Magnetism and Electromagnetism Engr. Faheemullah Shaikh

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• Magnetism and ElectromagnetismEngr. Faheemullah Shaikh

• Wire CoilNotice that a current carrying coil of wire will produce a perpendicular field

• Magnetic Field: CoilA series of coils produces a field similar to a bar magnet but weaker!

• Magnetic Field: Coil

• Magnetic FieldFlux can be increased by increasing the current I,I I

• Magnetic FieldFlux can be increased by increasing the number of turns N,I NN

• Magnetic FieldFlux can be increased by increasing the cross-section area of coil A,I ANA

• Magnetic FieldFlux can be increased by increasing the cross-section area of coil A,I ANA

• Magnetic FieldFlux is decreased by increasing the length of coil l,INA1ll

• Magnetic FieldTherefore we can write an equation for flux as,INANIAllor

• Where 0 is vacuum or non-magnetic material permeability0 = 4 x 10-7 H/mMagnetic Field

• Solenoid If a coil is wound on a steel rod and connected to a battery, the steel becomes magnetized and behaves like a permanent magnet.

• Magnetic Field: CoilPlacing a ferrous material inside the coil increases the magnetic fieldActs to concentrate the field also notice field lines are parallel inside ferrous elementflux density has increased

• Magnetic FieldBy placing a magnetic material inside the coil,INAlWhere is the permeability of the magnetic material (core).

• Magnetic FieldBy placing a magnetic material inside the coil,INAlWhere is the permeability of the magnetic material (core).

• Flux Density

• PermeabilityPermeability is a measure of the ease by which a magnetic flux can pass through a material (Wb/Am)Permeability of free space o = 4 x 10-7 (Wb/Am)Relative permeability:

• ReluctanceReluctance: resistance to flow of magnetic flux

Associated with magnetic circuit flux equivalent to currentWhats equivalent of voltage?

• Magnetomotive Force, FCoil generates magnetic field in ferrous torroidDriving force F needed to overcome torroid reluctance Magnetic equivalent of ohms law

• Circuit Analogy

• Magnetomotive ForceThe MMF is generated by the coilStrength related to number of turns and current, measured in Ampere turns (At)

• Magnetic Field IntensityThe longer the magnetic path the greater the MMF required to drive the fluxMagnetomotive force per unit length is known as the magnetizing force H

Magnetizing force and flux density related by:

• Electric circuit:Emf = V = I x R Magnetic circuit:mmf = F = x = (B x A) x= (B x A) x= B x= H x l= H x l

• Magnetic Force On A Current Carrying Conductor

• Magnetic Force On A Current Carrying Conductor The magnetic force (F) the conductor experiences is equal to the product of its length (L) within the field, the current I in the conductor, the external magnetic field B and the sine of the angle between the conductor and the magnetic field. In short F= BIL (sin)

• The force on a current-carrying conductor in a magnetic field: When a current-carrying conductor is placed in a magnetic field, there is an interaction between the magnetic field produced by the current and the permanent field, which leads to a force being experienced by the conductor:

• The magnitude of the force on the conductor depends on the magnitude of the current which it carries. The force is a maximum when the current flows perpendicular to the field (as shown in diagram A on the left below), and it is zero when it flows parallel to the field (as in diagram B, on the right):

• Fleming's left hand rule shows the direction of the thrust on a conductor carrying a current in a magnetic field.

The left hand is held with the thumb, index finger and middle finger mutually at right angles.Fleming's left hand rule (for electric motors)The First finger represents the direction of the Field. The Second finger represents the direction of the Current (in the classical direction, from positive to negative). The Thumb represents the direction of the Thrust or resultant Motion.

