Using Magnetism to Induce an Electric Current

1
Using Magnetism to Induce an Electric Current

• Electromagnetic Induction is used to generate
  most of the electrical energy used today

2
The Discovery of Electromagnetic Induction

• The production of electricity by magnetism is
  called electromagnetic induction.

• Michael Faraday first demonstrated that magnetism
  can produce electricity.
• Faraday showed that when a magnet approaches a
  coil, a current is induced in the coil.
• The direction of induced current depends on the
  pole of the magnet that approaches the coil.
• A stationary magnet will not induce current.
  There must be motion of the coil or magnet to
  induce current.

A galvanometer is a sensitive current detector.
This diagram illustrates electromagnetic
induction. As the magnet is moved into the wire
coil, current is generated in the coil.

• Faradays law of electromagnetic induction A
  changing magnetic field in the region of a
  closed-loop conductor will induce an electric
  current.

3
The Discovery of Electromagnetic Induction

• A wire moving in a magnetic field produces
  electromotive force (emf).

• Electromagnetic induction also involves the
  production of electric potential difference
  (emf).
• Faraday discovered that three factors influence
  the magnitude of emf and induced current in the
  wire
• The velocity of the wire the higher the
  velocity, the greater the emf and current.
• The strength of the magnetic field the stronger
  the magnetic field, the greater the emf and
  current.
• The length of the wire in the magnetic field
  the longer the wire, the greater the emf and
  current.

A segment of a closed loop of wire moves through
a magnetic field. Note that the wire must be
perpendicular to the magnetic field in order for
current and emf to be induced.
4
The Direction of Induced Current

• Lenzs law states that induced current and emf
  are in a direction that opposes the change that
  produced them.

• Lenzs law means that induced current creates a
  magnetic force that acts on the wire. This force
  always opposes the wire. Lenzs law thus obeys
  the law of conservation of energy it takes work
  to produce energy in a different form.

Holding the hand flat will determine the
following variables Thumb direction of
velocity of wire Fingers direction of magnetic
field Palm direction of induced conventional
current
5
The Direction of Induced Current

• Lenz knew the cardinal rule

• That nature likes to conserve things (like
  energy) you cant get something for free

• So he reasoned that

The induced current is such as to OPPOSE the
CHANGE in applied magnetic field.
This is Lenzs Law
6
The Direction of Induced Current

• Originally, when the magnet is not moving, the
  magnetic field is not changing.
• Suddenly, the magnet moves towards the coil and
  the field starts to increase.
• The current in the coil instantly starts up to
  counteract this increase.

N
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The Direction of Induced Current
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The Direction of Induced Current

• Right-Hand Rule for Induction in Solenoids

According to Lenzs law, the induced current
created by pushing a permanent magnet into a
solenoid will create a magnetic field in the
solenoid. The magnetic field creates a repulsive
force against the permanent magnet. Holding the
right hand with the fingers curled and the thumb
extended will determine the following
variables The thumb points in the direction
of the north pole of the solenoid. Fingers
curl in the direction of induced current.
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Summary

• As the magnet approaches the loop, the applied
  magnetic field in the centre increases. This is
  a change.
• An Induced Field is created which attempts to
  cancel the applied field to keep the total
  field at zero its original value.
• This induced field must be a associated with a
  current the INDUCED CURRENT in the loop. You
  can determine the direction of the current by the
  RHR.

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Example
Determine the direction of the induced current in
the solenoid shown below.