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# Induced Voltages And Inductance - PowerPoint PPT Presentation

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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Presentation Transcript

Chapter 20

Hans Christian Oersted

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

• Symmetry in nature helps scientists to make new discoveries.

• Could magnetic fields produce electric currents?

• Joseph Henry made this discovery

• Power plants were the result of his efforts

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

20.1

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

• 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

• q is the angle between the B field and

• the normal to the plane of the loop

• A simple demonstration

• Wire loop, galvanometer, magnet

20.4

• 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.

20.4

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

Since F = B.A.cosq

Ande = -N.DF/Dt

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

• 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.

• 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

• 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

• 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.

• Motion involving a closed conducting path

• Current flows through the circuit

162, 211

• Another example (Figure 20.17):

• A stationary conducting loop and a bar magnet

215, 20.13, 165

• Cassette tape recorders/players

• Magnetic tape

• VHS recorders/players

• Computer hard drives

1 - 7

Pg. 689

• 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

• Fossil Fuel

• Hydroelectric

• Nuclear

• 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

213

• The direct current (dc) generator

• Uses a split ring or commutator

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

20.22

• 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.

217

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

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

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

• These effects can be reduced

• Laminations are used

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

• This occurs in coils and solenoids

• Solenoid Video

• 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).

• Formula for inductance (L)

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

• Usually a closely wrapped coil of many turns

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

• Schematic symbol

20.27/20.28

• RL time constant (t)

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

• 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

8,10, 11, 13, 14

Pg. 689