Physics Presentation. Magnets.
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible and causes the most notable property of a magnet: a force that pulls on nearby magnetic materials, or attracts or repels other magnets. The structure of the invisible magnetic field of a magnet is made visible by the pattern formed when iron filings are scattered around the magnet, as in the accompanying figure.
- Hard or Permanent Magnetand- Soft or Impermanent Magnet
Eg. Liquid Oxygen
Eg. A traditional refrigerator magnet
Ferrimagnetic retain their magnetization in the absence of a field.
However, like antiferromagnets, neighboring pairs of electron spins like to point in opposite directions.
These two properties are not contradictory, due to the fact that in the optimal geometrical arrangement
A solenoid is a three-dimensional coil. In physics, the term solenoid
refers to a loop of wire, often wrapped around a metallic core, which
produces a magnetic field when an electric current is passed through it.
Solenoids are important because they can create controlled magnetic
fields and can be used as electromagnets.
The knowledge of static electricity dates back to the earliest civilizations, but for millennia it remained merely an interesting and mystifying phenomenon, without a theory to explain its behavior and often confused with magnetism. The ancients were acquainted with other curious properties possessed by two minerals, amber and magnetic iron ore. The former, when rubbed, attracts light bodies : the latter has the power of
Based on his find of an Olmec hematite artifact in Central America, the American astronomer John Carlson has suggested that "the Olmec may have discovered and used the geomagnetic lodestone compass earlier than 1000 BC". If true, this "predates the Chinese discovery of the geomagnetic lodestone compass by more than a millennium“.
Carlson speculates that the Olmecs may have used similar artifacts as a directional device for astrological or geomantic purposes, or to orientate their temples, the dwellings of the living or the interments of the dead. The earliest Chinese literature reference to magnetism lies in a 4th century BC book called Book of the Devil Valley Master: "The lodestone makes iron come or it attracts it."
Isaac Newton contended that light was made up of numerous small particles. This could explain such features as light's ability to travel in straight lines and reflect off surfaces. This theory was known to have its problems: although it explained reflection well, its explanation of refraction and diffraction was less satisfactory. In order to explain refraction, Newton's Opticks (1704) postulated an "Aethereal Medium"
transmitting vibrations faster than light, by which light, when overtaken, is put into "Fits of easy Reflexion and easy Transmission", which caused refraction and diffraction.
In 1752, Benjamin Franklin is frequently confused as the key luminary behind electricity. William Watson and Benjamin Frankilin share the discovery of electrical potentials. Benjamin Franklin promoted his investigations of electricity and theories through the famous, though extremely dangerous, experiment of flying a kite through a storm-threatened sky. A key attached to the kite string sparked and charged a
Leyden jar, thus establishing the link between lightning and electricity. Following these experiments he invented a lightning rod. It is either Franklin (more frequently) or Ebenezer Kinnersley of Philadelphia (less frequently) who is considered as the establisher of the convention of positive and negative electricity.
In 1800 Alessandro Volta constructed the first device to produce a large electric current, later known as the electric battery. Napoleon, informed of his works, summoned him in 1801 for a command performance of his experiments. He received many medals and declarations, including the Légion d'honneur.
Davy in 1806, employing a voltaic pile of approximately 250 cells, or couples, decomposed potash and soda, showing that
these substances were respectively the oxides of potassium and sodium, which metals previously had been unknown. These experiments were the beginning of electrochemistry, the investigation of which Faraday took up, and concerning which in 1833 he announced his important law of electrochemical equivalents, viz.: "The same quantity of electricity — that is, the same electric current — decomposes chemically equivalent quantities of all the bodies which it traverses; hence the weights of elements
separated in these electrolytes are to each other as their chemical equivalents." Employing a battery of 2,000 elements of a voltaic pife Humphrey Davy in 1809 gave the first public demonstration of the electric arc light, using for the purpose charcoal enclosed in a vacuum.
In the first half of the 19th century many very important additions were made to the world's knowledge concerning electricity and magnetism. For example, in 1819 Hans Christian Oersted of Copenhagen discovered the deflecting effect of an electric current traversing a wire upon- a suspended magnetic needle.
This discovery gave a clue to the subsequently proved intimate relationship between electricity and magnetism which was promptly
followed up by Ampère who shortly thereafter (1821) announced his celebrated theory of electrodynamics, relating to the force that one current exerts upon another, by its electro-magnetic effects, namely:
1. Two parallel portions of a circuit attract one another if the currents in them are flouring in the same direction, and repel one another if the currents flow in the opposite direction.
