- Slides: 73
NORTH Pole N MAGNETIC FIELD MAGNET SOUTH Pole S
Paper N Needle S Thumb Nail Copper Cable
MAGNETIC CIRCUIT, ELECTROMAGNETISM AND ELECTROMAGNETIC INDUCTION
The end of lesson, students should be ; q q q q Understand magnetism Understand the composite series magnetic circuit Understand the electrical and magnetic quantities Understand hysteresis Understand electromagnetism Determine the magnetic field direction. Understand electromagnetic induction
i. NTRo. DUct. ION MAGNET is the material that have two poles NORTH and SOUTH NORTH Pole N SOUTH Pole S
i. NTRo. DUct. ION MAGNET can be define as Needle Material that can attract piece of iron or metal N S Thumb Nail
i. NTRo. DUct. ION MATERIAL that ATTRACTED by the MAGNET is known as Needle MAGNETIC SUBSTANCES S Thumb Nail
i. NTRo. DUct. ION The ABILITY to ATTRACT the MAGNETIC SUBSTANCES is known Needle as MAGNETISM S Thumb Nail
i. NTRo. DUct. ION MAGNETIC FIELD is the force around the MAGNET which can attract any MAGNETIC MATERIAL around it.
FLUX MAGNET is the line around the MAGNET bar which form MAGNETIC FIELD. N S
TYp. Es of MAGNET There are 2 types of ¢ PURE MAGNET ¢ MANUFACTURE MAGNET
PURE MAGNET Known as MAGNET STONE The stone ORIGINALY have the NATURAL MAGNETIC Basically the stone is found in the form of IRON ORE
MANUFACTURE MAGNET There are 2 types of MANUFACTURE MAGNET ¢ PERMANENT MAGNET ¢ TEMPORARY MAGNET
PERMANENT MAGNET The ABILITY of the MAGNET to kept its MAGNETISM The basic shape of PERMANENT MAGNET U shape horseshoe ROD Cylinder BAR
PERMANENT MAGNET U shape Horseshoe Rod Cylinder Bar
PERMANENT MAGNET Permanent magnet can be obtained by: ◦ naturally or magnetic induction ( metal rub against natural magnet) ◦ placing a magnet into the coil and then supplied with a high electrical current.
PERMANENT MAGNET Permanent magnet used in small devices such as: speakers meter compass
TEMPORARY MAGNET BECOME MAGNET only when there is CURRENT SUPPLY to the metal It has magnetic properties when subjected to magnetic force and it will be lost when power is removed.
TEMPORARY MAGNET Example : relay electric bells
CHARACTERISTICS OF MAGNETIC FORCE LINES (FLUX). Magnetic flux lines have direction and pole. The direction of movement outside of the magnetic field lines is from north to south.
CHARACTERISTICS OF MAGNETIC FORCE LINES (FLUX). The strongest magnetic field are at the magnetic poles. DIFFERENT POLES ATTRACT each other N N S S SAME MAGNETIC POLES will REPEL each other N S S N
CHARACTERISTICS OF MAGNETIC FORCE LINES (FLUX). FLUX form a complete loop and never intersect with each other. FLUX will try to form a loop as small as possible. N S
MAGNETIC QUANTITY CHARACTERISTICS Magnetic Flux Magnetic flux is the amount of magnetic field produced by a magnetic source. The symbol for magnetic flux is . The unit for magnetic flux is the weber, Wb.
MAGNETIC QUANTITY CHARACTERISTICS Magnet Flux density The symbol for magnetic flux density is B. The unit is tesla, T the unit for area A is m 2 where 1 T = 1 Wb/m.
MAGNETIC QUANTITY CHARACTERISTICS Magnet Flux density Magnetic flux density is the amount of flux passing through a defined area that is perpendicular to the direction of flux
MAGNETIC QUANTITY CHARACTERISTICS Magnetic flux density = Tesla
MAGNETIC QUANTITY CHARACTERISTICS Area, A Example 3 Flux, Φ A magnetic pole face has rectangular section having dimensions 200 mm by 100 mm. If the total flux emerging from the pole is 150 Wb, calculate the flux density. B?
