Investigating motors and magnetism.
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.
Context for Use
Resource Type: Activities:Lab Activity
Grade Level: High School (9-12)
Description and Teaching Materials
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
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.
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.
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.