Physics 2212, Sim 09: Magnetic Fields

Eric Murray, Fall 2020

Question these experiments will enable you to answer: How does the magnetic field magnitude on the axis of a current loop depend on distance from the loop? How does the magnetic field magnitude on the axis of a permanent magnet depend on distance from the magnet?

Features: A current loop and a magnet are simulated seperately. The resulting magnetic field visualized by a grid of compass needles, and a tool is available to measure magnetic field.

Preliminaries: Open Magnets and Electromagnets simulation. In the Bar Magnet tab, uncheck Show Compass, but check Show Field Meter. A grid of compass needles indicate field direction and, by their contrast, field strength. Choose a background color under Options so that all the needles are visible. Position the magnet so that most of it is off the left or right side of the screen, and center the end on one grid needle. By centering the Field Meter on a grid needle that lies on the magnet's axis, small adjustments to the vertical position of the magnet may bring it into alignment with one row of grid needles.

Experiment 1—Magnetic Field on the Axis of a Permanent Magnet: Use the Field Meter to measure the magnetic field magnitude at all grid needles on the magnet's axis. Center the Field Meter carefully. You must judge the horizontal position by eye, but when the vertical position is on axis, the vertical, or y, component of the field will be zero. If it is difficult to make that component zero at a particular grid needle, it is sufficient that the field magnitude and the horizontal, or x component, of the field be the same. Record the field magnitude, which is in Gauss, and the distance from the magnet, which is in arbitrary grid units.

Experiment 2—Magnetic Field on the Axis of a Current Loop: Repeat Experiment 1 in the Electromagnet tab of the simulation, using a single loop and battery with 10 V potential.

Analysis: Since the natural log should only be taken of unitless quantities, divide the fields you found in each experiment by the field measured at 1 grid unit from the loop or magnet. (You should do the same for the distances, but dividing all the distances by 1 grid unit leaves the values unchanges, although they are now technically unitless.)

Make the graphs described in the prelab reading, and find the power relationship, with its uncertainty, between the field magnitude and distance along the axis. Are the power relationships as you expect? Can you make the relationship for the current loop closer to what you expect by eliminating some data from your analysis? If so, is there a reason that would justify eliminating that data?

Summary: Review your worksheet. Think about the goals of these experiments, your results, and the expectations from theory while writing your discussion.