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Electromagnetic Induction. emf is induced in a conductor placed in a magnetic field whenever there is a change in magnetic field. Moving Conductor in a Magnetic Field. http://www.ngsir.netfirms.com/englishhtm/Induction.htm.

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electromagnetic induction
Electromagnetic Induction
  • emf is induced in a conductor placed in a magnetic field whenever there is a change in magnetic field.
moving conductor in a magnetic field
Moving Conductor in a Magnetic Field

http://www.ngsir.netfirms.com/englishhtm/Induction.htm

  • Consider a straight conductor moving with a uniform velocity, v, in a stationary magnetic field.
  • The free charges in the conductor experience a force which will push them to one end of the conductor.
  • An electric field is built up due to the electron accumulation.
  • An e.m.f. is generated across the conductor such that

E = Blv.

induced current in wire loop
Induced Current in Wire Loop
  • An induced current passes around the circuit when the rod is moved along the rail.
  • The induced current in the rod causes a force F = IlB, which opposes the motion.
  • Work done by the applied force to keep the rod moving is
  • Electrical energy is produced from the work done such that

E = E It = W

E= Blv

lenz s law
Lenz’s Law

http://www.launc.tased.edu.au/online/sciences/physics/Lenz's.html

  • The direction of the induced current is always so as to oppose the change which causes the current.
copper pipe experiment
Copper Pipe Experiment
  • This is a simplified diagram showing the areas of attraction and repulsion in this experiment.

http://regentsprep.org/Regents/physics/phys08/clenslaw/default.htm

lenz s law and law of conservation of energy
Lenz’s Law and Law of Conservation of Energy
  • Where does all the kinetic energy of the bar magnet go?
    • Well in fact, the Ek is transformed into electrical energy.
    • So this is the source of the emf, transferred from other energy into electrical energy.
    • Energy would be created from nothing if the induced current acted differently.
magnetic flux

Magnetic Flux
  • The magnetic flux is a measure of the number of magnetic field lines linking a surface of cross-sectional area A.
  • The magnetic flux through a small surface is the product of the magnetic flux density normal to the surface and the area of the surface.

Unit : weber (Wb)

faraday s law of electromagnetic induction
Faraday’s Law of Electromagnetic Induction
  • The induced e.m.f. in a circuit is equal to the rate of change of magnetic flux linkage through the circuit.

The ‘-’ sign indicates that the induced e.m.f. acts to

oppose the change.

http://www.physics.uoguelph.ca/applets/Intro_physics/kisalev/java/indcur/

induced currents caused by changes in magnetic flux
Induced Currents Caused by Changes in Magnetic Flux
  • The magnetic flux (number of field lines passing through the coil) changes as the magnet moves towards or away from the coil.

http://micro.magnet.fsu.edu/electromag/java/lenzlaw/index.html

simple a c generator
Simple a.c. Generator
  • According to the Faraday’s law of electromagnetic induction,

http://www.walter-fendt.de/ph11e/generator_e.htm

back emf in motors
Back emf in Motors
  • When an electric motor is running, its armature windings are cutting through the magnetic field of the stator. Thus the motor is acting also as a generator.
  • According to Lenz's Law, the induced voltage in the armature will oppose the applied voltage in the stator.
  • This induced voltage is called back emf.
back emf and power

Armature coils, R

Back emf, Eb

Driving source, V

Back emf and Power
  • So the mechanical power developed in motor

MultiplyingbyI, then

variation of current with the steady angular speed of the coil in a motor

I

0

Variation of current with the steady angular speed of the coil in a motor
  • The maximum speed of the motor occurs when the current in the motor is zero.
variation of output power with the steady angular speed of the coil in a motor

Po

0

Variation of output power with the steady angular speed of the coil in a motor
  • The output power is maximum when the back emf is ½ V.
variation of current as a motor is started

I

t

0

Variation of current as a motor is started

The motor begins to move

  • As the coil rotates, the angular speed as well as the back emf increases and the current decreases until the motor reaches a steady state.

Larger load

Zero load

the need for a starting resistance in a motor
The need for a starting resistance in a motor
  • When the motor is first switched on,  =0.
  • The maximum current, Io=V/R, very large if R is small.
  • When the motor is running, the back emf increases, so the current decrease to its working value.
  • To prevent the armature burning out under a high starting current, it is placed in series with a rheostat, whose resistance is decreased as the motor gathers speed.
eddy current
Eddy Current
  • An eddy current is a swirling current set up in a conductor in response to a changing magnetic field.
  • Production of eddy currents in a rotating wheel
applications of eddy current 1
Applications of Eddy Current (1)

