• Michael Faraday and Joseph Henry discovered that a magnetic field could cause an electric current to flow in a conductor moving in the field

• The magnetic field exerts a force on the electrons in the wire, causing them to move

• This is called the GENERATOR EFFECT or ELECTROMAGNETIC INDUCTION

3 ways to induce a current:

Method 1: move the conductor in a magnetic field

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One end is positive and the other end is negative – a voltage is induced in the rod,

Use the third left hand rule: fingers in direction of magnetic field, thumb points in direction of WIRE MOTION, palm indicates the directions the electrons are forced

A current will flow in the conductor if a complete electrical circuit exists

For a moving conductor, the induced voltage is found by

V=LvB

where V = induced voltage L=length of conductor in the magnetic field in mv = speed in m/s B= magnetic field strength at right angles to the conductor

Method 2:A moving magnet will cause an induced voltage in a conductor placed in the field of the magnet

Magnet in coil

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Method 3: a changing magnetic field can cause an induced voltage in a conductor

insulated wire(primary coil)

insulated wire(secondary coil)

Faraday's Experimnt

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Iron Core

Metal Detector Animation and Explanation

• When the switch is closed, the current causes a magnetic field around the primary coil which increases as the current rises from zero to the maximum value

• The iron core concentrates the field lines so the secondary coil is in the magnetic field of the primary coil.

• The changing magnetic field around the primary causes a changing magnetic field around the secondary coil

• The changing secondary magnetic field will cause a voltage to be induced in the secondary coil

• The direction of current flow in the secondary will be in the opposite direction of current flow in the primary

•Once the primary current reaches its maximum, the magnetic field is no longer changing and no voltage is induced in the secondary

• When the switch is opened (current is turned off) the magnetic field around the primary begins to reduce and induces a voltage in the secondary coil again

• The magnitude of the induced potential difference depends on the:

☺ strength of the primary magnetic fields☺ how quickly the magnetic field is changing

• Faraday’s Law of Induction:☺An induced voltage is produced by a changing magnetic field.☺The induced potential difference depends on the rate of change of the magnetic flux through the coil.

Reading Magnetic Tape:

Writing to a Magnetic Tape:

Read pages 694 to 698; 738 to 740