Period 17 Activity Solutions: Induction Motors and Transformers Activity 17.1: How Can Current Be Induced in a Closed Circuit? a) Connect the tan coil of wire to the large galvanometer that measures electric current. Move a magnet near and into the wire coil. Describe what happens. A current is induced in the wire when the magnet moves near to and into the coil of wire. The galvanometer measures this current. b) Hold the magnet still and move the coil of wire. Describe what happens. The galvanometer again measures current flowing through the wire. The direction of current flow depends on the direction the wire moves relative to the magnet. c) What happens if neither the magnet nor the wire is moving? No current flows. One (either the magnet or the wire) must be moving. Activity 17.2: How Do Generators Work? a) 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 a current. The crank spins a coil of wire relative to a magnet located inside the generator. This movement causes the wire to experience a changing magnetic field, which induces current in the wire. b) 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 c) Connect one hand-cranked generator to a second hand-cranked generator and make the second generator spin. How is this activity similar to a generating plant? How is it similar to a motor? 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. Activity 17.3: Is Induced Current Alternating or Direct Current? a) Move a magnet into and out of the small coil of wire with red and green bulbs attached. How must you move the magnet so that the red bulbs light and then the green bulbs light? When you move the magnet into the coil one color of bulb lights, and when you move the magnet out of the coil the other color of bulb lights. The lights are designed to respond to current moving in one direction only. Thus, the red lights go on when the current flows in one direction, and the green lights go on when the current flows in the opposite direction. 65
b) Is the current that you induce as you move the magnet in and out of the coil direct current (DC) or alternating current (AC)? _AC_ How do you know? When you change direction of the motion of the magnet, you create a changing magnetic field that induces a changing (alternating) current. c) Connect the hand-cranked generator to the coil with red and green bulbs and turn the crank. Are you generating AC or DC current? If you turn the crank in one direction to light only one color of bulb, you have generated DC current. Turning the crank in alternating directions to light both colors of bulbs generates AC current. Activity 17.4: What Is Induced Magnetism? a) Your instructor will demonstrate a pendulum, which swings between the poles of a large magnet. On the end of the pendulum are discs of various shapes. 1) Which shape of disc causes the pendulum to stop abruptly? The solid disc. 2) Which shape of disc permits the pendulum to swing freely? The disc with a slit. b) Hold the long aluminum tube upright with a foam pad on the floor beneath it. Drop one blue slug down the tube. Now drop the other blue slug down the tube. 1) What is the difference between the two blue slugs? One of the slugs is a magnet. 2) Explain what you observed using the principles of induced magnetism. The falling magnet creates a changing magnetic field, which induces a changing current in the pipe. This current induces a second magnetic field inside the pipe. The two magnetic fields exert forces on each other, slowing the magnet s fall. c) Using your observations from parts a) and b), what can you conclude about the force between the induced magnetism and the magnet that induces it? The swinging pendulum stopped and the magnet falling through the tube slowed down, indicating that the force between the magnet and the induced magnetism opposes the motion of the moving object (the pendulum disc or falling magnet). d) Your instructor will demonstrate a large solenoid, which is connected to a variable current source. 1) What happens when a small light bulb is placed near the solenoid? The bulb lights. The alternating current through the solenoid creates a changing magnetic field around the solenoid. The changing magnetic field induces a current in any nearby conductor. The induced current flows through the bulb. 2) What happens when a solid ring is placed over the solenoid? The ring levitates (jumps up). The changing magnetic field created by the solenoid induces a changing current in the ring. This current creates a magnetic field around the ring. The two magnetic fields repel, causing the ring to jump. 66
3) What happens when a ring with a slit is placed over the solenoid? Nothing happens. The ring does not jump because the slit in the ring creates an open circuit. Current cannot flow around the ring, so the ring has no induced magnetic field around it. 4) Move a solid ring and a slit ring over the end of a large U-shaped magnet. What do you feel when you move the rings? As you move the solid ring over the end of the magnet, you feel a force. You feel no force when the slit ring moves over the end of the magnet. 