Magnetism and Electromagnetism Lecture 2 Magnet A substance

Magnetism and Electromagnetism Lecture # 2

Magnet: A substance that attracts pieces of iron and steel is called a magnet and this property of a material is called magnetism. Magnets are found in the natural state in the form of the mineral called magnetite. Types of Magnets: -Basically two types of magnets 1. Natural magnets 2. Artificial magnets. Natural magnets: Natural magnets are those iron ores (magnetite Fe 3 O 4) which are obtained from mines and have the property of attracting iron pieces naturally. However, natural magnets have no practical value because their magnetism is not strong enough to be utilized in the modern devices. Magnetic Field of earth

The Earth its self is a magnet. (What causes the Earth's magnetic field? ) Our planet’s magnetic field is believed to be generated deep down in the Earth’s core. Nobody has ever taken the mythical journey to the centre of the Earth, but by studying the way shockwaves from earthquakes travel through the planet, physicists have been able to work out its likely structure. Right at the heart of the Earth is a solid inner core, two thirds of the size of the Moon and composed primarily of iron. At a hellish 5, 700°C, this iron is as hot as the Sun’s surface, but the crushing pressure caused by gravity prevents it from becoming liquid. Surrounding this is the outer core, a 2, 000 km thick layer of iron, nickel, and small quantities of other metals. Lower pressure than the inner core means the metal here is fluid. This flow of liquid iron generates electric currents, which in turn produce magnetic fields. --The average distance to the centre of the Earth is 6, 371 km or 3, 959 miles. Natural : The Mariana Trench or Marianas Trench is the deepest part of the world's oceans. The trench is about 2, 550 kilometers (1, 580 mi) long. Artificial : The Kola Superdeep Borehole, an abandoned Cold War attempt by the Soviets to drill to the center of the Earth. Drilling began on 24 May 1970 reached 12, 262 meters (40, 230 ft) in 1989.

Distance between Earth and Sun 150 million kilometers

Artificial Magnets : The artificial magnets are those which are created by artificial means. These are Temporary magnets and Permanent magnets. A temporary magnet is that in which magnetism remains temporarily. If a wire is wound on a soft iron piece and current is passed through the wire, the soft iron behaves like a magnet. On the removal of current iron losses its magnetism. Permanent magnets are made from steel which is in general harder than soft iron. A part from steel, alloys like cobalt and tungsten are also used. These magnets are used in DC machines to create magnetic flux, electrical instruments and in loud speakers etc. Magnetic Induction : The phenomenon in which a magnetic substance becomes magnet when it is placed near a magnet is called magnetic Induction.

Magnetic Field : The space outside the magnet where its poles have a force of attraction or repulsion on a magnetic pole is called a magnetic field. Or The region around a magnet where it has a magnetic effect is called its magnetic field. Magnetic lines of force. 1. Magnetic lines of force start from the North Pole and end at the South Pole. 2. They are continuous through the body of magnet. 3. Magnetic lines of force can pass through iron more easily than air. 4. Two magnetic lines of force can not intersect each other. 5. At neutral point there cannot be any lines of force.

Magnetic effect due to a Current Carrying Conductor: Electro-magnetism: The magnetism effects produced due to the flow of electric currents is called electro-magnetism. When an electric current passes through a conductor, a magnetic filed is set up all along the length of the conductor. Following points are of importance 1. The magnetic lines of forces are circular in a plane perpendicular to the current. 2. The field near the conductor is stronger and becomes weaker as we go away from the conductor 3. The magnetic filed becomes stronger if current is increased and vice versa. 4. The direction of field is reversed if current is reversed. Magnetic field due to current through a straight conductor: A current carrying straight conductor has magnetic field in the form of concentric circles around it. Magnetic field of current carrying straight conductor can be shown by magnetic field lines. The direction of magnetic field is given by right hand Thumb Rule.

Magnetic field due to circular current carrying conductor: In case of a circular current carrying conductor, the magnetic field is produced in the same manner as it is in case of a straight current carrying conductor. In case of a circular current carrying conductor, the magnetic field lines would be in the form of concentric circles around every part of the periphery of the conductor. Since, magnetic field lines tend to remain closer when near the conductor, so the magnetic field would be stronger near the periphery of the loop. On the other hand, the magnetic field lines would be distant from each other when we move towards the centre of the current carrying loop. Finally; at the centre, the arcs of big circles would appear as a straight lines.

Magnetic flux : The total number of magnetic lines of force in a magnetic filed passing through a surface (such as a conducting coil) is called magnetic flux. Its is the amount of a magnetic field produced by a magnetic source. It is denoted by Greek letter (Φ) Phi. If 10 magnetic lines come out of the north pole or enter the south pole of a magnet, then magnetic flux Φ = 10 lines or 10 maxwells. The SI unit if flux is weber. Magnetic flux density: Flux per unit area at right angle to the flux is called flux density.

Magnetic Circuits Just as the closed path followed by electric current is called electric circuit , similarly closed path followed by magnetic flux is called magnetic flux. Magnetomotive force : In an electric circuit, an electromotive force is required to produce current flow. The magnetic counterpart of it is magneto motive force. The product of number of turns in the winding and the current flowing in it is called magnetomotive force (mmf). M. M. F = N I (unit amperes or ampere turns) Reluctance : The opposition that the magnetic circuit offers to magnetic flux is called reluctance. Magnetic Field Intensity Magneto motive force per unit length is called magnetic field intensity. It is denoted by H and its unit is ampere turns per meter.

