Download STEP Conference Activity

STEP Conference Activity
Bourns, Inc., Technology Team Leader: Gordon Bourns
Homopolar Motor
Exploring the properties of electricity and magnetism
Materials
Copper Wire
A. 1 Neodymium magnet
B. 1 piece copper wire
C. 1 washer
D. 1 nut
E. 1 AA battery
Nut
Battery
Magnet
Washer
Step 2
Step 3
Step 4
Step 6
Assembly:
1. Wrap plastic-coated magnet with aluminum foil (If not already wrapped).
2. Attach the washer to the magnet.
3. Attach the magnet to the negative (-) terminal of the battery.
4. Place the nut on the positive (+) terminal of the battery.
5. Shape the copper wire as illustrated in the Step 5 figure (if not already done).
6. Place the wire on top of the battery. Make sure that the wire is able to
rotate freely with minimal resistance while at the same time the two lower
sides of the wire are brushing against the bottom washer or the foil wrapped magnet.
Caution: Do not swallow magnets or other small parts.
How it works:
Step 5
This activity is called “Homopolar motor” because the electric current flows in the same direction on both
segments of the wire. This simple motor shows how moving charges inside the copper wire (electric
current) experience a force when they move through a magnetic field generated by the Neodymium
magnet.
When the wire is placed so that it connects the
“+” terminal of the battery to the “–“ terminal of
the battery (thus the circuit is closed) current
flows through the wire from + to – .
Observe in the picture on the right the shape
of the magnetic field generated by the magnet
attached at the bottom of the battery.
The lines of the magnetic field generated by the magnet form closed lines. (Note that in the picture not all
the lines are closed because of lack of space.) By convention, the arrows indicating the field direction are
taken to be outward from the North pole and in to the South pole of the magnet.
Because of the shape of the magnetic field, the negative charges moving inside the wire (the electrons)
experience a component of the magnetic field that is perpendicular to the direction of their movement along
the wire. This causes the charges to experience a force that is perpendicular to both the direction of the
current and the direction of the magnetic field (known as the Right Hand Rule). The force causes the wire
to rotate.
Note: only one connection to the magnet is sufficient for the motion to occur. Creating two connections
doubles the resulting force, creating a fasters spin.
To try: reverse the direction of the battery or reverse the direction of the magnet to see a change in the
direction of the rotation.
Warning: Because movement of charges is involved in this activity, note that both copper wire and battery
may heat up quite a bit!
Suggested Teaching Opportunities
Caution: Do not swallow magnets or other small parts.
Grade Two and Grade Three
Students learn that magnets can be used to make some objects move without being touched.
Grade Four to Grade Six
Students learn that electricity and magnetism are related and have many useful applications in everyday life.
They can explore the relationships between electricity and magnetism. Students learn about the relationship
between electricity and magnetism and how they interact regarding motion:
• Magnets moving relative to a conductor can produce electricity in the conductor.
• Electricity traveling through a conductor in a magnetic field can produce motion.
Students can learn how to make the motor spin the opposite direction.
Middle School
Students learn the physics of an electro-magnetic field interacting with the field of a permanent magnet, the
basis for all motors.
The Right Hand Rule can be demonstrated.
Students can consider ways to improve the performance of the motor.
Hint: Form the wire so that it touches both sides of the metal-covered magnet in order to keep it from
bouncing off of the magnet and breaking the electrical connection to the battery.
High School
Students evaluate the electro-mechanical system to determine opportunities to improve its performance.
Then they can collect test date to validate their assumptions and improve the performance of the system.
Students can study the interaction of the magnetic field of the permanent magnet and the electromagnetic
field generated by the current in the wire. Using the Right Hand Rule, they can use vector analysis to
determine mathematically why the wire rotates around the axis of the battery.
Additional information
To purchase the kit: Arbor Scientific – P/N P8-8350 – www.arborscientific.com
To purchase plastic coated magnets (for safety) – P/N D84PC-RB -- www.kjmagnetics.com
½” x ¼” Discs, N42, Red/Black Plastic
Batteries provided by Energizer
You Tube video for “Homopolar motor”: http://www.youtube.com/watch?v=3aPQqNt15-o
STEP Conference: myscienceeducation.com