Analyzing Simple Electric Motors in the ClassroomKeywords: electromagnetic, motor, modelingPACS codes: 01.50M, 41.01A, 85.90Abstract:Electromagnetic phenomena and devices such as motors are typically unfamiliar to both teachers and students. To better visualize and illustrate the abstract concepts (such as magnetic fields) underlying electricity and magnetism, we suggest that students construct and analyze the operation of a simply-constructed Johnson electric motor. In this paper, we describe a classroom activity that elicits student analysis to aid the comprehension and retention of electromagnetic interactions. (REF1)The motivation for teaching electricity and magnetism using experiential and hands-on activities is that by seeing and feeling the effects, students contextualize the information in reality and everyday experience (REF2; REF3). Magnetic field lines, electron flow, and cross products are abstract concepts and models that are often easier to present for memorization than to internalize in context. If students are kinesthetically supported in their learning by directly observing and touching concrete applications of concepts, the abstract and arcane becomes reality. Building and observing a simple motor will foster conceptual development and help students assimilate electromagnetism.(REF4) Students typically consider electric motors to be black boxes. Even individuals who have disassembled simple motors can have difficulty understanding the relationship between the wire coils, the magnets, and the electricity (REF5). Construction and operation of their own personal motor will not only empower the students, it will confirmthe reliability of the physical principles. If students with no previous motor expertise are able to construct functioning motors, then the concepts cannot possibly be too complex tounderstand. Instructional goals for this activity include the right hand rule and interactionof magnetic fields in devices.Materials for this activity are available from various vendors. The ceramic magnets shown are part number CB60 from Master Magnetics (http://www.magnetsource.com). Motor wire is widely available, and should be approximately 14-16 gauge, enamel coated, solid copper wire. Inexpensive wire may also be available (in shorter lengths) from a local motor winding factory. Standard D-Cell batteries and large paperclips are also necessary for this activity. D-Cell batteries are capable of supplying 5-8 amps of current during a short circuit, so please be careful.To construct a Johnson (REF6) motor, wrap insulated wire around a D cell battery forming a coil, extend the two wire ends outward from the loop for armatures, and selectively remove the insulation from the armatures. Current runs through the coil from paperclips connecting the two poles of the D cell battery to the armatures, and a magnet stuck to the side of the battery supplies a fixed magnetic field. To expedite the activity, 1/14/2019 1/8these coils can be pre-wrapped and prepared by a teacher or student assistant. There is a certain level of difficulty in winding multiple armatures. Perfect symmetry and weight balance are not essential, but the armature needs to be fairly balanced to function. While it is rewarding to see misshapen armatures spin, it requires a certain amount of practice and repetition to be able to create very reliable armatures. However, if students have sufficient time and guidance for building their own armatures, they are able to observe the construction process from start to finish, and may find complete construction more educational. Another solution to save time is to pre-fabricate all of the armatures, but manufacture one in front of the class.The students should be told to leave an interrupter (an area of intact insulation on one or both of the armatures) which will reduce the tendency of the armature to lock up in a specific position (REF7). This concept can be addressed later in the class discussion. Once basic armatures are fabricated and observed, variations include increasing or decreasing the number of loops in the coil, creating a bigger or smaller loop, and altering the shape of the coil. The benefit of these changes are to allow the students to make empirical observations, compare the behaviors with the standard motor, and begin to figure out how the different variables relate to each other. This is a precursor step to deriving equations based on their knowledge and experience. After initial observations and experiments have been made, a permanent fixture can be constructed or given to allow hands-free observations and manipulation.As a prerequisite activity to the Johnson motor, students should be made familiar withmagnetic field lines by having used compasses to map and sketch magnetic fields from permanent magnets. In a similar fashion, students should have mapped magnetic fields created by a current carrying wire to connect electrical current to magnetism. Using suspended iron fillings, a ferro-fluid, or several compasses, students will map the area surrounding a wire. A third pre-requisite hands-on activity is kinesthetically feeling the forces and resulting torques induced when two magnets are brought close together in various alignments. Following this activity, students should be given the opportunity to disassemble commercial motors and attempt to explain how they work. This can be used to confirm their understanding. (REF8, REF9)Activities and class procedure can proceed as follows: After the students assemble the motor apparatus, (Insert Figure 1) they make simple qualitative observations about the motor. If the teacher is intentionally non-specific regarding the orientation of the magnet, the contacts, the battery, and the armature, different students will have different orientations, which can lead to later discussion. The students describe the motion of the motor when spinning freely, then manually hold the motor at various positions and describe the force, or “push,” that they feel from the motor. A guiding question from the teacher can prompt the students to try reversing the magnet, the battery, the armature, or any combinations of these factors. Students can also try adding additional magnets or batteries in alignment with or against the original set. Each of these modifications will lead to additional qualitative observations that can be documented and used for reference when developing a working
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