Ch21_ListroR

Chapter 21: Electromagnetism toc

= Guiding Questions: = =** Magnetic Field Pre-Lab Questions: **= > The relationship between electric field strength and distance is an inverse cube. > The relationship will be an inverse cube because of the following equation, where B is the electric field strength and d is the distance. > > Using a magnetic field sensor, I will vary the distance of a set of magnets and the sensor. I will then record the data from the sensor and observe the relationship between the distance and field strength. > = = = **Motor Activity** =
 * 1) The objective is stated in the title. What is your hypothesis? (Attempt to answer the question, to the best of your knowledge.)
 * 1) What is the rationale for your hypothesis? (Provide detailed reasoning here. This may take the form of a list of what you already know about the topics, with a summary at the end.)
 * 1) How do you think you might test this hypothesis? (What might you measure and how?)
 * 1) Read the entire procedure through.
 * 2) Design __data table(s)__ in order to record your observations __and__ calculations. Do this in Excel and post this draft on your wiki.

Materials:Single D-cell battery, Two flat ceramic magnets, About 1 arms length of wire, Sand paper, Two paper clips, Tape

Procedure:
 * 1) To form the rotor coil, wind approximately 1.5 meters of magnet wire tightly around a test tube, film canister, or some other round object; the diameter of the coil should be between 2.5 to 5 cm. Leave about 7 cm of wire free at each end.
 * 2) Loop the ends of the wire around and through the rotor coil to hold it firmly together so it doesn’t unwind. It is critical that the two end wires are centered and bent parallel to each other across the coil.
 * 3) Completely remove the insulation from one end of the coil wire. To do this, lay the coil flat on the desk and rub the end with sandpaper, turning the wire to scrape off all insulation.
 * 4) At the other end of the coil wire, remove half the insulation.
 * 5) Tape a flat magnet to a D-cell. (To stabilize the D-cell, you may want to tape it to the desk or to an upside-down foam cup.)
 * 6) Bend the paper clips into hooks to support the rotor. Attach them to the D-cell with a rubber band.
 * 7) Place the coil in the paper clip supports. Spin the coil, and it should continue to spin by itself – a working motor! If it does not, proceed to the Troubleshooting Instructions.

media type="file" key="Movie on 2011-11-29 at 14.48.mov" width="300" height="300" > A galvanometer is an ammeter. When current passes through a metal coil in a device, the coil feels torque created by the force of the magnetic field. This then causes rotation at an angle that is proportional to the current. > Motor: device that converts electrical energy into mechanical energy. > Generator: device that converts mechanical energy into electrical energy. > My motor converts the electrical energy from the connected battery into mechanical energy, which is the movement of the coil. The movement is created by the interaction between the magnetic field of the loop of the wire and the magnetic field of the magnets. > If it was fully sanded, then the current would always be moving through the coil. If this happened, the coil would just move back and forth, rather than in a constant loop. When the non-sanded part of the rotor is touching the paper clips, there is no current through the coil and inertia allows the movement to continue. > In order for this to occur, something other than the current provided by the battery would have to turn the coil. One option would be wind. If the coil was then connected to a light bulb, the mechanical energy of the turning coil could be converted into electrical energy. > = **Magnetic Force Pre-Lab Questions** = Answers to these questions are to be written in your **group wiki** //BEFORE// coming to class on lab day. 1. The objective is stated as a question. What is your hypothesis? (Attempt to answer the question, to the best of your knowledge.) 2. How do you think you might test this hypothesis? (What might you measure and how?) Alter each part of the equation, one at a time. 3. Read the entire procedure through.
 * 1) How does a galvanometer work?
 * 1) Define motor and generator.
 * 1) A motor is a device which converts electrical energy into mechanical energy (motion). Explain how your motor does so.
 * 1) Why does the one rotor support have only ½ of its insulation sanded off?
 * 1) How could the motor you built in be converted to a generator? Describe carefully what would have to be changed and what the result would be.
 * Include the rationale for your hypothesis (Provide detailed reasoning here. This may take the form of a list of what you already know about the topics, with a summary at the end.)
 * The relationship between force and everything else in the equation will be directly proportional because of the equation F=BILsin(theta).

4. Design __data table(s)__ in order to record your observations __and__ calculations. Do this in Excel (preferable), and post a copy on your wiki.


 * F mag (N) || B (T) || I (A) || L (m) || Sin(Theta) ||

5. Answer the following questions:
 * How is the direction of the magnetic force oriented with respect to the directions of magnetic field and current which produced it?
 * It will be the direction of your palm in the right hand rule.
 * How do changes in the angle between the current and the magnetic field affect the force acting between them?
 * You will take the sin of the angle. Therefore, a 90 degree angle will give you the highest force, assuming that everything else is kept constant.
 * What angle between the current and the magnetic field produces the greatest force?
 * 90
 * What angle between the current and the magnetic field produces the least force?
 * 0
 * How is the magnitude of the force of magnetism related to the magnitude of the length of the wire carrying the current?
 * It is directly proportional.
 * A graph of force vs. current has a trendline with an equation of y = 0.00559x. What is the theoretical magnetic field strength of the magnet used in this experiment if the loop is 4.2-cm long? Show your work.
 * .00559/.042=.133 T
 * Find the magnetic force on the conducting loop described above, when the current is 0.352-A.
 * .00559*.352=.001974

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