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Magnetism Türmer Kata History of Magnetism The ancient Greeks > city of Magnesia > magnetit The early Chinese > the power to attract iron Around 1000 the Chinese found that such a needle, when freely suspended, pointed north-south. The magnetic compass soon spread to Europe. Columbus used it when he crossed the Atlantic ocean Around 1600 William Gilbert, physician to Queen Elizabeth I of England, proposed an explanation: the Earth itself was a giant magnet The Magnetosphere On Earth one needs a sensitive needle to detect magnetic forces, and out in space they are usually much, much weaker. But beyond the dense atmosphere, such forces have a much bigger role, and a region exists around the Earth where they dominate the environment, a region known as the Earth's magnetosphere. That region contains a mix of electrically charged particles, and electric and magnetic phenomena rather than gravity determine its structure. We call it the Earth's magnetosphere Only a few of the phenomena observed on the ground come from the magnetosphere: fluctuations of the magnetic field known as magnetic storms and substorms, and the polar aurora or "northern lights," appearing in the night skies of places like Alaska and Norway. Satellites in space, however, sense much more: radiation belts, magnetic structures, fast streaming particles and processes which energize them. All these are described in the sections that follow. Types of magnets There are three main types of magnets: Permanent magnets Temporary magnets Electromagnets Permanent Magnets Permanent magnets are those we are most familiar with, such as the magnets hanging onto our refrigerator doors. They are permanent in the sense that once they are magnetized, they retain a level of magnetism. As we will see, different types of permanent magnets have different characteristics or properties concerning how easily they can be demagnetized, how strong they can be, how their strength varies with temperature, and so on. Temporary Magnets Temporary magnets are those which act like a permanent magnet when they are within a strong magnetic field, but lose their magnetism when the magnetic field disappears. Examples would be paperclips and nails and other soft iron items. Electromagnets An electromagnet is a tightly wound helical coil of wire, usually with an iron core, which acts like a permanent magnet when current is flowing in the wire. The strength and polarity of the magnetic field created by the electromagnet are adjustable by changing the magnitude of the current flowing through the wire and by changing the direction of the current flow. Properties of a magnet For each north pole , there is a south pole as well! Monopoles do not exist! The opposite poles are attracted to each other, N S N S S N the same poles repel each other. N S Magnetic Field Michael Faraday realized that a magnet has a magnetic field distributed throughout the surrounding space. - if Earth itself is considered as a magnet, the south pole of that magnet would be the one nearer the north magnetic pole, and vice-versa. - the north magnetic pole is so named not because of the polarity of the field there but because of its geographical location Magnetic Field -Michael Faraday proposed a widely used method for visualizing magnetic fields. -Imagine a compass needle freely suspended in three dimensions, near a magnet or an electrical current. -We can trace in space (in our imagination, at least!) the lines one obtains when one "follows the direction of the compass needle." Faraday called them lines of force, but the term field lines is now in common use. Compass needles outlining field lines! -Field lines of a bar magnet are commonly illustrated by iron filings sprinkled on a sheet of paper held over a magnet. -Similarly, field lines of the Earth start near the south pole of the Earth, curve around in space and converge again near the north pole. -However, in the Earth's magnetosphere, currents also flow through space and modify this pattern: on the side facing the Sun, field lines are compressed earthward, while on the night side they are pulled out into a very long "tail," like that of a comet. -Near Earth, however, the lines remain very close to the "dipole pattern" of a bar magnet, so named because of its two poles. Magnetic field lines from an idealized model. Magnetic field lines. Magnetic field can be shown with the help of magnetic field lines. If we put a compass/needle in a magnetic field, than the needle will point along the field line. Field lines converge where the magnetic field is strong and spread out where it is weak. For example bar magnet the lines spread out from the north pole and converge and closes in the south pole. The magnetic field is the strongest where the lines are closer together. Magnetic field can be characterized with two physical quantities: Magnetic field strength vector H and magnetic induction Both are vector quantities which means they have direction and magnitude. B Diverse materials Ferromagnetic materials are the ones normally thought of as 'magnetic'; they are attracted to a magnet strongly enough that the attraction can be felt. Expl: refrigerator magnet. Ferrimagnetic materials, which include ferrites and the oldest magnetic materials magnetite and lodestone, are similar to but weaker than ferromagnetics. Paramagnetic substances such as platinum, aluminum, and oxygen are weakly attracted to a magnet. This effect is hundreds of thousands of times weaker than ferromagnetic materials attraction, so it can only be detected by using sensitive instruments, or using extremely strong magnets.. Diamagnetic means repelled by both poles. Compared to paramagnetic and ferromagnetic substances, diamagnetic substances such as carbon, copper, water, and plastic are even more weakly repelled by a magnet. The permeability of diamagnetic materials is less than the permeability of a vacuum. Untill now we talked about natural magnets and their magnetic fields. But not only magnets can have magnetic fields but moving electric charges also create magnetic field. Moving charges can be found in wires under electric current . Oersted’s experiment Until 1821, only one kind of magnetism was known, the one produced by iron magnets. Then a Danish scientist, Hans Christian Oersted, while demonstrating to friends the flow of an electric current in a wire, noticed that the current caused a nearby compass needle to move. The new phenomenon was studied in France by Andre-Marie Ampere, who concluded that the nature of magnetism was quite different from what everyone had believed. It was basically a force between electric currents: two parallel currents in the same direction attract, in opposite directions repel. Iron magnets are a very special case, which Ampere