2. Two portions of circuits crossing one another obliquely attract one another if both the currents flow either towards or from the point of crossing, and repel one another if one flows to and the other from that point.
3. When an element of a circuit exerts a force on another element of a circuit, that force always tends to urge the latter in a direction at right angles to its own direction.
Hans Christian Oersted
In 1853 Sir William Thomson (later Lord Kelvin) predicted as a result of mathematical calculations the oscillatory nature the electric discharge of a condenser circuit. To Henry, however, belongs the credit of discerning as a result of his experiments in 1842 the oscillatory nature of the Leyden jar discharge. He wrote:The phenomena require us to admit the existence of a principal discharge in one
direction, and then several reflex actions backward and forward, each more feeble than the preceding, until the equilibrium is obtained. These oscillations were subsequently observed by Fcddersen (1857) who using a rotating concave mirror projected an image of the electric spark upon a sensitive plate, thereby obtaining a photograph of the spark which plainly indicated the alternating nature of the discharge. Sir William Thomson was also the discoverer of the electric convection of heat (the "Thomson" effect). He designed for electrical measurements of precision his quadrant and absolute electrometers.
He designed for electrical measurements of precision his quadrant and absolute electrometers. The reflecting galvanometer and siphon recorder, as applied to submarine cable signaling, are also due to him.
In 1896 J.J. Thomson performed experiments indicating that cathode rays really were particles, found an accurate value for their charge-to-mass ratio e/m, and found that e/m was independent of cathode material. He made good estimates of both the charge e and the mass m, finding that cathode ray particles, which he called "corpuscles", had perhaps one thousandth of the mass of the least massive ion known (hydrogen).
He further showed that the negatively charged particles produced by radioactive materials, by heated materials, and by illuminated materials, were universal. The nature of the Crookes tube "cathode ray" matter was identified by Thomson in 1897.
current advocated by Westinghouse and Nikola Tesla. Tesla's patents and theoretical work formed the basis of modern alternating current electric power systems, including the polyphase power distribution systems.
short for magnetic levitation, is a type of high-speed train that hovers over the track and uses electromagnets to propel it along the track. There are three basic components of the trains: a large electrical power source, metal coils lining the track, and large magnets attached underneath the train, which are used to guide it along the track and allow it to levitate. Since the trains do not actually touch the track, there is no friction present. This enables the train to reach speeds of approximately 300 mph. This also reduces the noise and vibrations common on other types of similar transportation.
The large costs involved in building one of these systems is the issue currently prohibiting their widespread use. The first commercial maglev train was made available to the public in Shanghai, China in 2003. The idea behind magnetic levitation can be traced back to the early 1900's, but it was not actually created until the 1960's. The work of physicists Jim Powell and Gordon Danby in Brookhaven National Laboratory on Long Island, New York, sparked interest in these systems throughout the world. Research and development began for maglev trains in the United States, Japan, Germany, the United Kingdom, and Canada.
1. Electrodynamic Suspension System
2. Electromagnetic Suspension System
The electrodynamic system, also known as the Linear express, was developed in Japan. The electrodynamic system is based on three basic principles, Lenz's law (motional emf), moving current creates a magnetic field, and that like poles of a magnetic field repel while opposite poles attract. Superconducting wires are used in a basically solenoid shape to create the electromagnets on the train with a linear synchronous motor that provides the alternating current.
These electromagnets can be placed on the train in a variety of designs, one with only two large electromagnets at the front and back of each car. Unlike a traditional train track, the "maglev" (magnetically levitating) train track is dug into the ground, covered with metal sheets, and has special coils on the sides to allow the train to move and levitate. Since the train does not initially levitate, there must be wheels to allow the train to reach a speed where levitation occurs.
The three forces allowing the train to move and hover over the track are propulsion, levitation, and guidance.
Track of Linear Express train in a tunnel
Linear Express train, model MLX01-901
Trains propelled by electrodynamic suspension system levitate over the track with a distance of about 8-10 cm while the electromagnetic system only hovers at 1-2cm above the track. The levitation in the electrodynamic system depends upon induced currents caused by the movement of the train's electromagnets and the levitation coils on both sides of the track. Remember when a magnetic field changes over time, an induced current is created opposing the changing magnetic field.
