Chpt 25 – Magnetism and Electromagnetic Induction Broadneck Physics Electromagnetism and...
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Transcript of Chpt 25 – Magnetism and Electromagnetic Induction Broadneck Physics Electromagnetism and...
Chpt 25 – Magnetism and Electromagnetic Induction
Broadneck PhysicsElectromagnetism 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
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.
This iron is NOT
magnetized. The domains
are randomly arranged!
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!
“Maglev” train
Chpt 25 – Magnetism and Electromagnetic Induction
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 high-
energy 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.
The Voltmeter has high internal
resistance, so it just senses the electrical
“pressure”
Chpt 25 – Magnetism and Electromagnetic Induction
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
You are here!
Power Grid
Chpt 25 – Magnetism and Electromagnetic Induction
Or another view…
Chpt 25 – Magnetism and Electromagnetic Induction
Complicated controls in a nuclear plant!