5) Why is there a difference between the solid ring and the slit ring? The solid ring has an induced magnetic field around it. The slit ring has no current flowing, so no magnetic field is induced. Activity 17.5: What Is an Induction Motor? Your instructor will demonstrate several induction motors. a) What happens when your instructor holds a disc and a shield near the large solenoid? The disc rotates. The changing magnetic field created by the solenoid induces a current in the disc. The induced current flowing through the disc induces a magnetic field around the disc. The two magnetic fields interact and cause the disc to rotate. The shield covers part of the solenoid and produces a nonuniform magnetic field. b) Examine the small black induction motor on your table. Why doesn t this motor need a permanent magnet? Induction motors use magnetic fields from two electromagnets. One of the electromagnets is in the circular shell of the motor and a second electromagnet is induced around the rotor. c) Explain what causes the rotor in an induction motor to turn. Induction motors use a rotor made of a conducting, nonmagnetic material. The rotor is surrounded by a stationary electromagnet. The changing magnetic field from this electromagnet induces a current in the rotor. The induced current in the rotor induces a magnetic field around the rotor. The rotor s magnetic field interacts with the stationary magnetic field and the rotor spins. d) A watt hour meter is an example of an induction motor. How is the watt hour meter similar to the spinning disc your instructor held near the large solenoid? Current flowing through wires in the meter induces a changing magnetic field. This changing magnetic field induces a current in the meter s disc. The current induces a magnetic field around the disc, which interacts with the first magnetic field, causing the disc to spin. Activity 17.6: How Do Transformers Work? a) Your instructor will discuss transformers and the relationship between the number of turns of wire and the voltage of a transformer s secondary coil. 67
Describe the relationship between the number of turns of wire and the voltage in the transformer coil. The ratio of the number of turns in the secondary to the number of turns in the primary equals the ratio of the output voltage from the secondary to the input voltage to the primary. b) Make a transformer with the large coil of wire. Connect the coil to the power strip. Loop a piece of wire through the center of the coil several times. (Caution: Do not let the ends of the wire you loop touch one another.) Attach the ends of the wire to a 4 bulb tray. 1) Note the brightness of the bulbs. 2) Wrap more turns of wire around the coil. What happens to the bulb brightness? The brightness of the bulbs increases. The more turns of wire, the larger the voltage and the brighter the bulbs. 3) What happens to the bulbs when you use fewer turns of wire? the bulbs become dim or may go out 4) How many turns of wire are in the large coil? Loop the wire through the coil several times and attach the ends of the wire to a digital multimeter. Set the meter to AC voltage and read the output voltage, V s. Assume that the input voltage, V p, is 120 volts. Calculate the number of turns of wire in the large coil. Solve equation 17.1 for N p by multiplying both sides of the equation by N s and canceling: N p = V p N s V s 5) Wrap double the number of loops of wire around the coil that you used in activity 4) above. How does the output voltage change? Make a prediction and then measure the voltage V s using a multimeter. Prediction: Measurement: 6) How does a current in one coil of wire in a transformer induce a current in a second coil of wire? The changing current through the input coil of wire creates a changing magnetic field around that coil. This changing magnetic field induces a current in the output coil. 7) How is a transformer like a simple machine? Simple machines trade force for distance or distance for force, but keep the power the same (except for some energy wasted due to friction). Transformers trade voltage for current or current for voltage, but keep the power the same (except for some waste as joule heating). c) Your instructor will demonstrate large transformers and the high voltage they can produce with a Jacob s ladder. What evidence do you see that a large voltage exists between the two ladders? 68
The big sparks that jump between the wires indicate a large voltage. Since dry air has high resistance, a large voltage (25,000 volts in this case) is needed to cause current to jump between the wires. Activity 17.7: Superconductivity and Induced Magnetism 1) Your instructor will give you a small magnet a piece of superconducting material attached to an inverted cup. Very carefully pour small amounts of liquid nitrogen on the superconductor to cool it. (Caution: Liquid nitrogen can quickly freeze your skin.) Hold the small magnet above the cold disc with tweezers and release the magnet. What happens? The magnet floats above the disc. 2) What force holds the small magnet above the superconducting disc? The repulsive magnetic force between the magnet and the magnetic field around the disc. 3) How does the magnet induce a current in the superconducting disc? When the magnet is moved into place above the disc, its motion creates a changing magnetic field that induces a current in the disc. The current flowing in the disc induces a magnetic field around the disc. The magnet floats because it is repelled by the magnetic field around the disc. 4) Why is a superconductor needed for this activity? When the disc is very cold, its resistance is zero, and current easily flows through it. The small amount of current induced by the magnet would die out if the material had any resistance. As long as the disc is kept very cold, the disc continues to be a superconductor and current flows, producing the magnetic field that supports the magnet. 69