Physics 241 Lab: Motorshttp://bohr.physics.arizona.edu/~leone/ua_spring_2009/phys241lab.htmlName:____________________________“First Fig”My candle burns at both ends;It will not last the night;But ah, my foes and oh, my friends – It gives a lovely light. -Edna St. Vincent Millayfrom “The Retreat”Happy those early days! when IShined in my angel-infancy.Before I understood this placeAppointed for my second race,Or taught my soul to fancy oughtBut a white, celestial thought,-Henry VaughanImportant:- In this course, every student has an equal opportunity to learn and succeed.- How smart you are at physics depends on how hard you work. Work problems daily.- Form study groups and meet as often as possible.. - Join professional organizations. - Physicists help people: science => technology => jobs.Section 1:1.1. A magnet has a strong magnetic field near its surface, but this field becomes very weak away fromthe magnet surface.When a current carrying wire of length L passes through a magnetic field, the magnetic field provides aforce 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 productindicates 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 getthe 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 wrongdirection). You can use this simple concept to make the real motor. The motor is made of current carryingwire loops that can rotate. The part of the wire loop that passes through the magnetic field experiencesa magnetic force € r F fromB−field= I ⋅r L ×r B upon it. This magnetic force causes a net torque on the wire loopyielding angular acceleration. The wire loop will therefore rotate faster and faster until the torque fromthe magnetic force equals any frictional torque in the motor. At this time, the loop will reach aconstant angular speed w.In this picture, the magnetic force pushes the segment of the loop out of the page.How could you make a more powerful motor using an extra amount of wire? Your answer:How could you make a more powerful motor using an extra magnet? Provide a small sketch with youranswer. Your answer and sketch:1.2. The only difficulty in making a motor is to ensure that the current always travels in the samedirection as it passes through the magnetic field no matter how the loop itself is oriented. Examine thefollowing picture to better understand what this difficulty is if the current supplied to the wire loop isalways 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 nowpush the loop in the opposite direction because the direction of the current through the magnetic fieldwill be reversed. Unless you want to make a fancy electronic rocking chair, this is not good motordesign. Instead, you will need to design your motor so that the current always flows in the samedirection for the part of the wire loop inside the magnetic field as is shown in the next picture:In this motor set-up (previous picture), when the wire loop rotates 180o, the manner in whichcurrent is supplied to the loop is changed in order to get the current to flow in the same direction forthe part of the loop experiencing the magnetic force.1.3. The figure below shows three possible arrangements for a current carrying wire to passbetween two magnets. For each case, use the Lorentz force equation to compare the resulting force onthe segment of current carrying wire (relative magnitude and direction). You are not provided anumerical value of B so you need to give a qualitative description of the strength of the force. Apicture of an actual current carrying wire in a magnetic field is also provided to motivate your solution.Your answer:1.4. A generator is a motor in reverse. Instead of taking electrical power from a current and turningit into mechanical energy (the rotation of the motor), a generator takes mechanical energy and turns itinto electrical power by providing a current. Imagine taking a motor and disconnecting it from its DCpower supply and connecting it to a light bulb. If you motor had a hand-crank, then you could providethe mechanical energy to create the current in the light bulb. In today’s lab, what would you see on theoscilloscope screen if you connected your motor to one of the oscilloscope channels and turned themotor with your fingers? Your quick sketch:1.5. The following four sections provide information that is especially useful in completing the lab.1.5.1. Power & EfficiencyAt some time, the rotation rate w reaches an equilibrium where the opposing torques balancethe torque caused by the magnetic force, € r T fromB−field=r T opposing∑. These opposing torques come fromfriction and any load you place on the motor (work you make it do). In this experiment, it will bedifficult to directly analyze the magnetic force being applied to the motor so that the force-torqueperspective 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 bemeasured in the lab as € Pinputpower= IthroughmotorVacrossmotor and the output power is equal to thework energy that the motor performs per second. However, the motor will not always be able to make contact through the wire brushes so thatcurrent 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 veryuseful 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 thepower supply readout. A much better value for the current may be obtained by placing the motor inseries with a 1 W resistor and using the oscilloscope to
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