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Unformatted text preview: Phy 1033 Discovering Physics Laboratory #6 Magnetic Fields Objective This experiment has 2 parts: (1) you will map the magnetic fields around a bar magnet and a current-carrying coil, and (2) you will measure induced currents using a bar magnet and a galvinometer. Introduction All magnetic fields arise from the motion of electric charges. Steady magnetic fields like those we will study in this experiment are created by steady currents of charged particles. DC electrical circuits, charged particle beams, and even the electrons circulating around a nucleus create steady magnetic fields. Magnetic fields, like electric fields, are vector fields. They have a direction and strength that can vary at different points in space. The SI unit of magnetic field strength is the tesla (T). The units of gauss are also often used: 1T = 10 4 gauss. A long, straight, current-carrying wire creates a magnetic field like that shown in Fig. 1. The lines are called field lines or lines of ux. Here, they are circles. Their direction of circulation is given by right-hand rule #2. That is, with the thumb pointing in the direction of the current the field lines circulate in the direction of the curled fingers. The direction of the field lines at any point gives the direction of the field. The field is strongest near the wire and decreases as one moves further away. The strength of the field is normally demonstrated by the density of the lines. In stronger field regions the lines are more closely packed together. In an accurate field- line illustration the strength of the field is proportional to the number of lines crossing a unit area oriented perpendicularly to the lines. If one could isolate a short segment of current-carrying wire, the field would appear as shown in Fig. 2. Of course, since currents must always ow in a complete circuit, such a segment could not exist in isolation. However, any real current can be made by stringing a large number of such segments end-to-end, and the magnetic field at any point would be the sum of the fields due to each segment. The direction of the field from each segment may be Figure 1: The field lines for a long, straight, current-carrying wire (thick line). Viewed from above, the field lines (thin) circulate in a counterclockwise direction. Only a few of the arrows are shown. different, and of course the sum must be performed vectorially. The precise mathematical description of such calculations is given by the law of Biot and Savart. For this experiment, we will be concerned only with approximate determinations of the field. Note that the field lines dont start anywhere; they circulate around their sourcecurrent elements. This behavior is different from that of electric field lines which originate and terminate on their sourcecharged particles....
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