MAGNETIC QUANTITY CHARACTERISTICS Solution 3 Magnetic flux, = 150 Wb = 150 x 10 -6 Wb Cross sectional area, A = 200 mm x 100 mm = 20 000 x 10 -6 m 2 Flux density, = 7. 5 m. T
MAGNETOMOTIVE FORCE (MMF) The force which creates the magnetic flux in a magnetic circuit is called magnetomotive force (mmf) - The mmf is produced when a current passes through a coil of wire. The mmf is the product of the number of turns(N) and current (I) through the coil. Formula , Fm = N x I Unit = Ampere Turns (A. T)
MAGNETIC FIELD STRENGTH, H (MAGNETISING FORCE) Defined as magnetomotive force, Fm per metre length of measurement being ampere-turn per metre. number of turns magnetomotive force Current average length of magnetic circuit
MAGNETIC FIELD STRENGTH, H (MAGNETISING FORCE) Example 1 Current, I A current of 500 m. A is passed through a 600 turn coil wound of a toroid of mean diameter 10 cm. Turn, NCalculate the magnetic field strength. Diameter, d H?
MAGNETIC FIELD STRENGTH, H (MAGNETISING FORCE) Solution 1 I = N= l = 0. 5 A 600 x 10 -2 m
MAGNETIC FIELD STRENGTH, H (MAGNETISING FORCE) Example 2 An iron ring has a cross-sectional area of 400 mm 2. The coil resistance is 474 Ω and the supply voltage is 240 V and a mean diameter of 25 cm. it is wound with 500 turns. Calculate the magnetic field strength, H
MAGNETIC FIELD STRENGTH, H (MAGNETISING FORCE) Solution 2 I = V/ R = 240 / 474 = 0. 506 A l = π D = π (25 x 10 -2) = 0. 7854 m H= H= H= 322. 13 AT/m
PERMEABILITY For air, or any other nonmagnetic medium, the ratio of magnetic flux density to magnetic field strength is constant , This constant is called the permeability of free space and is equal to 4 x 10 -7 H/m. µ 0
PERMEABILITY For any other non-magnetic medium, the ratio For all media other than free space
PERMEABILITY r is the relative permeability and is defined as r varies with the type of magnetic material.
PERMEABILITY r for a vacuum is 1 is called the absolute permeability. The approximate range of values of relative permeability r for some common magnetic materials are : Cast iron r = 100 – 250 Mild steel r = 200 – 800 Cast steel r = 300 – 900
PERMEABILITY Flux density, B Example 4 H A flux density of 1. 2 T is produced in a piece of cast steel by a magnetizing force of 1250 A/m. Find the relative permeability of the steel under these conditions. µ r?
PERMEABILITY Solution 4
RELUCTANCE Reluctance, S is the magnetic resistance of a magnetic circuit to presence of magnetic flux. Reluctance, The unit for reluctance is 1/H or H-1 or A-T/Wb
RELUCTANCE Example 5 S? Determine the reluctance of a piece of metal of length 150 mm and cross sectional area is 1800 mm 2 when the relative permeability is 4 000. Find also the absolute permeability of the metal. Length, l µ? µr
RELUCTANCE Solution 5 Reluctance, = = 16 579 H-1 Absolute permeability, = = 5. 027 x 10 -3 H/m
ELECTROMAGNET Is a magnetic iron core produced when the current flowing through the coil. Thus, the magnetic field can be produced when there is a current flow through a conductor.
The direction of the magnetic field can be determined using the method: 1. Right Hand Grip Rules 2. Maxwell's screw Law. 3. Compass Three rules may be used to indicate the direction of the current and the flux produced by current carrying conductor.
