Electromagnetic Induction

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# Electromagnetic Induction - PowerPoint PPT Presentation

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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Presentation Transcript
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

• 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
• 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

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
• 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
• 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
• 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)

• The induced e.m.f. in a circuit is equal to the rate of change of magnetic flux linkage through the circuit.

oppose the change.

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

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
• According to the Faraday’s law of electromagnetic induction,

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

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.

Armature coils, R

Back emf, Eb

Driving source, V

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

MultiplyingbyI, then

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.

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.

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.

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
• 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)

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

• Metal Detector
Applications of Eddy Current (2)
• Eddy current levitator
• Smooth braking device
• Damping of a vibrating system
Applications of Eddy Current (3)
• Induction stove
• Critical damping in the armature
• of a moving-coil galvanometer.
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

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
• 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

Thick copper wire

Closed loop laminated iron core

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 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
• 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.
• Changing voltage with d.c. is more difficult and expensive.
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)
• 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)
• 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
• 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
• Since the magnetic flux density due to a solenoid is
• By the Faraday’s law of electromagnetic induction,
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
• 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
• 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
• 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
• 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
• 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.