• Flemings left-hand rule

• The directional relationship of I in the conductor, the external magnetic field and the force the conductor experiencesIFB

• Magnetic Field can produce an electric current in a closed loop, if the magnetic flux linking the surface area of the loop changes with time. The electric Current Produced Induced CurrentThis mechanism is called Electromagnetic Induction

• First ExperimentsConducting loopSensitive current meterSince there is no battery or other source of emf included, there is no current in the circuitMove a bar magnet toward the loop, a current suddenly appears in the circuitThe current disappears when the bar magnet stopsIf we then move the bar magnet away, a current again suddenly appears, but now in the opposite direction

• Discovering of the First ExperimentsA current appears only if there is relative motion between the loop and the magnet3. If moving the magnets N-pole towards the loop causes clockwise current, then moving the N-pole away causes counterclockwise.2. Faster motion produces a greater current

• Constant flux, no current is induced in the loop. No current detected by GalvanometerAn Experiment - Situation AConstant flux though the loopDC current I, in coil produces a constant magnetic field, in turn produces a constant flux though the loop

• An Experiment - Situation B: Disconnect battery suddenlyMagnetic field drops to zeroSudden change of magnetic flux to zero causes a momentarily deflection of Galvanometer needle.

• Link: http://micro.magnet.fsu.edu/electromag/java/faraday/index.htmlAn Experiment - Situation C: Reconnect BatteryMagnetic field becomes non-zeroDeflection of Galvanometer needle in the opposite direction

• Conclusions from the experiment

Current induced in the closed loop when magnetic flux changes, and direction of current depends on whether flux is increasing or decreasing

If the loop is turned or moved closer or away from the coil, the physical movement changes the magnetic flux linking its surface, produces a current in the loop, even though B has not changedIn Technical TermsTime-varying magnetic field produces an electromotive force (emf) which establish a current in the closed circuit

• 3. A combination of the two above, both flux changing and conductor moving simultaneously. A closed path may consists of a conductor, a capacitor or an imaginary line in space, etc.Electromotive force (emf) can be obtained through the following ways:1. A time-varying flux linking a stationary closed path. (i.e. Transformer)2. Relative motion between a steady flux and a close path. (i.e. D.C. Generator)

• Faraday summarized this electromagnetic phenomenon into two laws ,which are called the Faradays lawFaradays First LawWhen the flux magnet linked to a circuit changes, an electromotive force (emf) will be induced.

• Faradays Second LawThe magnetic of emf induced is equal to the time rate of change of the linked magnetic flux F.Minus Sign Lenzs LawIndicates that the emf induced is in such a direction as to produces a current whose flux, if added to the original flux, would reduce the magnitude of the emf(volts)

• Minus Sign Lenzs LawThe induced voltage acts to produce an opposing flux

• Minus Sign Lenzs LawThe induced voltage acts to produce an opposing flux

• Minus Sign Lenzs LawThe induced voltage acts to produce an opposing flux

• Heinrich F.E. Lenz Russian physicist (1804-1865)1834 Lenzs LawThere is an induced current in a closed conducting loop if and only if the magnetic flux through the loop is changing.Indicates that the emf induced is in such a direction as to produces a current whose flux, if added to the original flux, would reduce the magnitude of the emf

• There is an induced current in a closed conducting loop if and only if the magnetic flux through the loop is changing. The direction of the induced current is such that the induced magnetic field always opposes the change in the flux.

• Right Hand RuleIf you wrap your fingers around the coil in the direction of the current, your thumb points north.

• 2Direction of induced currentIn both cases, magnet moves against a force.Work is done during the motion & it is transferred as electrical energy.Induced I always flows to oppose the movement which started it.bLenz's law

• Applications of Magnetic InductionMagnetic Levitation (Maglev) TrainsInduced surface (eddy) currents produce field in opposite direction Repels magnet Levitates train

Maglev trains today can travel up to 310 mph Twice the speed of Amtraks fastest conventional train!

• Liner induction0-70 mph in 3 sec

• FALLING MAGNETThe copper tube "sees" a changing magnetic field from the falling magnet. This changing magnetic field induces a current in the copper tube. The induced current in the copper tube creates its own magnetic field that opposes the magnetic field that created it.

• Fleming Right Hand RuleDirection of Induced e.m.f, Magnetic Flux, Conductor Motion

• Fleming's right hand rule shows the direction of induced current flow when a conductor moves in a magnetic field.

The right hand is held with the thumb, first finger and second finger mutually at right angles, as shown in the diagram Fleming's right hand rule (for generators)The Thumb represents the direction of Motion of the conductor. The First finger represents

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