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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. – PowerPoint PPT presentation

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Title: 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
  • Consider a straight conductor moving with a
    uniform velocity, v, in a stationary magnetic
  • The free charges in the conductor experience a
    force which will push them to one end of the
  • An electric field is built up due to the electron
  • An e.m.f. is generated across the conductor such
  • 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
Lenzs Law
  • The direction of the induced current is always so
    as to oppose the change which causes the current.

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

Faraday Disk Dynamo
Simple a.c. Generator
  • According to the Faradays law of electromagnetic

Simple d.c. Generator
Eddy Current
  • An eddy current is a swirling current set up in a
    conductor in response to a changing magnetic
  • Production of eddy currents in a rotating wheel

Applications of Eddy Current (1)
  • Metal Detector

Applications of Eddy Current (2)
  • Eddy current levitator
  • Smooth braking device
  • Damping of a vibrating system

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
  • 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
Multiplying by I, then
  • So the mechanical power developed in motor

Variation of current as a motor is started
Larger load
Zero load
  • 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 initial current, IoV/R, very large if R is
  • When the motor is running, the back emf
    increases, so the current decrease to its working
  • To prevent the armature burning out under a high
    starting current, it is placed in series with a
    rheostat, whose resistance is decreases as the
    motor gathers speed.

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
  • The output power is maximum when the back emf is
    ½ V.

  • 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 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
  • 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
  • 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
  • Disadvantage
  • Changing voltage with d.c. is more difficult and

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

L is numerically equal to the emf induced in the
circuit when the current changes at the rate of
1 A in each second.
  • 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

Inductance of a Solenoid
  • Since the magnetic flux density due to a solenoid
  • By the Faradays 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
  • 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.
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