was also able to explain. In nature, magnetic fields are produced in the rarefied gas of space, in the glowing heat of sunspots and in the molten core of the Earth. Such magnetism must be produced by electric currents, but finding how those currents are produced remains a major challenge. In 1820 Oersted made an experiment: Without current: needle/compass showed to the Earth’s magnetic poles. With current: Needle/compass showed in the current generated magnetic field. An electric current produced the deflection of a compass needle. Another magnetic field than the Earth’s is affecting the needle Electric current produces magnetic field!!!!!!! Magnetic field of a current-carrying wire Straight wire: right-hand rule 0 I B 2R I R B Loop: I B 0 2R Solenoid: A coiled wire = lots of loops I N B 0 l In a magnetic field: Place a linear wire in the magnetic field of a horseshoe magnet. Switch current on the wire. The wire will move perpendicularly to the induction lines of the magnetic field. If we change the direction of the current than the movement’s direction will change also in the opposite direction. B S Direction of the movement F l B I I N - Lorentz force + F Force on wire under current F I l B Force on a moving charge F qv B In general (in case of a solenoid frame) we can determine the moment of rotation: The force acting on one side of the frame is: Torque: The angle between the solenoid frame and the magnetic field direction of the shoemagnet. F I l B M F d sin A l d M B I l d sin B I A sin If we have more turns in our solenoid, the torque will change like this: M N B I A sin B F I l B F B I l Magnetic induction Unit: Vs B 2 T m tesla B A Magnetic flux In homogenous magnetic field the magnetic flux is the number of the induction lines passing through the surface A perpendicular to the induction lines. Vs Tm 2 W weber Electromagnetic induction If we move a wire in a magnetic field and it crosses induction lines than electric potential can be measured at the ends of the wire. S Direction of the movement l B I N - + Electric potential This phenomenon is called electromagnetic induction and the measured potential is called induced potencial. Induced voltage Vi=Blv v-velocity of the movement (m/s) L-length of the conductor (m) In case of an solenoid which has N coils: Vi=BlvN Lenz’s law The direction of the potential enducing movement is opposite to the direction of the movement enduced by the changing current. The direction of the induced current is such that the magnetic influence of it inhibits the inducing movement or changes. S S l B l B I I N N moving - + currentflow - + An induced current is always in such a direction as to oppose the motion or change causing it. (according to energy conservation law) Magnitude of the induced potential U B l v N t Neumann’s law: the induced potential is directly proportional with magnetic induction, with the length of the wire and the movements speed perpendicular to the induction lines. The induced electromotive force or EMF in any closed circuit is PROPORTIONAL to the time rate of change of the magnetic flux through the circuit. The electromotive force is directly proportional to the change of magnetic flux in by wire surrounded area and inverse proportional with the time needed for the change. Self-induction If the current is changing in a conductor, the flux of the conductor’s magnetic field will change too, which induces potential in the conductor itself. IN A 2 BA N A I I l s N N N L t t t l t t Vs L H A H [ Henry] self-induction constant Electromagnets In order to concentrate the magnetic field generated by a wire, it is commonly wound into a coil, where many turns of wire sit side by side. Much stronger magnetic fields can be produced if a "core" of ferromagnetic material, such as soft iron, is placed inside the coil. The magnetic field of all the turns of wire passes through the center of the coil. A coil forming the shape of a straight tube, a helix (similar to a corkscrew) is called a solenoid; a solenoid that is bent into a donut shape so that the ends meet is a toroid. The ferromagnetic core magnifies the magnetic field to thousands of times the strength of the field of the coil alone, due to the high magnetic permeability μ of the ferromagnetic material. This is called a ferromagnetic-core or iron-core electromagnet. The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be rapidly manipulated over a wide range by controlling the amount of electric current. However, a continuous supply of electrical energy is required to maintain the field. Electromagnet in use Casette Hard disks work similar way! Generator, Electromotor Electromagnetic waves Faraday not only viewed the space around a magnet as filled with field lines, but also developed an intuitive (and perhaps mystical) notion that such space was itself modified, even if it was a complete vacuum. His younger contemporary, the great Scottish physicist James Clerk Maxwell, placed this notion on a firm mathematical footing, including in it electrical forces as well as magnetic ones. Such a modified space is now known as an electromagnetic field. Today electromagnetic fields (and other types of fields as well) are a cornerstone of physics. Their basic equations, derived by Maxwell, suggested that they could undergo wave motion, spreading with the speed of light, and Maxwell correctly guessed that this actually was light and that light was in fact an electromagnetic wave. Heinrich Hertz in Germany, soon afterwards, produced such waves by electrical means, in the first laboratory demonstration of radio waves. Nowadays a wide variety of such waves is known, from radio (very long waves, relatively low frequency) to microwaves, infra-red, visible light, ultraviolet, x-rays and gamma rays (very short waves, extremely high frequency). Radio waves produced in our magnetosphere are often modified by their environment and tell us about the particles trapped there. Other such waves have been detected from the magnetospheres of distant planets, the Sun and the distant universe. X-rays, too, are observed to come from such sources and are the signatures of high-energy electrons there. Electromagnetic waves If we have a capacitor with the capacity of C, and a solenoid with the inductivity of L connected the electric energy of the capacitor and the magnetic energy of the solenoid periodically (quarter period) changes into each other. If we open the plates of the capacitor to open oscillating circuit the electric „space” runs free into the environment and a periodocally changing electric (E) and magnetic (B) „space” appears. This „space” has energy: 2 1 1B 2 w 0E 2 2 0 w: energy density (E/V) These alternating electric (E) and magnetic (B) field converts from one to the other and so propagates in space. Thank you for the attention!