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Physics 241 Lab Motors Worksheet: 1. A magnet has a strong magnetic field near its surface, but this field becomes very weak away from the magnet surface. When a current carrying wire of length L passes through a magnetic field, the magnetic field provides a force on the wire given by ! r F fromB"field= I #r L $r B . The force on the wire here is out of the page as determined by the right hand rule.Note that L is written as a vector to describe the direction of the current, and the cross product indicates that the right hand rule must be used to find the direction of the force. We could also write ! r F fromB"field= L #r I $r B giving the direction information to the current variable I, instead of L and still get the same answer. But whatever you do, don’t switch the order of I with B! In other words, don’t use ! r F fromB"field# L $r B %r I or you will be off by a negative sign (your force will point 180o in the wrong direction). You can use this simple concept to make the real motor. The motor is made of current carrying wire loops that can rotate. The part of the wire loop that passes through the magnetic field experiences a magnetic force ! r F fromB"field= I #r L $r B upon it. This magnetic force causes a net torque on the wire loop yielding angular acceleration. The wire loop will therefore rotate faster and faster until the torque from the magnetic force equals any frictional torque in the motor. At this time, the loop will reach a constant angular speed ω. In this picture, the magnetic force pushes the segment of the loop out of the page. 1.a. How could you make a more powerful motor using an extra amount of wire? Your answer: 1.b. How could you make a more powerful motor using an extra magnet? Provide a small sketch with your answer. Your answer and sketch:The only difficulty in making a motor is to ensure that the current always travels in the same direction as it passes through the magnetic field no matter how the loop itself is oriented. Examine the following picture to better understand what this difficulty is if the current supplied to the wire loop is always the same for each lead of the wire loop: In this before-and-after picture, you can see that when the wire loop rotates 180o the force will now push the loop in the opposite direction because the direction of the current through the magnetic field will be reversed. Unless you want to make a fancy electronic rocking chair, this is not good motor design. Instead, you will need to design your motor so that the current always flows in the same direction for the part of the wire loop inside the magnetic field as is shown in the next picture: In this motor set-up, when the wire loop rotates 180o, the manner in which current is supplied to the loop is changed in order to get the current to flow in the same direction for the part of the loop experiencing the magnetic force.1.c. The figure below shows three possible arrangements for a current carrying wire to pass between two magnets. For each case, use the Lorentz force equation to compare the resulting force on the segment of current carrying wire (relative magnitude and direction). You are not provided a numerical value of B so you need to give a qualitative description of the strength of the force. A picture of an actual current carrying wire in a magnetic field is also provided to motivate your solution. Your answer: 1.d. A generator is a motor in reverse. Instead of taking electrical power from a current and turning it into mechanical energy (the rotation of the motor), a generator takes mechanical energy and turns it into electrical power by providing a current. Imagine taking a motor and disconnecting it from its DC power supply and connecting it to a light bulb. If you motor had a hand-crank, then you could provide the mechanical energy to create the current in the light bulb. In today’s lab, what would you see on the oscilloscope screen if you connected your motor to one of the oscilloscope channels and turned the motor with your fingers? Your sketch:1.e. The following four sections provide useful material that is especially useful in completing the open-ended section of the lab. Power & Efficiency At some time, the rotation rate ω reaches an equilibrium where the opposing torques balance the torque caused by the magnetic force, ! r T fromB"field=r T opposing#. These opposing torques come from friction and any load you place on the motor (work you make it do). In this experiment, it will be difficult to directly analyze the magnetic force being applied to the motor so that the force-torque perspective will not be useful. Therefore, it is better to examine the energy perspective of the motor. Specifically, ! Pinputpower= Poutputpower+ Pfriction where the input power can easily be measured in the lab as ! Pinputpower= IthroughmotorVacrossmotor and the output power is equal to the work energy that the motor performs per second. However, the motor will not always be able to make contact through the wire brushes so that current does not flow to the motor at all times. If the motor is only in contact with the source for 30% of the time, then ! Pinputpower= 0.3" IthroughmotorVacrossmotor. The oscilloscope in the lab is very useful in determining what percentage of the time current is flowing through the motor. In order to measure the current through the motor, a rather imprecise value is obtained from the power supply readout. A much better value for the current may be obtained by placing the motor in series with a 1 Ω resistor and using the oscilloscope to find the current through the resistor (and therefore the motor). The efficiency of the motor is given by ! "efficiency=PoutputpowerPinputpower. If the motor is made to lift objects vertically, then the output power can be found via the force of gravity: ! Eoutput= mgh so that Poutputpower="Eoutput"t= mg"h"t. Speed of Rotation The rotational speed of a motor is typically measured in revolutions per minute (RPM). You will need to use the oscilloscope to find the time of one full oscillation (the period), turn this into frequency, and finally into RPM. Use simple factor labeling with ! f cyclesseconds"60 seconds1 minute=60 " f cyclesminute=60 " f revolutionsminute= 60f RPMs. Warning about Motor Destruction The wire insulation melts at approximately 5 amps of intermittent current. Once the insulation melts, all the


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UA PHYS 241 - Motors

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