MnSTEP Teaching Activity Collection > MnSTEP Activities > Investigating motors and magnetism.

Investigating motors and magnetism.

David A. Reierson, Roosevelt High School.
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Summary

Students will build a simple DC motor out of metal coat hangers, a 24-guage wire armature and field magnet, 14-guage wire brushes, and build the motor so that it rotates when connected to a 10-volt DC power supply. Students will understand the principles of operation of the DC motor, to include: induction of an electromagnetic field Via current flowing through a conductor (electromagnetism), and become familiar with the notion that Forcetotal of the motor is proportional to charge, proportional to speed, proportional to the induced magnetic field (B), and dependent on the angles between the rotation of the armature in the field magnetism. Forcetotal = qv X B = (mv2/R), where q is the amount of charge, v is velocity of charge, B is the magnetic field strength, m is the mass of the charge, and R is the radius of the armature loop. Students will be able to solve one variable magnetism problems, describe how their motors operate, and write up a lab report on their findings. During the lab report, students will discuss how they got their motors to rotate faster than the initial trial after building it.

Learning Goals

From above, students should be able to construct a motor that rotates, understand magnetism, and become familiar and be able to manipulate one variable problems using the magnetic force equation, Forcetotal = qv X B = (mv2/R). This activity is designed for students to: use higher order thinking skills (critical thinking, math use in science, data analysis, synthesis of ideas, and modeling) that are developed by the motor activity. The other skills would be (observations, field techniques, Logger-Pro technology operation, strobe light operation, questioning, and other skills and equipment operations). The concepts are Magnetic Force, magnetism, the right hand rule for magnetism, and using math to predict the motor operation and understand the theory of motor operation. The following is the format that might be considered when designing the motor lesson plan.

Context for Use

This activity could include students in grades 9-12. It would require some modification to meet the needs of all learners and their experience base in science and math. It requires some lecture, lab procedures, math skills demonstrations, and motor assembly modeling and demonstrations by the teacher. You should expect about one full week to develop all the concepts and building the motor. It would be encouraged to build the motor in steps (field magnet one day, armature one day, final assembly one day, troubleshooting one day, etc.). Students should be able to solve a one variable algebraic problem, understand that electrical current flows in a conductor, and have been introduced to Logger-Pro and the use of a magnetic pick-up probe. Additionally, students should have some experience with small hand tools required to build the DC motor.

Subject: Physics:Electricity & Magnetism
Resource Type: Activities:Lab Activity
Grade Level: High School (9-12)

Description and Teaching Materials

Concepts:
Background and history; principles of operation; motor vocabulary and measurements; common errors; and classroom assessments.

Background and history:
At the basic level, DC motors convert electrical energy into mechanical energy. This is accomplished by interacting with a stationary field magnet, a rotating armature magnet, and a commutator. The basic principles of motor operation (electromagnetic induction) date back to the early 1800's when scientists Michael Faraday, Joseph Henry, Hans Christian Oersted, Carl Friedrich Gauss, Andre Marie Ampere, and William Sturgeon, each introduced and compounded ideas that moved principles of electromagnetic induction to the simple rotating DC motor.

Principles of operation:
The DC motor in this demo works by simple electromagnetism. A DC power supply transfers DC current into the field magnet and armature which both create external magnetic fields. The field magnet creates a magnetic field with a fixed magnetic field, and the armature creates a field in which the magnetic poles alternate twice during each complete rotation. As the armature rotates, the commutator forces the polarities (north and south) of the magnetic poles to switch. Switching the magnetic field polarities causes the magnetic fields to attract and repel forcing the armature to rotate with respect to the fixed polarity field magnet.

Motor vocabulary and measurements:
Electromagnetism , dc motor, commutator, field magnet (stator), armature, torque, electromotive force (EMF), inductance, and revolutions/radians per minute (RPM).

Teaching Notes and Tips

Common errors: After students build the motor, most often the armatures do not rotate smoothly. Causes include poor commutator contact with the armature, one or both windings broken on the field magnet and armature, and/or geometric misalignment of the energized field magnet windings and armature windings not allowing a smooth "flip" of the rotor's magnetic field, therefore limiting rotation.

Assessment

Using a DC power supply and fixed output voltage, the armature spins freely for 15-seconds without aid, and quantified measurements using Logger-Pro with magnetic probes and frequency probes. High level assessments could include comparisons between theoretical torques and actual motor torques (stall torque, and measured torque) to discover the motor's optimal operating efficiency.

During the past motor activities, the Logger-Pro and magnetic probes ware not used to measure the magnetic field magnetism and frequency of the armature rotation. This tool provides students with a refreshing look at how technology is used to measure science in society.

During the activity, using a rubric to assess gives important feedback to both the individual student and the student group performing the activity. Having high expectation from the beginning encourages student motivation and expectations of the motor project. As of last year, 7-years after beginning this activity, 95% of the groups build a motor that works well enough to perform measurements using Logger-Pro. Compared to the first year of teaching motors, student groups roughly performed at about 25% success rate. This in mind, it is important for teachers pursuing this activity to become very skilled at building motors and understanding the operational physics of the motor before having students perform it.

Standards

History and nature of science:

The student will understand the nature of scientific ways of thinking and that scientific knowledge changes and accumulates over time.

Scientific enquiry: Apply mathematics and models to analyze data and support conclusions. Identify possible sources of error to analyze the motors and make them work better.

Scientific enterprise: Provide an example of a need or problem identified by science and solved by engineering or technology.

Historic perspectives: Be able to trace the development of a scientific advancement, invention or theory and its impact on science.

Physical science:

Energy Transformations: Differentiate between kinetic energy and potential energy and identify situations where kinetic energy is converted to potential energy and vice versa. Differentiate between AC and DC current.

Motion: Use Newton's three laws of motion to qualitatively describe the interaction of objects. Describe the effect of friction and gravity on the motion of the motor. Identify the forces in the interactions between the field magnet, and armature.

References and Resources

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