Physics 241 Lab: Magnetism and Electrostaticshttp://bohr.physics.arizona.edu/~leone/ua/ua_spring_2010/phys241lab.htmlName:____________________________Section 1.1.1. Charged particles create electric fields that can push on other charged particles with an electricforce € r F electric= qr E . These electric fields are represented by drawing electric field lines that show thedirection of the electric force. Magnetic fields are more complex because they are created by movingcharges (currents). That may seem counterintuitive since a macroscopic magnet looks stationary, but amagnet is often modeled as a valence electron circling around each atom creating a tiny magnetic field.Though this description is incomplete and magnetism can only be explained using quantum mechanics,the basic idea is correct: only moving charges create magnetic fields. An electric field pushes on a charged particle in a direction parallel to the electric field, but a magneticfield pushes a moving charged particle in a direction perpendicular to the direction of the magneticfield. This magnetic force is described mathematically with € r F magnetic= qr v ×r B . The appearance of thevelocity of the charge in the force equation indicates that the force is proportional to the speed of thecharged particle while the use of the vector cross product indicates that the force is perpendicular toboth the direction of the magnetic field and the direction of the particles motion. These are microscopic descriptions of nature, but we will now examine what happens with themacroscopic magnetism of a bar magnet. A bar magnet being comprised of many tiny moving charges(~Avogadro’s number!) creates a sizeable magnetic field near it’s surface. From experience we knowthat a magnet has two different sides because magnets can attract or repel. We call these kinds of sidesNorth and South poles. These sides can be determined microscopically by examining the direction ofthe current:The magnetic field lines created by moving charges begin at the north pole of a magnet and end on asouth pole whether or not they belong to the same magnet:example 1: example 2:Note that the same poles of a magnet will experience a repulsive force. This corresponds to themagnetic field lines “repelling” each other (for north poles the field line arrows would be reversed):The right-hand-wrap rule is useful for finding the poles of the magnetic field when you know thedirection of the current. But you cannot see the microscopic currents in a bar magnet so you must findthe poles of the magnet experimentally by using a pole-finding device: a compass. A compasstypically has a marked tip pointing to the geographic north pole of the Earth. However, the geographicnorth pole of the Earth is really a south magnetic pole. That means that the marked tip of the compassis a north magnetic pole because it is attracted to the Earth’s south magnetic pole (which is thegeographic north pole):1.2. Sources of magnetism have only been found experimentally to come in north/south pairs. Thismeans that the magnetic lines of force (field lines) always begin at a north pole and end at the southpole. Note: one thing that is rarely studied is the strength of attraction/repulsion between two magnets.Usually we are interested in the effect of the magnetic field produced by the magnet on nearby movingcharges.Use your compass to check the labeling of the magnetic poles of your magnetized soft iron bar magnet.If your magnet is labeled incorrectly, let your instructor know and maybe they can use a strongmagnetic field or DC current to remagnetize it correctly, otherwise use a pencil to lightly label itcorrectly. Be sure to first check that your compass is magnetized correctly using the Earth’s magneticfield. Your results:1.3. Sketch the magnetic field produced by your bar magnet by placing it underneath this worksheetand sprinkling some iron flakes onto the top of your page. The flakes will show you the field lines, butyou will need to sketch the direction of the field lines by identifying the magnetic poles using yourcompass. Don’t let the magnet under the paper touch the filings or things will get messy. Your sketch:1.4. For the following double bar magnet arrangements, predict the magnetic field lines by sketchingwhat you think they will look like in the entire area surrounding the bar magnets. (Some of the fieldlines will disappear out of the drawing area only to reenter in another location of the drawing area.)1.5. Use your compass to test your prediction for each of the above arrangements. Explain anyinconsistencies between your measurements and predictions. Your results and explanations:Section 2.2.1. Electrostatics is the study of stationary charges. That means you try to understand physical systems where excess charge has been placed on an object, or systems where the net charge is zero (neutral) but there is some degree of charge separation.The first kind of system to describe is the conductor, which is a system where charges can movearound freely (usually a metal). In this system, if you deposit excess charge on the conductor, theexcess charges will repel each other and spread out uniformly over the surface of the conductor:If a neutral conductor comes into the presence of an electric field (say from another charged object),the charges already present on its surface will redistribute so that there is macroscopic chargeseparation across the entire conductor:The other kind of material we will study is that of the insulator. Charges cannot move around on thesurface of an insulator. If any excess charge is placed on an insulator, it is stuck at the location where itwas placed:Some insulators are also dielectrics, materials comprised of polar molecules that can rotate at theirposition in the material when placed in the presence of an electric field. This leads to microscopiccharge separation:2.2. Rub a glass rod with some spare paper. Electrons will be transferred from the rod to the paperleaving a positively charged rod with which to experiment. Now tear up some paper into tiny piecesand use your charged rod to pick up the (neutral) pieces. The pieces are uncharged yet are stillattracted to the rod. If the rod could transfer some of its positive charge to the paper, then they wouldboth be positive and
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