Electromagnetism

Electromagnetism surrounds us in the modern world, we use it to travel places, to diagnose diseases, and in some of the newest research in the field of physics, but how does it work in different devices. Before explaining how it is used, it is important to understand what electromagnetism is. Electromagnetism is made up of electric and magnetic forces, in the case of a current traveling through a single wire the current creates a magnetic field that has equipotential lines that are circular and decrease in strength as the distance from the wire increases. For a single coil of wire the magnetic field spirals around the wire with the strongest point at the center of the circle, the effect can be magnified by increasing the number of turns of wire. In transportation electromagnetism is used in electric trains called maglev trains. In the forms of this train that are being developed in the United States the undercarriage contains permanent magnets and the tracks that the train rides over contain coils of wire that both repel the magnets on the train and move it down the tracks. The propulsion system uses alternating current which allows the magnetic field that is produced by the coils to switch from pulling the train to pushing the train many times per second , allowing it to reach extremely high speeds. In the Japanese trains liquid helium is used to cool the wire coils in the train to less than two-hundred degrees below zero Fahrenheit. These extreme temperatures decrease the resistance of the wire to nearly zero allowing for enormous magnetic fields to be produced and for the train to reach over three hundred miles per hour. Another emerging transportation technology that uses electromagnetism is all electric cars. The motor in the car uses thin copper wire coils on the inner portions of the motor that is connected to the shaft create a changing magnetic field and a set of outer field coils that generate a constant magnetic field. The outer coil is wrapped around an iron or highly magnetic steel magnetic circuit with an air-gap. A magnetic circuit is the loop that is formed by the magnetic flux lines and making a magnetic circuit out of iron forces the magnetic flux lines to be evenly spaced throughout the circuit. The air-gap is a space in which there is not any of the material that the circuit is made of, but rather air where the magnetic flux lines would pass through, and if the gap is small enough the flux will continue through to the other side of the circuit with minimal changes in the magnetic flux density. The rotating coils of wire are placed in the air-gap introducing a strong and constant magnetic field on the coils. The torque generated by the motor is fed through a mechanical transmission with gears to the wheels. Electromagnetism is used in countless different ways in the medical field and one of the most recognizable is Magnetic Resonance Imaging, or MRI. The magnetic fields in MRI are extremely powerful, between 1.5 and 3 Tesla, or forty-thousand times stronger than Earth’s magnetic field. Within the machine there are thousands of loops of wire that are wrapped around the super-cooled core in order to create a powerful and uniform magnetic field. But in order of the field to be as uniform as possible the coils are not evenly spaced, also at the time of instillation there are steel blocks added throughout the parts near the main coil to set the magnetic field as uniform as possible, this is called passive shimming. However the placement of anything inside of a magnetic field will disrupt the field, so to counter this there is a secondary set of coils that is not super cooled that lie outside of the main coil and are automatically switched on and off in sections by a computer that monitors the magnetic field inside of the machine. This magnetic field interacts with the hydrogen atoms in the molecules that are spread throughout the body. According to Stroman this works for three reasons, “The first is that it is magnetic, with a north pole like any magnet, and the second is that it spins on its axis. These properties are actually related, and the axis that the nucleus spins around is the same as the magnetic field axis, which runs between the north and south magnetic poles.” The Magnetic Resonance Imaging machine aligns the poles of the hydrogen atoms, therefor causing the hydrogen atoms to make their own magnetic field, which is what shows up in the image. However due to the fact that the hydrogen atoms are spinning means that it would take a long time for the axes of the hydrogen atoms to align with that of the MRI machine, this makes where the total angular momentum must remain constant, this rotation about an axis that is not parallel to the magnetic field of the machine is called wobbling. The rate of the wobble is known and is relative to the magnetic field that the hydrogen atoms are in and is recognized as the Larmor frequency. In 1.5 Tesla to 7 Tesla magnetic fields similar to those used in Magnetic Resonance Imaging machines is anywhere from sixty-four megahertz to three hundred megahertz, putting them in the radio frequency spectrum of electromagnetic radiation giving these pulses there name, radio frequency pulses. To get most if not all of the hydrogen atoms to align to the primary magnetic field axis of the machine a secondary much smaller field is introduced perpendicular to the primary field, on the order of one Gauss or about one ten thousandth of a Tesla, and is then rotated about the axis of the primary axis at the Larmor frequency. This rotation causes the overall magnetic field generated by the hydrogen atoms to rotate as well and this rotation is the resonance part of Magnetic Resonance Imaging. The secondary field is only applied for a fraction of a millisecond, but that is all that it takes for each image to be taken. This is only touching on the principles of Magnetic Resonance Imaging since the programs of the computers that control the machine and read the echoes from the magnetic fields produced by the body are outside of the scope of this article. Principles of electromagnetism are used in one of the leading areas of modern physics. The Large Hadron Collider (LHC) is a seventeen mile long circular tunnel located on the border of Switzerland and France, and has aided in the discovery of Higgs Particle, which is a sub particle of protons. The Large Hadron Collider uses extremely powerful electromagnets to accelerate a beam of protons to nearly the speed of light (99.9%). The particle accelerator uses multipole magnetic fields (dipole, quadrupole, and sextupole) to both accelerate the beam of particles and to compress the beam into a more focused area, to achieve this the magnets turn on and off at a specific frequency depending on the particle. Each of the coils that generate these fields are shielded from the rest of the coils to prevent undesirable currents from being generated in one coil from another, the existence of the induced currents in the coils would make the magnetic fields in the machine difficult to control. Recently the instillation if super conducting cables throughout the Large Hadron Collider, when these cables are cooled to extreme temperatures the resistance drops to nearly zero ohms allowing for much larger magnetic fields to be generated. This works because as the material is cooled the atoms inside begin to slow the rate at which they vibrate, thus allowing the electrons that are flowing in the wire to have fewer things impeding their movement through the material. Once the particles are accelerated to the desired speed the particles are collided into other particles or a thin layer of atoms in the form of foil. The majority of the uses of these machines is actually not in the discovery of sub-atomic particles, but rather in medicine, industry, and even national security. In medicine the beams that are generated are used to make radioactive isotopes that are used in radiation therapy and the beams themselves, rather alpha or beta particles or carbon ions, to destroy cancerous tissues. In industry to etch computer chips, radioactively sanitize objects, and to harden the surfaces of artificial joints. Finally in national security the military is looking at using electron beams in several ways along with its current use to inspect radioactive waste containers. Rather we recognize it or not the principles of electromagnetism touches nearly every aspect of our modern lives. Everything including communication, transportation, medicine, military, manufacturing, some theoretical anti-gravity devices, and nearly all of modern physics. Due to the cost of some of this new technology makes sticking with traditional technology more appealing, especially in the case of maglev trains. In medicine it diagnoses and can treat many different diseases and in the future may even hold the cure to some of the most vigorous diseases. In modern physics electromagnetism is used to smash sub-atomic particles to brake them down even farther, possibly leading to the discovery of dark matter and other sub-atomic particles.