http://micro.magnet.fsu.edu/electromag/java/detector/index.html

  • Metal Detector
applications of eddy current 2
Applications of Eddy Current (2)
  • Eddy current levitator
  • Smooth braking device
  • Damping of a vibrating system
applications of eddy current 3
Applications of Eddy Current (3)
  • Induction stove
  • Critical damping in the armature
  • of a moving-coil galvanometer.
how an induction stove works
How an induction stove works
  • The element's electronics power a coil that produces a high-frequency electromagnetic field.
  • The field penetrates the metal of the ferrous (magnetic-material) cooking vessel and sets up a circulating electric current, which generates heat.
  • The heat generated in the cooking vessel is transferred to the vessel's contents.
  • Nothing outside the vessel is affected by the field--as soon as the vessel is removed from the element, or the element turned off, heat generation stops.
transformer
Transformer

http://micro.magnet.fsu.edu/electromag/java/transformer/index.html

  • A transformer is a device for stepping up or down an alternating voltage.
  • For an ideal transformer,
    • (i.e. zero resistance and no flux leakage)
transformer energy losses
Transformer Energy Losses
  • Heat Losses
    • Copper losses- Heating effect occurs in the copper coils by the current in them.
    • Eddy current losses- Induced eddy currents flow in the soft iron core due to the flux changes in the metal.
  • Magnetic Losses
    • Hysteresis losses- The core dissipates energy on repeated magnetization.
    • Flux leakage- Some magnetic flux does not pass through the iron core.
designing a transformer to reduce power losses
Designing a transformer to reduce power losses

Thick copper wire

Closed loop laminated iron core

designing a transformer to reduce power losses27
Designing a transformer to reduce power losses
  • Thick copper wire of low resistance is used to reduce the heating effect (I2R).
  • The iron core is laminated, the high resistance between the laminations reduces the eddy currents as well as the heat produced.
  • The core is made of very soft iron, which is very easily magnetized and demagnetized.
  • The core is designed for maximum linkage, common method is to wind the secondary coil on the top of the primary coil and the iron core must always form a closed loop of iron.
transmission of electrical energy29
Transmission of Electrical Energy
  • Wires must have a low resistance to reduce power loss.
  • Electrical power must be transmitted at low currents to reduce power loss.
  • To carry the same power at low current we must use a high voltage.
  • To step up to a high voltage at the beginning of a transmission line and to step down to a low voltage again at the end we need transformers.
direct current transmission
Direct Current Transmission
  • Advantages
    • a.c. produces alternating magnetic field which induces current in nearby wires and so reduce transmitted power; this is absent in d.c.
    • It is possible to transmit d.c. at a higher average voltage than a.c. since for d.c., the rms value equals the peak; and breakdown of insulation or of air is determined by the peak voltage.
  • Disadvantage
    • Changing voltage with d.c. is more difficult and expensive.
self induction
Self Induction
  • When a changing current passes through a coil or solenoid, a changing magnetic flux is produced inside the coil, and this in turn induces an emf.
  • This emf opposes the change in flux and is called self-induced emf.
  • The self-induced emf will be against the current if it is increasing.
  • This phenomenon is called self-induction.
definitions of self inductance 1
Definitions of Self-inductance (1)
  • Definition used to find L

The magnetic flux linkage in a coil  the current flowing through the coil.

Where L is the constant of proportionality for the coil.

L is numerically equal to the flux linkage of a circuit when unit current flows through it.

Unit : Wb A-1 or H (henry)

definitions of self inductance 2
Definitions of Self-inductance (2)
  • Definition that describes the behaviour of an inductor in a circuit

Lis numerically equal to the emf induced in the circuit

when the current changes at the rate of 1 A in each second.

inductors
Inductors
  • Coils designed to produce large self-induced emfs are called inductors (or chokes).
  • In d.c. circuit, they are used to slow the growth of current.
  • Circuit symbol

or

inductance of a solenoid
Inductance of a Solenoid
  • Since the magnetic flux density due to a solenoid is
  • By the Faraday’s law of electromagnetic induction,
energy stored in an inductor
Energy Stored in an Inductor
  • The work done against the back emf in bringing the current from zero to a steady value Io is
current growth in an rl circuit
Current growth in an RL circuit
  • At t = 0, the current is zero.
  • So
  • As the current grows, the p.d. across the resistor increases. So the self-induced emf ( - IR) falls; hence the rate of growth of current falls.
  • As t
decay of current through an inductor
Decay of Current through an Inductor
  • Time constant for RL circuit
  • The time constant is the time for current to decrease to 1/e of its original value.
  • The time constant is a measure of how quickly the current grows or decays.
emf across contacts at break

-

+

emf across contacts at break
  • To prevent sparking at the contacts of a switch in an inductive circuit, a capacitor is often connected across the switch.

The energy originally stored

in the magnetic field of the coil

is now stored in the electric

field of the capacitor.

switch design

-

+

Switch Design
  • An example of using a protection diode with a relay coil.
  • A blocking diode parallel to the inductive coil is used to reduce the high back emf present across the contacts when the switch opens.
non inductive coil
Non-Inductive Coil
  • To minimize the self-inductance, the coils of resistance boxes are wound so as to set up extremely small magnetic fields.
  • The wire is double-back on itself. Each part of the coil is then travelled by the same current in opposite directions and so the resultant magnetic field is negligible.