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MIT 8 02 - Magnetic Fields of a Bar Magnet and Helmholtz Coil

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8.02 Spring 2009Experiment 3: Magnetic Fields of a Bar Magnet and Helmholtz CoilINTRODUCTION3. Magnetic Field SensorGENERALIZED PROCEDUREThis lab consists of three main parts. In each you will measure the magnetic field generated either by bar magnets or by current carrying coils.Part 2: Constructing a Magnetic Field DiagramA pair of bar magnets are placed so that either their opposite poles or same poles are facing each other and you will map out the field lines from these configurations.In this part you will use the magnetic field sensor to measure the amplitude of the magnetic field generated from three different geometries of current carrying wire loops.END OF PRE-LAB READINGIN-LAB ACTIVITIESEXPERIMENTAL SETUPNOTE: When working with bar magnets, please do NOT force a north pole to touch a north pole (or force south poles to touch), as this will demagnetize the magnets.MEASUREMENTSPart 1: Mapping Magnetic Field Lines Using Mini-CompassesPart 2: Constructing a Magnetic Field Diagram2A: Parallel Magnets2B: Anti-Parallel MagnetsPart 3: Helmholtz Coil3A: Using a Single Coil3B: Helmholtz Configuration3C: Anti-Helmholtz ConfigurationMASSACHUSETTS INSTITUTE OF TECHNOLOGY Department of Physics 8.02 Spring 2009 Experiment 3: Magnetic Fields of a Bar Magnet and Helmholtz Coil OBJECTIVES 1. To learn how to visualize magnetic field lines using compasses and a gauss meter 2. To examine the field lines from bar magnets and see how they add 3. To examine the field lines from a Helmholtz coil and understand the difference between using it in Helmholtz and anti-Helmholtz configurations. PRE-LAB READING INTRODUCTION In this lab we will measure magnetic field lines using two methods. First, we will use small compasses that show the direction, but not magnitude, of the local magnetic field. Next we will use a gauss meter, which measures the magnitude of the magnetic field along a single, specific axis and thus does not allow as easy a visualization of the magnetic field direction. We will measure fields both from bar magnets and from a Helmholtz coil. APPARATUS 1. Mini-Compass You will receive a bag of mini-compasses (Fig. 1a) that indicate the magnetic field direction by aligning with it, with the painted end of the compass needle pointing away from magnetic north (i.e. pointing in the direction of the magnetic field). Conveniently, the magnetic south pole of the Earth is very close to its geographic north pole, so compasses tend to point North (Fig. 1b). Note that these compasses are cheap (though not necessarily inexpensive) and sometimes either point in the direction opposite the way they should, or get completely stuck. Check them out before using them. (a) (b) Figure 1 (a) A mini-compass like the ones we will be using in this lab. (b) The painted end of the compass points north because it points towards magnetic south. E03-12. Science Workshop 750 Interface As always, we will use the Science Workshop 750 interface, this time for recording the magnetic field magnitude as measured by the magnetic field sensor (gauss meter). 3. Magnetic Field Sensor The magnetic field sensor measures the strength of the magnetic field pointing into one of two white dots painted at its measurement end (far left in Fig. 2). Selecting “radial” mode records the strength of the field pointing into the dot on the side of the device, while “axial” records the strength of the field pointing into the dot on the end. There is also a tare button which sets the current field strength to zero (i.e. measures relative to it). Figure 2 Magnetic field sensor, showing (from right to left) the range select switch, the tare button, and the radial/axial switch, which is set to radial. 4. Helmholtz Coil Consider the Helmholtz Coil Apparatus shown in Fig. 3. It consists of two coaxial coils separated by a distance equal to their common radii. The coil can be operated in 3 modes. In the first, connections are made only to one set of banana plugs, pushing current through only one of the coils. In the second, a connection is made between the black plug from one coil to the red plug from the other. This sends current the same direction through both coils and is called “Helmholtz Mode.” In the final configuration “Anti-Helmholtz Mode” a connection is made between the two black plugs, sending current in the opposite direction through the two coils. Figure 3 Helmholtz Coil Apparatus E03-24. Power Supply Because the Helmholtz coils require a fairly large current in order to create a measurable field, we are unable to use the output of the 750 to drive them. For this reason, we will use an EZ dc power supply (Fig. 4). This supply limits both the voltage and the current, putting out the largest voltage possible consistent with both settings. That is, if the output is open (no leads connected, so no current) then the voltage output is completely determined by the voltage setting. On the other hand, if the output is shorted (a wire is placed between the two output plugs) then the voltage is completely determined by the current setting (V = IRshort). Figure 4 Power Supply for Helmholtz Coil E03-3t flow). The power supply allows independent control of current (left knob) and voltage (right knob) with whichever limits the output the most in control. The green light next to the “CV” in this picture means that we are in “constant voltage” mode – the voltage setting is limiting the output (which makes sense since the output at the bottom right is not hooked up so there is currently no curren GENERALIZED PROCEDURE This lab consists of three main parts. In each you will measure the magnetic field generated either by bar magnets or by current carrying coils. Part 1: Mapping Magnetic Field Lines Using Mini-Compasses Using a compass you will follow a series of field lines originating near the north pole of a bar magnet. Part 2: Constructing a Magnetic Field Diagram A pair of bar magnets are placed so that either their opposite poles or same poles are facing each other and you will map out the field lines from these configurations. Part 3: Helmholtz Coil In this part you will use the magnetic field sensor to measure the amplitude of the magnetic field generated from three different geometries of current carrying wire loops. END OF PRE-LAB READINGIN-LAB ACTIVITIES EXPERIMENTAL SETUP 1. Download the LabView file and start up the program. 2.


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