Transcript Electromagnetism and ElectroMagnetic Induction

Broadneck Physics
Electromagnetism and
ElectroMagnetic Induction
Chpt 25 – Magnetism and Electromagnetic Induction
Iron filings dropped
onto a piece of paper
below a magnet will
demonstrate the
magnetic field lines
If you break a magnet,
each piece will establish
a N and S pole!
Chpt 25 – Magnetism and Electromagnetic Induction
Placing a magnet in
an existing magnetic
field produces a
torque (a twist!) on
the magnet if it isn’t
aligned with the
field.
Chpt 25 – Magnetism and Electromagnetic Induction
Opposite poles
nearest to each other
Similar poles nearest
to each other
We get a more complicated pattern with two
magnets!
Chpt 25 – Magnetism and Electromagnetic Induction
We believe magnetism
is caused by our old
friends – electrons –
as they both spin on
their axis and rotate
about the nucleus.
Chpt 25 – Magnetism and Electromagnetic Induction
Chpt 25 – Magnetism and Electromagnetic Induction
This iron is
NOT
magnetized.
The domains
are randomly
arranged!
The magnetic field of an individual iron atom
is so strong that interactions among adjacent
atoms cause large clusters of them to line up
– these clusters are called magnetic domains.
Chpt 25 – Magnetism and Electromagnetic Induction
But if we
subject this
iron to a strong
magnetic field,
it will become
magnetized.
The domains will
“line up”.
Chpt 25 – Magnetism and Electromagnetic Induction
Chpt 25 – Magnetism and Electromagnetic Induction
Wai Tsan plays
with magnets…!
(actually, nails
which have been
temporarily
magnetized).
Wai Tsan is one
cool dude…
Chpt 25 – Magnetism and Electromagnetic Induction
Electrical current – the flow of electrons –
produces a magnetic field. There is a
fundamental connection between electricity
and magnetism!
Chpt 25 – Magnetism and Electromagnetic Induction
Current
(I)
Current moving through a wire “induces” a
magnetic field about the wire.
Chpt 25 – Magnetism and Electromagnetic Induction
We can use our iron filings to show this…!
Chpt 25 – Magnetism and Electromagnetic Induction
Ceramic superconductors can produce
extremely strong magnetic fields because
the magnetic field cannot penetrate into the
ceramic materials.
Suspended
magnet!
Chpt 25 – Magnetism and Electromagnetic Induction
“Maglev” train
Magnetic Forces on Moving Charges
A magnetic field can change the
direction of an electron beam
Chpt 25 – Magnetism and Electromagnetic Induction
The Earth’s
magnetic field
deflects many
harmful highenergy charged
particles.
Chpt 25 – Magnetism and Electromagnetic Induction
Engineers designing high-voltage power towers
have to worry about the forces of the Earth’s
magnetic field on the large currents being
carried in the wires!
Chpt 25 – Magnetism and Electromagnetic Induction
Since the flow of electricity produces
magnetism, we should be able to use a
magnet and a coil of wire to test for the
presence (and the amount of!) electrical
currents.
Chpt 25 – Magnetism and Electromagnetic Induction
The same basic device
– called a
“Galvanometer” – is
the basis for meters
which test for
current, voltage and
resistance in circuits!
Chpt 25 – Magnetism and Electromagnetic Induction
An Ammeter measures Current.
It lets a small % of the current
flow through the Galvanometer.
Chpt 25 – Magnetism and Electromagnetic Induction
The Voltmeter has
high internal
resistance, so it just
senses the electrical
“pressure”
Electric Motors
The current moving
through the wire
induces a magnetic
field, which is
attracted & repelled
by the magnetic field
from the big magnet.
Chpt 25 – Magnetism and Electromagnetic Induction
Chpt 25 – Magnetism and Electromagnetic Induction
MRI machine
MRI visualization
of a ruptured
spinal disk
Chpt 25 – Magnetism and Electromagnetic Induction
Magnetic Induction
Moving a current through a wire causes a
magnetic field to be formed.
So…does moving a magnetic field past a
wire cause a current to be formed???
The answer is YES! Lots of things in
physics (and nature!) work this way…
Chpt 25 – Magnetism and Electromagnetic Induction
Moving the
magnet (and thus
its field!) inside
the coil of wire
will induce a
current, which
we measure with
the meter
Chpt 25 – Magnetism and Electromagnetic Induction
It doesn’t matter whether the wire moves
past the magnet, or the magnet past the wire.
It’s the relative motion of one to the other
which induces the current.
Chpt 25 – Magnetism and Electromagnetic Induction
And the more wires we have “intercepting”
the magnetic field, the more current we get!
Chpt 25 – Magnetism and Electromagnetic Induction
Mr. Faraday stated this another way:
“The induced voltage in a coil is proportional
to the number of loops, multiplied by the
rate at which the magnetic field changes
within those loops.”
So are we producing
voltage or current…?
Chpt 25 – Magnetism and Electromagnetic Induction
Electric Guitars
The metal guitar string –
slightly magnetized –
induces a current in the coil!
Chpt 25 – Magnetism and Electromagnetic Induction
Generators and Alternating Current
It is easier to
move the wire (or
coil of wire) than
it is to move the
magnet. If we do
this we produce
current that
changes direction
with each turn.
Chpt 25 – Magnetism and Electromagnetic Induction
A graph of the current vs. time….
Chpt 25 – Magnetism and Electromagnetic Induction
NEWS FLASH !!
Generators do not “produce” energy... They
change one type of energy (mechanical) into
another type (electrical), and only with about
60% efficiency
Chpt 25 – Magnetism and Electromagnetic Induction
A power plant converts thermal energy
to steam to mechanical energy to
electrical energy…
Chpt 25 – Magnetism and Electromagnetic Induction
Since the current direction is changing 60
times per second in AC current, the
associated magnetic field is constantly
changing as well.
Wires wrapped around a conductor will
induce a current in that conductor as this
happens. Nothing has to “move”…
Recall the amount of current produced
depends on the number of wires wrapped
around it!
Chpt 25 – Magnetism and Electromagnetic Induction
The ratio of the input side (primary) turns to
the output side (secondary) turns tells us
how much the voltage is decreased or
increased!
Chpt 25 – Magnetism and Electromagnetic Induction
The iron “core” help to focus the
magnetic field.
Chpt 25 – Magnetism and Electromagnetic Induction
Power Grid
You are here!
Chpt 25 – Magnetism and Electromagnetic Induction
Or another view…
Chpt 25 – Magnetism and Electromagnetic Induction
Complicated controls in a nuclear plant!
Chpt 25 – Magnetism and Electromagnetic Induction