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Lab 9 Magnetic Forces on Currents and Magnetic Induction

Lab 9 Magnetic Forces on Currents and Magnetic Induction -...

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Physics 1BL M AGNETIC F ORCES ON C URRENTS AND M AGNETIC I NDUCTION Spring 2009 ©UCSD-PERG (2005) Page 1 Introduction Magnetism and electricity are closely related. Moving charges make magnetic fields. Wires carrying electrical current in a part of space where there is a magnetic field experience a force. This is the principle behind the electric motor (e.g., the coffee grinder in your kitchen, your car’s starter motor, the fan motor and disc spinner in your PC, etc.) This interaction between moving charges and magnetic fields is an essential part of the process of transforming electrical energy into mechanical energy. Electrical conductors moved through a region of space where there are magnetic fields generate currents which are called ‘induced currents’ (in the sense of being caused, or induced , by the combination of the motion and the field). The electrical energy we consume each time we plug an appliance into an electrical outlet is generated by induced currents in conductors moving through magnetic fields at the power plant. This is part of the process of transforming mechanical energy into electrical energy. In this lab you will observe and measure forces on currents in magnetic fields, and make a prototype motor. You will then observe currents generated by moving conductors through magnetic fields and make a prototype electrical generator, which will light a light bulb. CAUTION: the magnets used in this lab are very strong (about 10,000 times the earth’s magnetic field). The strongest fields are concentrated between the magnets’ poles. Keep your credit cards etc. more than 3 feet away from the magnets. Do not place ferromagnetic materials near the magnets. If you have a heart pace maker, you should NOT do this lab, but see your TA for advice. There is otherwise no danger associated with strong magnetic fields. Pre-lab Questions: Please review chapters 19 and 20 of your textbook For these questions use the standard convention that a vector coming out of the plane of the page straight toward you is written as a circle with a dot in it, and a vector going into the plane of the page is written as a circle with an “X” in it. 1. In the arrangements shown, assume a positive electrical charge is moving with velocity v through a magnetic field B . Answer the following questions about the indicated Figure: Figure (a): Show the direction of the force Figure (b): Show the direction of the force Figure (c): Show the direction of the velocity Figure (d): Show the direction of the magnetic field. 2. Repeat each part of Question #1 for a negative charge.
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Physics 1BL M AGNETIC F ORCES ON C URRENTS AND M AGNETIC I NDUCTION Spring 2009 ©UCSD-PERG (2005) Page 2 3. Consider a wire loop that is (a) moving into, (b) through, and (c) out of a magnetic field in the dashed region that is directed into the page. At the instances of times shown below, explain whether the current that is induced in this wire loop will circulate clockwise, counterclockwise, or if there is no current induced at all.
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