Physics 8B Lab 3 – Electric Motors & Generators rev 4.0 Lab 3 – Electric Motors & Generators Part 1: Applications of Torque on a Magnetic Dipole We have already seen that a magnetic dipole (e.g. a compass) will experience a torque that tends to line the dipole up with an external magnetic field. Here we consider two applications of that. 1) Galvanometer Place the cylindrical magnet attached to the white plastic axle above the large wooden framed coil of wire. The magnet is suspended so that it is free to swing above the coil. A pointer has been attached to the magnet so that a small swing of the magnet will result in a large deflection of the pointer. Attach the coil to the DC power supply. DC Power Supply: With the power supply OFF, set it up as follows • Readout switch should be set to read AMPS • The voltage dials should be turned all the way to maximum (CLOCKWISE). • The current dials should be turned all the way to minimum (COUNTER-CLOCKWISE) • The High/Low button should be OUT in the HIGH setting. When there is no current through the coil, the magnet is horizontal and the pointer is vertical. a) Predict the deflection of the pointer (if any) when the power supply is turned on. b) Turn on the power supply and slowly increase the current using the “ FINE ” current dial. How does your prediction compare to your observations?
Physics 8B Lab 3 – Electric Motors & Generators rev 4.0 c) What happens to the pivot as the current through the coil is varied? d) With the current at some setting, would you say the magnet is lined up with the coil’s magnetic field? If so, why does the pivot move when the current is varied? If not, why not? The device above is called a galvanometer and can be used to detect current. If the pointer moves along a scale that has been calibrated to measure amperes, the device is called an ammeter. 2) Electric Motor A small coil of wire is suspended above a magnet by two copper bars resting on support posts. One of the copper bars has a non-conductive coating on half of its surface. On one side of the plastic spool of the coil is an arrow. This arrow shows the direction current will flow around the coil when it comes in that side’s copper bar. DC Power Supply : With the power supply OFF, set it up as follows • Disconnect all wires from power supply, then turn supply ON • Turn current dials all the way CLOCKWISE to maximum. • The HIGH/Low button should be OUT in the HIGH position • Move digital readout switch to “Volts” and adjust voltage to 3.5 V. • Do NOT adjust voltage dial after this point.
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