Period 15 Activity Solutions: Magnetic Forces and Electromagnets
Caution: Our class activities use strong magnets. While
these magnets are not dangerous to your health, they can
permanently damage objects with magnetic information.
Keep watches, credit and ID cards, calculators, and
computer discs away from the magnets on your table.
Activity 15.1: What Are the Properties of Magnets?
Experiment with the magnets, magnetic toys, compasses, and other materials on your table
to answer the questions.
a)
Are magnets always attracted to one another, or can they repel each other? Describe or
draw diagrams to show when two magnets attract and when they repel.
A magnet can attract or repel another magnet. When magnets are aligned so
that the south pole of one magnet is near the north pole of another magnet,
the magnets attract. When like poles of magnets are aligned (the north
poles or the south poles of two magnets), the magnets repel.
b)
Can a magnet attract any materials other than another magnet? List some materials that
are attracted to a magnet.
Magnets attract iron, steel (which contains iron), and nickel.
c)
Do you think a dollar bill is attracted to a magnet? Prediction: ______ Watch as your
instructor holds a dollar bill and gently brings a magnet near the bill. What happens?
If a strong magnet is held near a dollar bill, the bill moves slightly toward the
magnet because the ink on the bill has magnetic properties.
d)
If a magnet is broken into pieces, what happens to the pieces?
Each piece becomes a magnet with its own north and south pole.
Activity 15.2: How Do Magnetic Domains Store Information?
a)
Hang a paper clip from a permanent bar magnet. Touch another paper clip to the end of
the first clip. Now touch a third clip to the end of the second clip. How long a chain of
paper clips can you make? ____________ Explain what holds the clips together.
The paper clips are attracted to one another because the permanent magnet
temporarily aligns the magnetic domains in the clips, making them magnetic.
The permanent magnet induces magnetism in the metal clips.
b)
Your instructor will demonstrate a magnet placed against a cassette tape. Describe what
happens. Why does the magnet affect the tape?
When magnetic tape is recorded, the magnetic domains on the tape are aligned
in a pattern that stores information. When a magnet is placed against a section
of tape, the magnet realigns the domains and erases the info on the tape.
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Activity 15.3: Magnetic Forces and Magnetic Fields
a)
Magnetic Forces
1) Place two square magnets on a wooden dowel so that one magnet “floats” above the
other. What force(s) act on the floating magnet?
A repulsive magnetic force between the magnets pushes them apart. The
gravitational force acting on the top magnet pulls it down toward the bottom
magnet. The magnet “floats” when these forces are balanced.
2) What happens if you press the floating magnet down? Does the strength of the
magnetic force change as you move the magnets closer together?
As you push the floating magnet down, you feel the resistance of the
repulsive force between the magnets increasing as the top magnet nears the
bottom one. When you release the top magnet, it floats again.
3) How does the magnetic force depend on the distance between the magnets?
The magnetic force increases as the distance between the magnets
decreases.
4) If the floating magnetic has a mass of 5.0 grams, how large is the magnetic force that
holds it up?
The magnetic force holding it up equals the gravitational force pulling it down.
Gravitational Force = M g = 0.005 kg x 9.8 m/s2 = 0.049 newtons
b)
Magnetic Fields
1) Move a magnet on the 2-dimensional square filled with liquid and iron filings. Place a
cylindrical magnet inside the 3-dimensional shape with iron filings. Why do the filings
clump in certain places?
The filings arrange themselves in the shape of the magnetic field of the
permanent magnet. More filings clump where the magnetic field is stronger.
2) Draw a diagram of a magnet and its field.
3) Place the small compass magnaprobe on the model of the Earth. If like magnetic poles
repel and unlike poles attract, why does the north end of the magnaprobe point toward
the geographic North Pole of the globe?
The geographic North Pole is actually very close to the Earth’s magnetic
south pole. The geographic South Pole is very close to the Earth’s magnetic
north pole.