Absolute and Relative Permeability: Permeability of a material is its conductivity for flow of magnetic flux. Air or Vacuum is the poorest conductor of magnetic flux. The absolute permeability of air or Vacuum has constant value of μo = 4. π. 10 -7 H/m Relative Permeability, symbol μr is the ratio of μ (material permeability) and μo the permeability of free space and is given as. The absolute permeability of any magnetic material is much greater than the absolute permeability of air or vacuum. For example permeability of iron is 8000. Due to high permeability of magnetic materials such as iron, steel and other magnetic alloys they are widely used for the cores of all electromagnetic equipments.

Relation between B and H (Magnetic flux density and Magnetic field strength) The flux density B in a material is directly proportional the applied magnetizing force H. B ∝ H μ The ration B/H in a material is always constant and is equal to the absolute permeability μ (= μo μr ) Therefore B = μo μr H in a medium B = μo H in air Suppose a magnetizing force H produces a flux density Bo in air. Clearly Bo = μo H. If air is replaced by some other material and magnetizing force H is applied then flux density in the material will be Bmaterial = μo μr H. Therefore Bmaterial = μo μr H. Bo μo H = μr Hence the relative permeability of a material is equal to the ration of flux density produced in that material to the flux density produced in air by the same magnetizing force.

Classifications of Materials The magnetic properties of a material depend upon its relative permeability. (i) Materials that have relative permeability slightly greater than 1 but much less than 1. 1 are called paramagnetic materials. Aluminum and platinum are such materials. Paramagnetic materials are weakly attracted by a magnetic field. (ii) There are few materials with relative permeability slightly less than 1. These materials are called diamagnetic materials. Carbon, copper and silver are such materials. Diamagnetic materials are very slightly repelled by a magnetic field. This effect is so slight that it can only be detected with an extremely intense field and intense instruments. (iii) Materials that have relative permeability much greater than 1 are called ferromagnetic materials. Ferromagnetic materials are strongly attracted by a magnetic field. They are used in the construction of motors, transformers, relays and other magnetic devices. Materials with relative permeability very close to 1 are often just called nonmagnetic materials. Thus both paramagnetic and diamagnetic can be classified as nonmagnetic. For practical purposes, they are neither attracted nor repelled by magnetic field.

Permeability Problems: Formulas: Relative Permeability Absolute Permeability of air or Vacuum = μo = 4. π. 10 -7 H/m Absolute Permeability of other materials = μ B = μo H in air B = μo μr H in a medium Bmaterial / Bo = μr Ex 1. The relative permeability of ferromagnetic material is 10000. Its absolute will be? Ex 2. The absolute permeability of a material having flux density of 1 Wb/m 2 is 10 -3 H/m. The value of magnetizing force will be? Ex 3. The magnetic flux density in an air cooled coil is 10 -2 Wb/m 2. With a cast iron core of relative permeability 100 inserted, the flux density will become? Ex 4. The absolute permeability of a material having flux density of 2 Wb/m 2 is 10 -4 H/m. The value of magnetizing force will be? Answers (4π× 10 -3 H/m) (1000 AT/m) (1 Wb/m 2)

Magnetic Force on a Current-Carrying Conductor When an electrical wire is exposed to a magnet, the current in that wire will experience a force—the result of a magnet field. When an electrical wire is exposed to a magnet, the current in that wire will be affected by a magnetic field. The effect comes in the form of a force. The expression for magnetic force on current can be found by summing the magnetic force on each of the many individual charges that comprise the current. Since they all run in the same direction, the forces can be added. The force (F) a magnetic field (B) exerts on an individual charge (q) traveling at drift velocity v is: F=qv. Bsinθ

In this instance, θ represents the angle between the magnetic field and the wire. If B is constant throughout a wire then for a wire with N charge carriers in its total length l, the total magnetic force on the wire is: F=Nqv. Bsinθ.

Given that N=n. V, where n is the number of charge carriers per unit volume and V is volume of the wire, and this volume is calculated as the product of the circular cross-sectional area A and length (V=Al), yields the equation: F=(nq. Av)l. Bsinθ. The terms in brackets are equal to current (I), and thus the equation can be rewritten as: F=Il. Bsinθ The direction of the magnetic force can be determined using the right hand rule, The thumb is pointing in the direction of the current, with the four other fingers parallel to the magnetic field. Curling the fingers reveals the direction of magnetic force. The charge carriers have a charge (q) and move through the material at a velocity (v). This velocity is called the drift velocity. Within the material not all the charged particles are free to move, the carrier density (n) is the number of charge carriers free to move per cubic meter. Thus the total charge passing each second is n. Avq - this is the current This is known as the transport equation I=nq. Av

Ex: A wire of length 2 m carries a current of 10 A. What is the force acting on it when it is placed at an angle of 45 degree to the uniform magnetic field of 0. 15 T? Ex: A wire of length 2500 mm carries a current of 1500 m. A. A force of 4 N is acting on it an angle of 45 degree. What will be the value of magnetic field ? Ex: A force of 2. 5 N is acting on a wire of length 3 m and it carries a current of 17 A at an angle of 65 degree. What will be the value of magnetic field ?