Chapter 24: Magnetism

Magnetic Fields constrain the flow of plasma such as is seen in structures seen on the Sun known as Prominences, they can force nuclei to absorb preferred energies of radiation such as in Magnetic Resonance Imaging (MRI) machines, they can align magnetics embedded in their fields, such as in Compasses, and they can force particles to flow in predetermined paths, such as the cricular paths at the Cern Collider near the Swiss/French border.

Magnetic fields and their Effects are not a new type of phenomenon (force), they are manifestations of electrical forces. The twist is that magnetic fields are the results of charges in motion, that is, they are the results of electrical currents.

  • They arise from large-scale currents driven by batteries, such as in electromagnets

  • They arise from small-scale currents that they arise in charged particles such as electrons and protons. In certain materials known as Ferro-magnets large groups of particles align in domains, but the domains are usually oriented randomly. The domains are microscopic in size, 1 to 100 microns (0.000001 meters to 0.0001 meters).

Permanent magnets (such as bar magnets and horseshoe magnets) arise from the alignment of the domains of particles such as electrons. The alignment can be broken by high temperatures or by striking a permanent magnet.

Maglev Trains

The basic idea of maglev trains is to use magnetic forces to eliminate the wheel/rail contact in trains thereby greatly reducing friction:

Maglev trains typically use one of the left two ideas; the magnets on the rail repel the car making it float while magnets attached to the car pull the car up toward the track lifting the car up.

Maglev trains are propelled by a time varying magnetic field. Recall that unlike poles of magnets attract and that like poles of magnets repel. If the fields in the tracks alternately change direction by changing the direction of the current flow, then track can alternately pull the train forward, shift to neutral and then push the train forward.

Maglev trains due to the small friction between the wheels and rail, can reach very high speeds. The Japanese maglev train has reached speeds of greater than 350 miles per hour, speeds almost twice as fast its conventional trains have reached.

JR Maglev Train

German Maglev Train

Chinese Maglev Train

Magnetic Resonance Imaging Machines

Magnetic Resonance Imaging (MRI) is an noninvasive probe which relies on the nuclear magnetic resonance phenomenon. To understand what MRI entails, note that atomic nuclei and particles have a property known as spin and because they are charged then generate currents (as already noted). The currents have magnetic fields associated with them and so interact with applied fields:

Randomly oriented particles tend to be aligned by applied magnetic fields. In MRI machines, the applied fields are on the order of a few Tesla, very intense fields. (Reminder, the average magnetic field of the Earth is around 0.00005 Tesla.) The alignment is not complete, however, and misalignments may occur. The misalignments lead to an effect known as precession:

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This effect is similar to the effect that arises from the interaction of the Earth and its spin and the Sun and Moon. The interesting thing is that for particles, the misalignments are quantized and only certain misalignment states are possible.

So when the aligned particles are irradiated with low-frequency electromagnetic radiation, only the preferred energies can be absorbed (particular energies determined by the strength of the applied magnetic field). This allows researchers to determine where the particles are concentrated allowing structure unseen to be mapped:

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Electric Motor

An electric motor is a way to convert electrical energy into mechanical energy, kinetic energy. A simple example is shown to the right.

    A wire loop connected to a battery is placed between the ends of a horseshoe magnet. The left picture shows iron filings sprinkled around a horseshoe magnet. The field lines can be seen to flow between the N and S poles of the horsehoe.

    The voltage source is connected and current flows through the loop. If the current flows in the counter-clockwise sense, the force on the top half of the loop pushes out of the paper. The force on the bottom half of the loop pushes into the paper. This causes the loop to spin in the sense shown.

    This converts the electrical energy into spinning (mechanical energy) of the wire.

Note the break in the contact at the left end of the loop. This causes the driving potential on spinning wire loop to always remain in the same sense so that the current always flows in the clockwise sense when we view the picture (Why is this so?).