The train is moving, so the magnetic field into the sides of the track changes as it crosses each levitation coil, always creating an induced current.The train does not levitate until a velocity of about 100 km/hr is reached, and at that point the change in magnetic flux in the levitation coils causes an upward magnetic force strong enough to counteract the force of gravity and levitate the train. The train has wheels so it can move until levitation takes over.
So, since there's an attractive, opposite magnetic field on the top half of the levitation coils, why doesn't the train get stuck on the side of the track?
The reason the train doesn't get stuck on the side of the track isbecause of Lenz's law and induced currents! Guidance, the name for this induced current, opposes sideways motion of the train and keeps the train in the middle of the track. Guidance coils may be the levitation coils, or located underneath the levitation coils. Since the electromagnet is a solenoid, recall the intensity is largest very close to the center of the solenoid. When the electromagnet moves close to one side of the track, an induced current occurs because the intensity of the magnetic field increases.
An induced current from the guidance coils is created on both sides of the train. If the train is too close to one side of the track, the train is repelled on that side and on the opposite side of the train the guidance coils exerts an attractive force attracting the train back to its previous position. These two restoring forces push the train back to the center of the track, causing the train to remain a constant distance in the middle of the track.
The propulsion system moves the train forward, and is different from the levitation and guidance system. The propulsion coils in the track have an ac current that propagates down the track in front of the train. This alternating current creates an opposite magnetic field to the electromagnet, and since opposite poles attract, the electromagnet pulls the train moves forward. The alternating current also creates a repulsive force at the back of the electromagnet, helping push the train forward.
Since the ac current changes directions, once the train reaches the propulsion coil generating the attractive magnetic field, when the train's propulsion electromagnet is at about the halfway point of the coil the current changes, so now the train is repelled at that spot and pulled forward to the next propulsion coil, a cycle that repeats down the track. Since the electrical current propagating down the track use AC current and the electromagnets on the train also use AC current, calculating of the needed current to attract the train as it accelerates are complex.
Opposing forces to the forward movement are induced currents causing magnetic drag and air resistance. An induced currents oppose the forwards motion of the train and will eventually help slow the train down to a constant speed, along with the air resistance which becomes significant at high speeds. (Notice that when the train levitates, no friction occurs between train and track, a huge advantage from the traditional train!)
The Electromagnetic Induction Suspension System, also known as the Transrapid System (to go commercial In 2005), was tested and developed in Germany after being first conceived by Graeminger with a patent in 1934 by Herman Kamper. The first test vehicle called the "Principle Vehicle" had a maximum velocity of 90 km/h and was developed in 1971 on a 600 meter track. Starting in 1972 Germany also developed and tested an Electrodynamic System (EDS) which could be considered the forefather of
the Japanese EDS (Linear Express) which had a maximum velocity of 401.3km/h. After these initial trains -- a series of test vehicles capturing both EDS and EMS concepts were exercised between the early 70s and the present culminating in today's Transrapid 7 (an EMS based system) with a speed of 450 km/h on its test track in 1998.
A train that uses permanent magnets is in an unstable equilibrium. As such, the train will not return to the center of a rail. Rather, it will, without secondary assistance (such as with an EDS system), kilter off to one side. So it is a combination of magnets as well as technology that allows the Maglev machines that ARE feasible to work. It is the use of electromagnets that allows an unstable equilibrium to become a stable equilibrium.
Although some people are concerned about the effect of the electromagnetic field upon humans, according to scientists the magnetic field felt in the cabin is not strong enough to have a significant effect upon humans, especially when compared to the magnetic field created by other electrical appliances. Since the magnetic field induced by the solenoid is weak in the area above the solenoid, the magnetic field experience in the cabin is much weaker than the magnetic field between the sides of the train and track.
The magnetic field in the cars will vary, however, depending upon the arrangement, type, and composition of electromagnets.
Superconductors are expensive, but since at optimal, cool temperature there is no resistance in the superconducting wires, the generator required to create the needed magnetic force is smaller and lighter than the generator would need to be with normal wires. Other materials could also be used, but their effectiveness would not be as good as the superconductors. One method to keep the superconductor is kept cool by a small helium refrigerator surrounding each solenoid.
Sometimes the coils are cooled before the train moves. The downside, of course, is that if the wires get too hot, resistance increases, the train may not create a magnetic field strong enough for levitation or propulsion, and people could get stuck!