Chpt 25 – Magnetism and Electromagnetic Induction Broadneck Physics Electromagnetism and ElectroMagnetic Induction

Chpt 25 – Magnetism and Electromagnetic Induction

Broadneck PhysicsElectromagnetism 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!


Placing a magnet in an existing magnetic field

produces a torque (a twist!) on the magnet if it isn’t aligned with the

field.


Opposite poles nearest to each

other

Similar poles nearest to each other

We get a more complicated pattern with two magnets!


We believe magnetism is caused by our old friends – electrons – as they both spin on their

axis and rotate about the nucleus.



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!


But if we subject this

iron to a strong

magnetic field, it will become magnetized. The domains will “line up”.



Wai Tsan plays with

magnets…! (actually, nails

which have been

temporarily magnetized). Wai Tsan is

one cool dude…


Electrical current – the flow of electrons – produces a magnetic field. There is a

fundamental connection between electricity and magnetism!


Current (I)

Current moving through a wire “induces” a magnetic field about the wire.


We can use our iron filings to show this…!


Ceramic superconductors can produce extremely strong magnetic fields

because the magnetic field cannot penetrate into the ceramic materials.

Suspended magnet!

“Maglev” train


Magnetic Forces on Moving Charges

A magnetic field can change the direction of an electron beam


The Earth’s magnetic field deflects many harmful high-

energy charged particles.


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!


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.


The same basic device – called a

“Galvanometer” – is the basis for

meters which test for current, voltage and resistance in

circuits!


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”


Electric Motors

The current moving through the wire

induces a magnetic field, which is attracted &

repelled by the magnetic field from

the big magnet.



MRI machine

MRI visualization of

a ruptured spinal disk


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…


Moving the magnet (and thus its field!) inside the coil

of wire will induce a

current, which we measure

with the meter


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.


And the more wires we have “intercepting” the magnetic field, the

more current we get!


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…?


Electric Guitars

The metal guitar string – slightly magnetized – induces a current in the coil!


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.


A graph of the current vs. time….


NEWS FLASH !!

Generators do not “produce” energy... They change one type of energy (mechanical) into another type

(electrical), and only with about 60% efficiency


A power plant converts thermal energy to steam to mechanical energy to electrical energy…


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!


The ratio of the input side (primary) turns to the output side (secondary) turns tells us how much the voltage is decreased or

increased!


The iron “core” help to focus the magnetic field.


You are here!

Power Grid


Or another view…


Complicated controls in a nuclear plant!

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Chpt 25 – Magnetism and Electromagnetic Induction Broadneck Physics Electromagnetism and ElectroMagnetic Induction