Physics Dept. Demonstration Manual

DEM82 - The Simple "Dangling" Homopolar Motor


To demonstate how a simple homopolar motor works.

  • Neodymium disc magnet
  • Ferromagnetic screw
  • D-cell battery
  • Copper wire


One side of a small neodymium disc magnet is stuck to the level head of a ferromagnetic screw. The screw, in turn, becomes magnetised owing to the strength of the of the neodymium magnet. The pointy end of the screw can now be stuck to the bottom terminal of a D-cell battery where it hangs freely under gravity since the battery's casing is ferromagnetic and provides a very low fricion connection between the hanging magnet and the battery. If one end of a copper wire is pressed against the top terminal of the battery using your finger. brushing the other end of the wire aqgainst the rim of the disc magnet completes the circuit and not only causes current to flow but leads to a spinning in the disc.



So what causes the disc to rotate?

Figure 2: Current, magnetic field and magnetic force directions. Here the exerted torque causes the disc to rotate in an anti-clockwise direction when viewed from above.

Referring to Figure 2, on brushing the wire up against the rim of the disc magnet, current flows over its chrome-plated surface to the connection point at the head of the attached screw. As the disc magnet itself now takes on a current-carrying function, a Lorentz force acts tangentially to the inwardly flowing current in accordance with the right-hand rule. Hence a resultant torque acts on the disc about the axle formed by the attached screw and causes the disc to spin (for directions, refer to Figure 2). It is important to recognise that the disc magnet will only rotate provided it is mechanically decoupled from the closing wire as relative motion between the two is necessary; a fact that can be readily verified experimentally by taping the end of the wire to the rim of the disc magnet. Importantly, brushing the wire up against the rim of the disc magnet allows the latter to slide past the former whilst maintaining electrical contact between the two.