Transcript Using Magnetism to Induce an Electric Current

Electromagnetic Induction is used to
generate most of the electrical energy
used today
• 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.
• Faraday’s law of electromagnetic induction: A changing
magnetic field in the region of a closed-loop conductor will
induce an electric current.
• 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.
• Lenz’s law states that induced current and emf are in a
direction that opposes the change that produced them.
– Lenz’s law means
that induced
current creates a
magnetic force
that acts on the
wire. This force
always opposes
the wire. Lenz’s
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
• Lenz knew the cardinal rule…
– That nature likes to conserve things (like energy) – you can’t
get something for free
• So he reasoned that…
“The induced current is such as to OPPOSE the CHANGE
in applied magnetic field.”
This is Lenz’s Law
• 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.
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• Right-Hand Rule for Induction in Solenoids
According to Lenz’s 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.
• 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.
Determine the direction of the induced current in the solenoid shown
below.