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 Could magnetic fields produce electric currents? Joseph Henry made this discovery Power plants were the result of his efforts

Induced emf And Magnetic Flux Faraday’s experiment A current can be produced by a changing magnetic field. 20.1

Primary and Secondary Coils A steady magnetic field in the primary coil cannot produce a current in the secondary coil.

Induced emf And Magnetic Flux Magnetic flux  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

Magnetic Flux Formula   is the angle between the B field and  the normal to the plane of the loop

Faraday’s Law of Induction A simple demonstration Wire loop, galvanometer, magnet 20.4

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. 20.4

Faraday’s Law of Induction The induced emf (  ) in a circuit equals the rate of change of the magnetic flux through the circuit.

Formula for Faraday’s Law of magnetic induction

Faraday’s Law of Induction Since   A. cos  And  = -N.  /  t Then  = -N.   A. cos  /  t

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.

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

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

Formula:

Motional emf A potential difference (  V) 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.

Motional emf Motion involving a closed conducting path Current flows through the circuit 162, 211

Important Formulas :

Lenz’s Law Revisited Another example (Figure 20.17): A stationary conducting loop and a bar magnet 215, 20.13, 165

Applications of Magnetic Induction Cassette tape recorders/players Magnetic tape Recording head Playback head VHS recorders/players Computer hard drives

Questions Pg. 689

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

Types of Commercial Power Plants Fossil Fuel Hydroelectric Nuclear

Total emf Formula for total emf: Note  t =  and  f (f = 60 Hz in USA)  is 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

DC Generators The direct current (dc) generator Uses a split ring or commutator This insures that the output voltage does not change polarity

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. 217

Back emf vs. Speed

Equation for back emf

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

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

Effects of Eddy Currents The effects of eddy currents are undesirable in motors and generators. These effects can be reduced Laminations are used

Laminations in an Armature

Self-Inductance A changing flux through the circuit arises from the circuit itself. This occurs in coils and solenoids Solenoid Video Solenoid

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).

Inductance Formula for inductance (L)

Inductors An inductor is a circuit element which is used to provide inductance. Usually a closely wrapped coil of many turns

Inductance Inductance (L) is a measure of the opposition to the rate of change of current. Schematic symbol 20.27/20.28

RL Circuits RL time constant (  ) The time that it takes for the current in the circuit to reach 63.2 % of its maximum value.

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

Questions 8,10, 11, 13, 14 Pg. 689