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Induced Voltages And Inductance. Chapter 20 Hans Christian Oersted. Introduction. Oersted’s discovery was the first evidence of a link between electricity and magnetism. Symmetry in Physics. Symmetry in nature helps scientists to make new discoveries. Generating Electricity.

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Induced Voltages And Inductance

Chapter 20

Hans Christian Oersted

Introduction l.jpg

  • Oersted’s discovery was the first evidence of a link between electricity and magnetism.

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Symmetry in Physics

  • Symmetry in nature helps scientists to make new discoveries.

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Generating Electricity

  • Could magnetic fields produce electric currents?

    • Joseph Henry made this discovery

      • Power plants were the result of his efforts

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Induced emf And Magnetic Flux

  • Faraday’s experiment

    • A current can be produced by a changing magnetic field.


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Primary and Secondary Coils

  • A steady magnetic field in the primary coil cannot produce a current in the secondary coil.

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Induced emf And Magnetic Flux

  • Magnetic flux (F)

    • The flux is proportional to the number of lines passing through the loop

    • Changes in the flux induce a change in the emf.

  • 20.2

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Magnetic Flux Formula

  • q is the angle between the B field and

  • the normal to the plane of the loop

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Faraday’s Law of Induction

  • A simple demonstration

    • Wire loop, galvanometer, magnet


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Induced Current

  • A current is set up in the loop as long as there is relative motion between the magnetic field and the loop.

    • This is an induced current.


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Faraday’s Law of Induction

  • The induced emf (e) in a circuit equals the rate of change of the magnetic flux through the circuit.

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Faraday’s Law of Induction

Since F = B.A.cosq

Ande = -N.DF/Dt

Thene = -N.D (B.A.cosq) /Dt

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Lenz’s Law

  • The induced current tries to maintain the original flux through the circuit.

    • It sets up a magnetic field that opposes any change in the original magnetic field.

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Applications of Faraday’s Law

  • Electric guitar

    • Metal strings

    • Pickup coil

  • Cruise control

  • Ground fault interrupter (GFI)

    • Protects against electrical shock

    • Where is it used?

    • How does it work?

  • SIDS monitor

    20.10a, 209, 20.8, 214, 20.5

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Motional emf

  • Motional emf is the emf induced in a conductor moving through a magnetic field.

    • Example: A straight conductor moving through a magnetic field

      20.8, 20.13

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Motional emf

  • A potential difference (DV) is maintained across a conductor as long as there is motion through the field.

    • If the direction of the motion is reversed, the polarity also reverses.

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Motional emf

  • Motion involving a closed conducting path

    • Current flows through the circuit

      162, 211

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Lenz’s Law Revisited

  • Another example (Figure 20.17):

    • A stationary conducting loop and a bar magnet

      215, 20.13, 165

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Applications of Magnetic Induction

  • Cassette tape recorders/players

    • Magnetic tape

    • Recording head

    • Playback head

  • VHS recorders/players

  • Computer hard drives

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1 - 7

Pg. 689

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AC Generators

  • The alternating current (ac) generator

    • Converts mechanical energy to electrical energy

    • Has a coil rotating in a magnetic field

      • Slip rings

        • Insure that the output voltage changes polarity

      • Stationary brushes

        20.20, 216

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Types of Commercial Power Plants

  • Fossil Fuel

  • Hydroelectric

  • Nuclear

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Total emf

  • Formula for total emf:

    Note: wt = q and w = 2pf (f = 60 Hz in USA)

    qis measured between the magnetic field and the normal to the loop.

  • Maximum emf occurs when the plane of the loop is parallel to the magnetic field


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DC Generators

  • The direct current (dc) generator

    • Uses a split ring or commutator

      • This insures that the output voltage does not change polarity.


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Motors and Back emf

  • A motor is the opposite of a generator.

    • Because of Lenz’s Law, back emf is generated in a motor.

      • Back emf increases with rotational speed.

      • Back emf tends to reduce current flow in the windings.

        • Maximum current is present when the motor starts up.


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Eddy Currents

  • Eddy currents are circular currents which occur in a piece of metal when it moves through a magnetic field

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Magnetic Damping

  • The magnetic fields produced by the eddy currents try to prevent motion of the metal through the field.

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Effects of Eddy Currents

  • The effects of eddy currents are undesirable in motors and generators.

    • These effects can be reduced

      • Laminations are used

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  • A changing flux through the circuit arises from the circuit itself.

    • This occurs in coils and solenoids

      • Solenoid Video

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Self-Induced emf

  • The self induced emf is proportional to the time rate of change of current

    L is a constant representing the inductance of the device and is measured in Henries (H).

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  • Formula for inductance (L)

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  • An inductor is a circuit element which is used to provide inductance.

    • Usually a closely wrapped coil of many turns

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  • Inductance (L) is a measure of the opposition to the rate of change of current.

    • Schematic symbol


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RL Circuits

  • RL time constant (t)

    • The time that it takes for the current in the circuit to reach 63.2 % of its maximum value.

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Energy Stored In A Magnetic Field

  • Current flowing through a solenoid produces a magnetic field.

    • The battery must do work to produce a current in a coil.

    • This energy is stored in the magnetic field of the coil.

      37-1, 14, 78

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8,10, 11, 13, 14

Pg. 689