What is a homopolar motor?

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A homopolar motor is a direct current (DC) electric motor which produces constant circular motion. This device is easy to create and the figure below illustrates the basic concepts behind its operation. A permanent magnet is attached to one terminal of a DC power supply, in this case a AAA battery. A conducting wire connects the other terminal to the magnet, thus completing the circuit. This wire should be free to rotate whilst always maintaining contact with both the terminal and magnet. The current (I) flowing through the wire will produce a magnetic field. This field will interact with the magnetic field (B) produced by the permanent magnet, and a Lorentz force (F) will be exerted perpendicular to the directions of I and B. As you look at the diagram, the force on the left section of the wire is acting into the screen and the force on the right section is coming out of the screen. As the wire can move freely, these forces cause the wire to rotate in a clockwise motion. Because the polarities of the magnetic fields do not change (hence the name homopolar), the direction of the force will not change and the wire will rotate in a constant circular motion.

As mentioned above, it is the Lorentz force acting on the wire which causes it to turn. This force results from the interaction of the electric and magnetic forces in electromagnetic fields. In the presence of a magnetic field, a moving charged particle, such as an electron carrying the current in the wire, will experience a force.The direction of the force can easily be determined using the right hand rule; the index finger points in the direction of the current flow, the middle finger points in the direction of the magnetic field and the thumb reveals the direction of the force.

The discovery of the homopolar motor was surrounded by controversy. Michael Faraday was the first to successfully build the device in 1821. However, this achievement came after his discussions with the scientists Humphry Davy and William Hyde Wollaston, who had also been trying to create electric motors. Faraday’s design featured continuous circular motion that was produced by the magnetic force around a wire which stretched into a pool of liquid mercury containing a permanent magnet. Faraday presented his creation to the Royal Society and published his results without acknowledging the work of the other two scientists. This scandal damaged Faraday’s reputation at the Royal Society, limiting his subsequent involvement in electromagnetic research.

Based on the same general principles as the homopolar motor, rail guns have been of interest to the military since the early 1900s. A simple rail gun differs from a standard homopolar motor in that it does not use an external magnetic field. A pair of conducting rails connected to a power supply is joined by a sliding armature (or conductive projectile) to complete the circuit. A current flows down one rail, through the armature and back along the other rail. The current produces a magnetic field within the loop formed by the power supply, rails and armature. Similarly to the homopolar motor diagram above, a magnetic field will be produced perpendicular to the current and the plane of the armature and rails. As both the current and magnetic field act in opposite directions along each rail, the Lorentz force produced will accelerate the armature along the rails. This acceleration can launch a projectile to hypersonic speeds. However very large currents, and therefore large power supplies, are required and so much heat is generated that the rails can be eroded. Due to these practical complications, rail guns are still at the developmental stage. However, the U.S. Navy have been able to accelerate a 7 pound projectile to velocities of up to 8,600 km/h using experimental technology.

It is possible to turn a homopolar motor into a homopolar generator. If the conducting wire is physically turned, this motion will produce a current and a DC voltage between the battery terminals. This phenomenon was also discovered by Michael Faraday, who invented the Faraday disc. Whilst this device was incredibly inefficient and not suitable for use as a practical power source, it did pave the way for the development of the modern dynamos found in a variety of gadgets today, such as wind up radios and flashlights.

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