The concept of field can be applied to both electric charge and gravity. It also is a useful concept when talking about magnetism. Usually, we are first introduced to a magnetic field in a solid magnet, but this is not a magnetic field's most simple form, and not what we use to define a magnetic field. In fact, a magnetic field is more accurately the resulting field created by a moving charge. It is under this context that Feynman (Feynman, 1963: 2-4) deems magnetic and electric forces as "two different aspects of exactly the same thing" and can both "be attributed to one field."
Although on a small scale electric fields and magnetic fields are very closely related, on a large scale they actually seem a little less similar. The first major difference is that an electric field can be isolated, whereas a magnetic field cannot; magnets always come in dipoles (Lehrman, 1984: 234). The reason for this can be seen from what is occurring in a permanent magnet that creates the magnetic field. Two types of subatomic charge movements create a permanent magnetic field. They are the spin of the electron as it orbits the atom, and its actual orbit around the nucleus (Zemansky, 1960: 697). In a ferromagnetic material these create larger magnetic domains "which are at most about 1 mm in length or width" (Giancoli, 1991: 508). When a magnet is placed in a stronger magnetic field, the magnetic domains that oppose the external magnetic field are weakened and the domains that coincide with the external magnetic field are strengthened (Ibid: 509). The reasons these domains are obtained from the simpler electron spin and orbit, are not quite fully understood, however, it is believed it has something to do with the atomic structure of the ferromagnetic material and how electrons are passed in between atoms (Field Management Services, Inc. [online]).
Feynman's belief in the tight bond between electric and magnetic fields is reinforced when we see that not only does a moving charge create a magnetic field, but also a moving magnetic field creates a separation in charge. This is the basic premise for eddy currents (Exploratorium [online]). The induced charge separation, or emf, "always gives rise to a current whose magnetic field opposes the original change in flux" (Giancoli, 1991: 540). This law is called Lenz's law since Heinrich Friedich Emil Lenz first noticed it. However, Lenz's experiments were made usually using currents confined to well-defined paths using thin strips of wire. My experiment, on the other hand, will be using a more complicated scenario of induced currents moving throughout a thick metal plate (thick compared to a wire). Currents in this situation, due to their "general circulatory nature," are "referred to as eddy currents (Zemansky, 1960: 672). What I wanted to find was what exactly is the nature of these currents, but that was too broad of a topic. So a more specific goal would be to find the relationship between the counter force created by these currents, the velocity of the plate and the strength of the magnetic field.
Magnetic eddy currents have many useful applications. Often, in new amusement park rides the stopping mechanism is based on these currents. These rides use thick copper vanes as the conductor and then use either electromagnets or permanent magnets for the stopping rails. The magnetic rails create eddy currents in the copper and eventually bring the cart to a stop. This concept is also used in making efficient transformers. Transformer cores are laminated to prevent strong eddy currents by forcing the currents down well-defined paths (as in Lenz's experiments), thus decreasing the loss of energy due to heat and increasing the power flow (Zemansky, 1960: 673). Many weighing devices and galvanometers also have magnets to create such currents. This allows the needle to stop at the right point, but settle down quickly (Giancoli, 1991: 547).
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