Stony Brook University
7:30 pm
Harriman Hall, P137
Friday, November 8, 2024
Can A Magnetic Field Do Work?
Prof. Philip Allen
When you let go of a rock, it falls. Physicists attribute this to the force of gravity and describe it as the earth's gravity "doing work" on the rock. The amount of work is the increase of the rock's "kinetic energy" as it falls. In a similar way, a large stationary magnet will attract a smaller movable magnet, increasing its kinetic energy and doing "work" on the smaller magnet. This bothers people trying to learn or teach elementary physics, because (1) it is a firm law of nature that a magnet can not do work on a moving charged particle. It can make the charged particle move in a circle, but this doesn't change the particle's speed or kinetic energy. The "bother" is (2) the small magnet is made up of charged particles. If the big magnet can't do work on any of those particles, how can it do work on the small magnet? There are lots of interesting and tricky issues that arise when trying to answer this. And there isn't universal agreement on what the complete answer is. Prof. Allen will enjoy trying to answer and explain these things.
Prof. Allen got a BA degree in 1964 from Amherst College, with a thesis trying to answer this question about magnets (using sophomore-level "classical mechanics" and senior-level "quantum mechanics"). Since then he moved on to other things: a PhD from Berkeley in 1970 studying "solid state physics", a two-year grind at Bell Labs in NJ, and then 52 years of doing research at Stony Brook. The first 42 of those years included teaching, both undergraduate and graduate students. This work was a lot of fun, but the curious topic of work by magnetic fields was put aside until 2024. Now he is hoping to understand why this topic is still interesting