Right Hand Grip Rule is a physics principle applied to electric current passing through a solenoid, resulting in a magnetic field.
Right Hand Grip Rule When you wrap your right hand around the solenoid your thumb points in the direction of the magnetic north pole your fingers in the direction of the conventional current
Right Hand Grip Rule It can also be applied to electricity passing through a straight wire thumb points in the direction of the conventional current (from +ve to -ve) the fingers point in the direction of the magnetic lines of flux.
MAXWELL’S SCREW LAW Another way to determine the direction of the flux and current in a conductor is to use Maxwell's screw rule.
MAXWELL’S SCREW LAW a right-handed screw is turned so that it moves forward in the same direction as the current, its direction of rotation will give the direction of the magnetic field.
Electromagnetic Effect Direction of Current going INside Solenoid Direction of Magnetic Flux around Solenoid Right Hand Grip Rule Direction of Current going OUTside Solenoid
Electromagnetic Effect Direction of Current going OUTside Solenoid Direction of Current going INside Solenoid Maxwell Screw Law Same Direction of Magnetic Flux around Solenoid Different Direction of Magnetic Flux around Solenoid
Electromagnetic Effect Factors that influence the strength of the magnetic field of a solenoid The number of turns The value of current flow Types of conductors to produce coil The thickness of the conductor
ELECTROMAGNETIC INDUCTION Definition : When a conductor is moved across a magnetic field so as to cut through the flux, an electromagnetic force (emf) is produced in the conductor. This effect is known as electromagnetic induction. The effect of electromagnetic induction will cause induced current.
ELECTROMAGNETIC INDUCTION 2 laws of electromagnetic induction: i. Faraday’s law ii. Lenz’z Law
Faraday’s law It is a relative movement of the magnetic flux and the conductor then causes an emf and thus the current to be induced in the conductor. Induced emf on the conductor could be produced by 2 methods ◦ flux cuts conductor or ◦ conductor cuts flux.
Faraday’s law Faraday’s First Law : Flux cuts conductor When the magnet is moved towards the coil, a deflection is noted on the galvanometer showing that a current has been produced in the coil.
Faraday’s law Faraday’s Second Law : Conductor cuts flux When the conductor is moved through a magnetic field. An emf is induced in the conductor and thus a source of emf is created between the ends of the conductor.
Faraday’s law This induced electromagnetic field is given by E = Blv volts B=flux density, T l =length of the conductor in the magnetic field, m v =conductor velocity, m/s If the conductor moves at the angle to the magnetic field, then E = Blv sin volts
Faraday’s law Example A conductor 300 mm long moves at a uniform speed of 4 m/s at right-angles to a uniform magnetic field of flux density 1. 25 T. Determine the current flowing in the conductor when : a. its ends are open-circuited b. its ends are connected to a load of 20 resistance.
Faraday’s law Solution a. If the ends of the conductor are open circuited no current will flow.
Faraday’s law Solution b. E. m. f. can only produce a current if there is a closed circuit. When a conductor moves in a magnetic field it will have an e. m. f. induced. Induced e. m. f. , E = Blv =(1. 25)(0. 3)(4) = 1. 5 v From Ohm’s law
Lenz’z law The direction of an induced emf is always such that it tends to set up a current opposing the motion or the change of flux responsible for inducing that emf
Formula MAGNETOMOTIVE FORCE (MMF), Fm = N x I MAGNETIC FIELD STRENGTH MAGNETIC FLUX DENSITY PERMEABILITY RELUCTANCE
Composite magnetic circuit A series magnetic circuit that has parts of different dimensions and material is called composite magnetic circuit. Each part will have its own reluctance. Total reluctance is equal to the sum of reluctances of individual parts.
Comparison between magnetic and electric circuit
Similarities & dissimilarities between magnetic circuit and electric circuit
Similarities & dissimilarities between magnetic circuit and electric circuit
Hysterisis and hysterisis loss Figure 7. 6