Force on a Dipole

Located at: MIT, Cambridge, Massachusetts

The Force on a Dipole lab consists of a small magnet suspended vertically by a spring in the center of two horizontally mounted coils. A video camera is set in a position to observe the oscillation of the magnet. LabView software from National Instruments is used to both generate and allow the remote choice of control voltages, waveforms, and for monitoring current and outputting a data stream. The student can choose to have current in the coils run both in the same sense or in the opposite sense in the two coils, and can select a variety of the voltages and waveforms. Once the experiment is started, the magnet oscillates vertically (or not) in response to magnetic fields generated in the coils. By using image analysis, the position of the magnet is constantly monitored up to 30 times per second. Both the position of the magnet and the current in the coils are transmitted in real time across the web to drive a virtual representation of the experiment on the student's computer. This visualization is augmented by magnetic field lines computed in real-time from the real-time current and position of the magnet. If desired, the student can also see a visual image of the magnet from the video camera, although because of data rate limitations, the refresh time of that image will be low (e.g. once per second).

By knowing the current in the coils, the displacement of the magnet, and the spring constant of the coils, the student can off-line compute the magnetic dipole moment of the magnet. The student also can quite clearly see from the field lines generated in the virtual experiment that for there to be a force on a dipole there must be a gradient of the magnetic field at the center of the coils (for current in the coils in the same sense there is no gradient at the center and thus no force; for current in opposite senses there is a large gradient and thus a force.