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Activity 15.4: How Are Magnetic Forces Related to Moving Charges?
a)
Magnetic Force on a Current
1) Place a wire between the ends of a large C shaped magnet. Briefly touch the ends of
the wire to both terminals of a 3 battery tray. What happens to the wire?
The wire will either jump into the “C” of the magnet or out of it, depending
on the direction of the current flowing through the wire.
2) Change the direction of the current flowing through the wire by switching the leads to
the battery. Describe what happens.
The wire jumps in the opposite direction.
3) Your instructor will demonstrate a metal swing placed near a large magnet.
happens when the swing is connected to a current source?
What
When moving charges from the battery flow through the metal swing, the
swing is either attracted to the magnet or repelled from it, depending on the
direction of the current flow.
b)
Magnetic Force on a Moving Charge
Cathode ray tubes (CRTs), used in computer screens and television tubes, shoot a
beam of electrons from the back of the tube to the screen. A dot of light appears
where the moving beam of electrons hits the screen. Locate the green dot on the
screen of the cathode ray tube. Now move a strong magnet near the screen. Explain
what happens. Caution: Do not try this at home! A strong magnet will permanently
damage a TV or computer screen.
The electron beam of moving charges is a current. This current of electrons
experiences a magnetic force when a strong magnet is held nearby. The
image on the screen is distorted as the beam of electrons is bent.
c)
A Current-Carrying Wire Induces a Magnetic Field
1) Press the black button on the board with a small compass connected to a battery.
Make sure that the board is level and no large magnets are close to it. Describe what
happens.
When current flows through the wire, the magnetic compass needle moves.
2) Your instructor will demonstrate a flicker light. What causes the light to flicker?
The flicker light has a permanent magnet in its center and a filament wire
that can move. There is a force on the current flowing through the filament
wire due to the permanent magnet. The alternating current flowing through
the wire reverses the direction of its flow, thus reversing the force on the
filament. The filament moves back and forth, giving the impression of
flickering.
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3) Your instructor will demonstrate the force between two parallel current-carrying wires.
a) Do the currents in the wires flow so that the wires attract or repel?
The wires move apart, indicating that current flows so that the magnetic
fields surrounding the wires repel one another.
b) If the direction of the currents in both wires were reversed, would the wires attract
or repel?
Since the direction of current in both wires is reversed, the wires still
repel.
c) If the direction of current in one wire was reversed, but the current in the other
wire was kept the same, would the wires attract or repel?
When the currents flow in opposite directions, the magnetic fields
surrounding the wires attract. (We do not demonstrate this because if
the wires moved too close together, the large currents in the wires could
cause sparking.)
4) In each of these examples, what is the source of the magnetic force? Explain why the
objects moved.
As current flows through a wire, a magnetic field is created around the
wire. This magnetic field experiences a force from the magnetic fields of
other nearby magnets. The forces cause the object to attract or repel.
d)
Solenoids
1) Place an iron rod partially inside the coil of a solenoid. Connect the solenoid to a 3battery tray. Describe what happens.
A solenoid consists of a coil of wire surrounding a metal core. When current
flows through the solenoid, a strong magnetic field is created around the
wire. The iron core is attracted to this field and drawn into the center of the
solenoid.
2) Repeat the activity using rods made of aluminum, brass, and steel.
Which rod produces the strongest magnetic field? _the steel rod__
3) Does a coil of wire or a single straight wire have a stronger magnetic field? _The coil
of wire has a stronger magnetic field._
5) Your instructor will demonstrate a large electromagnet capable of lifting several
hundred pounds. How can a battery make a magnet?
Current flowing from the battery through the wires creates a magnetic
field around the wire. If the wire is wrapped in a coil, each loop of wire
contributes to the magnetic field of the electromagnet. If an iron core is
placed inside the coil, the magnetic domains in the iron core align with
the magnetic field of the wire, creating a much stronger magnetic field.
Thus a strong electromagnet can be made with a small amount of current.
e)
Group Discussion Question: Some circuit breakers use a solenoid to open and close the
circuit. How could a solenoid act like a switch to open and close a circuit?
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