shaft b) Spinning rotors. In this activity, we see why the rotor of an electric motor spins. Place a permanent magnet on a plastic spinner and make the magnet spin by holding another magnet nearby. The magnets simulate a motor. 1) 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 move to keep the second magnet spinning. 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. 1) 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. 2) What type of current is required to make a motor run? _a changing current_ 3) How are electric motors and electric generators similar? How do they differ? Both motors and generators use changing magnetic fields. Motors convert electrical energy into kinetic energy, while generators convert kinetic energy into electrical energy. Generators use kinetic energy to spin coils of wire near magnets, creating a changing magnetic field. The changing field induces a current in the wires. Motors use a changing electric current to produce a changing magnetic field, which spins a rotor by attracting and repelling it. 61

2 16.2 How Do Direct Current (DC) Electric Motors Work? a) St. Louis motor: Connect the St. Louis motor to a DC power supply. 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 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_ b) Synchronous motor: Connect a solenoid to the screws on the base of a step down 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. 1) Explain why the magnaprobe continues to spin. The alternating current (AC) from the power strip reverses direction 60 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. The magnaprobe spins in synch with the reversals in direction of the AC current. Thus, this is called a synchronous motor. 62

3 Activity 16.3: 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 a 3 battery tray to each side of the metal 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? 1) What keeps the wire coil spinning? Coil of wire End view of wire with coating scraped from one side of one end. A strong magnet held near the coil of wire provides an unchanging magnetic field. When you push the coil of wire to start it spinning, a changing magnetic field is induced in the coil. These two magnetic fields exert forces on each other. The coil is alternately attracted to and repelled by the permanent magnet, causing it to spin. 2) What provides a changing magnetic field in this motor? A changing current flowing through the coil of wire creates a changing magnetic field that is alternately attracted to and repelled by the permanent magnet. 3) 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? Scraping the coating off of the wire is necessary to make a conducting pathway. If there is coating on only one side of the wire, the conducting pathway is connected and disconnected as the wire spins. The spinning wire acts like a commutator, turning the current on and off. The changing current creates a changing magnetic field. Activity 16.4: Building a Buzzer a) Components List the components a DC motor requires, such as a power source, connecting wires to the power source, etc. 1. a DC power source that provides a steady, unchanging current 2. a way to make the steady DC current into a changing current 3. one electromagnet and either a permanent magnet, material that is attracted to a magnet, such as iron or steel, or a second electromagnet 4. connecting wires 63

4 b) 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. 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 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 nonmagnetic wire. Make sure the cleaned portion of the copper wire is in contact with the nonmagnetic wire. After wrapping the wire, you may want to secure it to the board with a staple. 2) Attach one clip lead from a 6-volt battery or DC power source to the far end of the paper clip 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 c) 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 which 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, label the components of a DC motor that you listed in activity 16.4 (b). produces changing current as clip opens and closes the circuit magnetic material connecting wires electromagnet 6 V DC current source 65