Instructor Outline:
Motors and Generators Lab
UM Physics Demo Lab 07/2013
Lab length: 70 minutes
Lab objective: To instruct the students about magnetic forces on current-carrying
wires, motors, generators and Faraday induction.
Materials
Motor Board
Form for Rotor
Wire for Rotor
2 Ceramic Magnets (black)
Magnetic Compass
Battery Board
Alligator Leads
Multimeter
Needle Nose Pliers
Band Steel Wire Scraper
Wooden Block (backstop for scraping wires)
Vector force mode
Analysis - Motors: 10 minutes – Lecture
The lecturer explains how motors can be built to exploit the magnetic force on a currentcarrying wire, with emphasis on finding the direction of the magnetic force using the RHR.
The RHR for finding the direction of the magnetic force is reviewed and reinforced with
demos.
Exploration – Motors and Generators: 50 minutes – Group Lab Work
The students build motors according to the apparatus described in the Analysis stage. It takes
them a lot of time to build a well balanced motor, but their motors will work much better if
carefully built. They explore how to determine the direction of rotation by using the right hand
rule to determine the force on the wire and hence the torque about the rotor axis. The
students unplug their motors and observe a voltage across the posts when the motor is
rotating to demonstrate that motors can be used as generators. They double up the magnets,
and observe a higher voltage.
Analysis - Generators: 10 minutes – Lecture
The lecturer next develops the concept of a generator, emphasizing how a generator is the
inverse of a motor. The concepts of magnetic flux and Faraday induction are introduced.
Concepts Developed:
1. Motors use the magnetic force on a current-carrying wire to convert electrical energy
into mechanical energy.
2. Generators convert mechanical energy into electrical energy.
3. Electric motors also function as generators by converting mechanical energy (work
done to rotate the shaft) into electrical energy.
4. Magnetic flux through a surface is defined as the produce of the surface area and the
component of magnetic field perpendicular to the surface. Qualitatively this is
equivalent to the number of field lines passing through the loop.
5. If the magnetic flux through a coil of wire changes with time it induces a current in the
wire. This process is called Faraday induction and is the process by which all electrical
power is generated.
Suggested Demos:
5H40.11 - Right Hand Rule
5H40.71 - Rolling Rod in a Magnetic Field
5K10.21 – Electromagnetic Induction
5K40.83 – Bicycle Generator
Faraday Induction—Intro lab magnet and
pickup coil with oscilloscope
Faraday Flashlight
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