Magnetic - 2019

Properties Of A Magnetic Field (Magnets)

       

  Magnets are made from alloys of iron, cobalt and nickel. Nipermag, an alloy of iron nickel, aluminums and titanium is used to make magnets. We have also learnt that alnico is a magnetic alloy of aluminums, nickel and cobalt. A magnet is a material or object that produces a magnetic field. This magnetic field is inviable but is responsible for the most notable property of a magnet : a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, cobalt, etc. and attracts or repels other magnets. A permanents magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include the elements iron, nickel and cobalt and their alloys, some alloys of rare-earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic , all other substances respond weakly to a magnetic field, by one of several other types of magnetism.

  

          Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically "hard" material's, which do. Permanent magnets are made from "hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a a saturated magnet, a certain magnetic field must be applied, and this threshold depended on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity. The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization.

       A Permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include the elements iron, nickel and cobalt and their alloys, some alloys of rare-earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism. Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically "hard" materials, which, which do. Permanent magnets are made from " hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure , making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity. The overall strength of a magnet is measured by its magnetic moment or, alternate the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization. 

          An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of "soft" ferromagnetic material such as mild steel, which greatly enhances the magnetic field produced by the coil.

Earth : A Gigantic magnet The scientist William Gilbert gave a scientific explanations, based on experiment, of the observation that a freely suspended magnet always settles in the north - south direction only. He gave a round shape to a naturally occurring magnetic rock. He suspended this spherical magnet so that it could turn freely and brought the north pole of a bar magnet near it. The south pole of the magnetic sphere was attracted towards it. 

       The north pole of a freely suspended magnet settles in the direction of the geographic north pole of the earth. It means that the south pole of some gigantic magnet must be near the geographic north pole of the earth and the north pole of that magnet, near the geographic south pole of the earth. Gilbert inferred from this that the earth itself is a gigantic magnet. However, the south pole of this magnet must be near the geographic north pole of the earth while the magnetic north pole is near the geographic south pole.

Earth's magnetic field : also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream a stream of charged particles emanating from the Sun. The magnetic field is generated by electric currents due to the motion of convection currents of a mixture of molten iron and nickel in Earth's outer core : these convection currents are caused by heat escaping from the core, a natural process called a geodynamics. 

          The magnitude of Earth's magnetic field at its surface ranges from 25 to 65 T (0.25 to 0.65 G ). As an approximation, it is represented by a field of a magnetic dipole currently tilted at an angle of about 11०  With respect to Earth's rotational axis,  as if there were an enormous bar magnet placed at that angle through the center of Earth.  The North geomagnetic pole actually represents the South pole of Earth's magnetic field, and conversely the South and conversely the South geomagnetic pole correspond to the north pole of Earth's magnetic field (because opposite magnetic poles attract and the north end of a magnet, like a compass needle, points toward Earth's South magnetic field, i.e., the North geomagnetic pole near the Geographic North Pole). As of 2015, the North geomagnetic pole was located on Ellesmere island, Nunavut, Canada.

        While the North and South Magnetic poles are usually located near the geographic poles, they slowly and continuously move over geological time scales, but sufficiently slowly for ordinary compasses to remain useful for navigation. However, at irregular intervals averaging several hundred thousand years, Earth's field reverses and the North and South Magnetic Poles respectively abruptly switch places. These reversals of the geomagnetic poles leave a record in rocks that are of value to paleo magnetisms in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors in the process of plate tectonics.

        The magnetosphere is the region above the ionosphere that is defined by the extent of Earth's magnetic  field in space. It extends several tens of thousands of kilometers into space, protecting Earth from the charged particles of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects Earth from the harmful ultraviolet radiation.

Magnetic Field : A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field. A permanent magnet's magnetic field pulls on ferromagnetic materials such as iron, and attracts or repels other magnets. In addition, a magnetic field that varies with location will exert a force on a range of non-magnetic materials by affecting the motion of their outer atomic electrons. Magnetic field surround magnetized materials, and are created by electric currents such as those used in electromagnets, and by electric fields varying in time. Since both strength and direction of a magnetic field may vary with location, it is described mathematically by a function assigning a vector to each point of space, called a vector field. In electromagnetics, the term "magnetic field " is used for two distinct but closely related vector fields denoted by the symbols B and H. In the International System of Units, H , MAGNETIC FIELD STRENGTH, IS measured in the SI base units of ampere per meter (A/m). B, magnetic flux density, is measured in tesla (in SI base units: kilogram per second per ampere), which is equivalent to newton per mater per ampere. H and B differ in how they account for magnetization. In a vacuum, the two fields are related through the vacuum permeability.

         Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. Magnetic fields and electric fields are interrelated and are both components of the electromagnetic force, one of the four fundamental force of nature. The interaction of magnetic fields in electric devices such as transformers is conceptualized and  investigated as magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall effect. The Earth produces its own magnetic field, which shields the Earth's ozone layer from the solar wind and is important in navigation using a compass.

Apparatus : A bar magnet, pins, cardboard, iron filing, plastic bottle, bucket, water.

Procedure : Take a bar magnet and some pins. Place them at such a distance from each other that they do not stick to each other. Now slowly move the magnet towards the pins. Observe the pins as they get pulled to the magnet. The magnet attracts the needles from afar. 

TRY : Take a small cardboard. place a bar magnet at its Centre. Sprinkle iron filing on the cardboard around the magnet. Tap the cardboards gently. Observe the iron filings. What is the inference from the above experiments ? The British researcher Michael Faraday named these lines, going from one end of the bar magnet to the other, 'magnetic lines of force'. The region around a magnet where the magnetic force acts on an object is called a magnetic field. The magnetic field around a magnet can be shown by means of magnetic lines of force. The intensity of the magnetic field at a place can be gauged by the number of line of force that pass through a unit area at that place, perpendicular to that area. Michael Faraday, imagined that there might be invisible lines of force going from one pole of a magnet to the other, and that magnetic attraction or repulsion might be taking place through the medium of these lines of force. If Faraday's idea is accepted, the intensity of the magnetic field can be obtained from the number of lines of force, as explained above.

          Take intensity of a magnetic field is low where the lines of force are sparse, and the intensity is high where the lines of force are concentrated.

Properties magnetic lines of force : While proposing the concept of lines of force, Michael Faraday argued that, if all observed effects are to be explained satisfactorily, then the lines of force must have certain properties. 1) Magnetic lines of force are imaginary connecting lines and Faraday introduced the concept of lines of force in order to explain magnetic attraction and repulsion. 2) Magnetic lines of force always run from the north pole to the south pole. The south pole may be of the same magnet or a different one. 3) Magnetic lines of force are in a state of tension like a stretched spring. 4) Magnetic lines of force repel each other. 5) Magnetic lines of force do not intersect each other. 6) The number of the magnetic lines force at a particular point determiners the strength of the magnetic field there.

          You can now see from the figure, how the properties given above help to explain the repulsion between like poles and attraction between opposite poles. According to the third property, the lines of force joining the north and south poles of a magnet, being in a screeched state like a spring, pull the two opposite poles towards each other. By the fourth property they give rise to repulsion between like poles.

Penetrating ability of the magnetic field : Procedure : Spread some pins on a table. Hold a cardboard at a small distance above these pins. Place a bar magnet on the cardboard and observe. Now move the magnet slowly over the cardboard and observe. Repeat this procedure, increasing the layers of cardboard, and observe.

Procedure : Fill water in a plastic bottle. Drop a few pins in the water. Take a bar magnet near the bottle and observe. Move the magnet through a small distance near the bottle and observe. From the above observation, we see that a magnetic fielsdcan pass through a cardboard, a bottle or water. However, in each case, the intensity of the magnetic field is found to decrease.

Procedure : Take water in a big basin. Place a bar magnet on a plastic lid and float it on the surface of the water. Magnetic a needle or pin. Stick this needle firmly to a small piece of thick cardboard by means of a sticking tape. place the magnetized needle stuck to the cardboard, in the water near the magnet. Observe the direction in which the needle moves. Repeat this, placing the magnet at difference places around  the magnet and observe.

Metal Detectors : The function of these machines is based on electromagnets. Metal detectors are used in very important places like an airport, bus station, certain  and buildings. They are used for inspection of persons entering these places. Metal detectors are used to detect very pervious articles and also in the food-processing industry to detect any iron / steel objects mixed unknowingly in foodstuff as these would be harmful to health. In geology, these machines are used to detect the presence and quantity of  metals.

        The simplest form of a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field. If a piece of electrically conductive metal is close to the coil, eddy currents will be induced (inductive sensor) in the metal, and this produces a magnetic filed of its own. If another coil is used to measure the magnetic field (acting as a magnetometer,) the change in the magnetic field due to the metallic object can be detected. The first industrial metal detectors were developed in the 1960s and were used extensively for mineral prospecting and other industrial applications. Uses include detecting land mines, the detection of weapons such as knives and guns (especially in airport security), geophysical prospecting, archaeology and treasure hunting. Metal detectors are also used to detect foreign bodies in food, and in the construction industry to detect  steel reinforcing bars in concrete and pipes and wires buried in walls and floors.