Name ____________________________ Section _____________________
Period 17 Activity Sheet: Induction Motors and Transformers
Activity 17.1: What Is Induced Current?
a)
Inducing Current
1) Connect a solenoid to the large galvanometer that measures electric current. Move a
magnet near and into the solenoid. Describe what happens.
A current is induced in the wire when the magnet moves near to and into the
solenoid. The galvanometer measures this current.
2) Hold the magnet still and move the solenoid 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.
3) What happens if neither the magnet nor the solenoid wire is moving?
No current flows. One (either the magnet or the solenoid) must be moving.
4) 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?
Turning the generator handle spins a coil of wire near a magnet, which
induces a current in the coil of wire. A generating plant uses this principle by
spinning large coils of wire near large magnets.
A motor is the opposite of a generator. A generator induces a current by
moving a wire coil near a magnet (converts kinetic energy into electrical
energy). A motor induces a magnetic field around a wire caused by the
current flowing through the wire (converts electrical energy to kinetic
energy).
b)
Direct Current and Alternating Current
1) 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.
2) 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.
3)
Connect the hand-cranked generator to the coil with read and green bulbs and turn
the crank. Is the current you that induce DC or AC? _DC (if you turn the crank in
only one direction)__
How do you know?
Since only one color of bulb lights, the current you generate by turning the
crank in one direction must be DC. You can induce an AC current by turning
the crank in one direction and then in the opposite direction.
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Activity 17.2: What Is Induced Magnetism?
a)
Your instructor will demonstrate a large solenoid.
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 disc and a shield are held near the 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.
3) 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.
4) 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.
5) 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.
6) 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 around it, so no magnetic field is induced.
b)
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.
c)
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. The current induces a second magnetic field in
the pipe. The two magnetic fields exert forces on each other, slowing the
magnet’s fall.
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d)
Your instructor will demonstrate several induction motors.
1) 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.
2) Explain what causes the rotor in an induction motor to turn.
Some induction motors use a rotor made of a conducting, nonmagnetic
material. The rotor is surrounded by a circular metal shell, which contains an
electromagnet. The electromagnet induces a magnetic field that moves
around the shell. The rotating magnetic field in the shell 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 shell’s magnetic field
and the rotor spins.
3) 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 17.3: How Do Transformers Work?
a)
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. 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, Vs. Assume that the input voltage, Vp, 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:
Np
=
Vp Ns
Vs
5) How does a current in one coil of wire in a transformer induce a current in a second coil
of wire?
The current through one coil of wire creates a magnetic field around that
coil. The magnetic field induces a current in the other coil.
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6) 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).
7) 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”?
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 17.4: Superconductivity and Induced Magnetism
1) Your instructor will give you a small magnet and a cup with a piece of superconducting
material attached to the bottom. 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 disc.
3) How does the magnet induce a current in the superconducting disc?
When the magnet moves a little, it 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, which repels the magnet.
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 could not
flow though material that had much resistance. As long as the disc is kept
very cold, current flows and induces the magnetic field, which supports the
magnet.
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