Magnetic Field
Magnetic Field – PHY 202 Introduction The purpose of this laboratory is to explore, qualitatively, the magnetic fields produced by a bar magnet and by a current solenoid. Magnetic field lines, the effect of the magnetic field on a moving charge and the dependence of field strength on solenoid current and distance from the solenoid are investigated. Theory Certain materials found in nature, called permanent magnets, exert forces on each other that are different than electrical forces or any other type of interaction. This new interaction is called Magnetism. If a bar-shaped permanent magnet, or bar magnet, is free to rotate, one end points north; this end is called a north pole or N pole and the other end (that points south) is called a south pole or S pole. Opposite poles attract each other, and like poles repel each other. Differently than electric charges, it is not possible to isolate a north or a south pole, they always come in pairs. Similarly, to the case of electric charges, we use the concept of magnetic field to describe magnetic interactions as well as magnetic field lines to characterize the magnetic field. At any position in space, the direction of the magnetic field vector is the direction in which the north pole of a compass needle points. We then draw the magnetic field lines so that the line through any point is tangent to the magnetic field vector at that point. Magnetic field lines never intersect because the direction of the magnetic field is unique at each point in space. Magnetic fields exert forces on moving charges, the force on a moving charge being perpendicular to the magnetic field direction and the particle velocity direction. The magnetic field produced by a bar magnet is very similar to the magnetic field produced by a current carrying solenoid. This is not just a coincidence but a reflection of the fact that magnetic fields are created by moving charges, the macroscopic current in the case of the solenoid and the microscopic electron orbital motion in the case of the permanent bar magnet. Procedure Open the PhET simulation Magnets and Electromagnets (Java version): https://phet.colorado.edu/en/simulation/magnets-and-electromagnets 1. Magnetic Field and Magnetic Field Lines Select Bar Magnet, increase strength to 100%, keep ‘see inside magnet’ and ‘show planet earth’ OFF. a) Move the compass around the bar and observe its behavior. How is the compass changing? What is the compass telling us? b) What do the compass needles, drawn all over the screen, tell us? c) Keep the bar magnet in the center of the screen and turn the compass OFF. In a separate piece of paper make a diagram of the magnetic field lines in the plane of the screen. 2. Electromagnet and Forces on a Moving Charge • • • Select Electromagnet, select DC and increase battery voltage to 10 V. Place the electromagnet near the center of the screen, as shown below and answer the following questions. Note: o Current is still flowing, just the ‘show electrons’ box has been unselected. o The magnetic field and the charge velocity are in the plane of the page. a) What will be the direction of the magnetic force on a positively charged particle moving with velocity v in the direction of arrow A? b) What will be the direction of the magnetic force on a positively charged particle moving with velocity v in the direction of arrow B (exactly along the axis of the solenoid? c) What will be the direction of the magnetic force on a positively charged particle moving with velocity v in the direction of arrow C? d) What will be the direction of the magnetic force on a positively charged particle moving with velocity v in the direction of arrow D? For the next activity we want to obtain a qualitative relationship between field strength and current. The units are not relevant, just the functional dependence. 3. Field Strength Relationship, Magnetic Field vs. Solenoid Current. Use 4 coils for the electromagnet. Position the electromagnet with the coil near the center of the screen and the Field Meter at the center of the coil (note that By should be zero along the axis of the coil) as shown below: a) Change the battery voltage from 0 V to 10 V in increments of 1 V, recording the magnetic field strength for each case. b) Create a table with two columns and the corresponding graph. c) Enter the values of voltage in the first column and the corresponding values of magnetic field strength in the second column. Remember that the magnetic field at the center of a solenoid is given by:
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