Transcript Magnetism

Magnetism
Part 2
Review: What elements are magnetic?
• Unlike electricity, NOT all metals can be
magnetic:
– Of the about 100 elements we know about,
ONLY iron, nickel, and cobalt can become
magnetic (whereas almost all metals are good
electrical conductors).
– Note aluminum can NOT become magnetic.
Don’t be confused by references to Alnico
magnets made of aluminum, nickel, and
cobalt. Aluminum is only a filler in those
magnets!
Review: HOW do iron, nickel, &
cobalt become magnetic? ….
• The “electron-spin” of electrons in atoms is called
the “domain”. In all of the elements, the electronspins are going every which way, and essentially
canceling each other out. (See figure a at right.)
• In iron, nickel, and cobalt, the electron-spins or
domains can become aligned. (See figure b at
right.)
• When the electron-spins/domains of iron, nickel, or
cobalt become aligned, then they become
magnetic.
• Note – you can NOT say just “electrons” become
aligned. It is either “electron-spins” or “domains”.
Be very careful about that!
• Remember - NOBODY really understands what’s
going on! Why do those three elements electronspins do that? Why only those three? Find out
yourself, and get a Nobel prize in Physics!
Review: Magnetic Domains aligned & not…
“stroking”
technique;
what’s the
other?
How could you
get domains
unaligned?
By dropping or
heating the
magnet!
Electricity vs Magnetism:
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2 kinds of charge: + and –
Can have single charges
alone
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Opposite charges attract
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Like charges repel
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Neutrals are ALWAYS
attracted through “induction”
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Electric fields do exist;
closer field lines = more
powerful/more forceful there;
field lines around +, , +&,
&, and +&+ you already
know
2 kinds of poles: N and S
Can NOT have single poles alone
(no “monopoles”; if you break a
magnet in half, you just get 2
smaller magnets that both have
N&S poles.)
Opposite poles attract
Like poles repel
“Neutrals” (meaning in this case
samples of iron, nickel, & cobalt
that are NOT yet magnetized, or
do NOT yet have their electronspins/domains aligned) are
ALWAYS attracted.
Magnetic fields do exist;
closer field lines more
powerful/forceful there; field lines
around a single bar magnet are
closed loops from N to S;
Electricity & Magnetism are “wed”:
•
*Electricity causes magnetism....
– There are 2 ways to make iron, nickel or cobalt
domains aligned:
a. “stroke” a needle/nail over & over again in the same direction
with a magnet (see bottom left of slide 4)
b. but also by wrapping a needle/nail with a wire (several coils
all going in the same direction) and attach the wire to a
power supply like a battery (an “electromagnet”)
•
– Cst 5f=“Students know magnetic materials
AND electric currents (moving electric charges) are
sources of magnetic fields and are subject to
forces arising from the magnetic fields of other
sources.”
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“All magnetic fields are actually caused by the motion of
_____ particles inside the metal. A spinning electron
constitutes a “charged particle in _____.”
More on electricity causes magnetism:
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Compasses laid out around a currentcarrying wire will point in the direction of the
magnetic field, which happens to form a
circle around the wire.
Forms concentric
circles with
magnetic field
tangential to circle
at all points
Electricity & Magnetism are “wed”
• *Magnetism causes electricity....
• Watch us spin the handle to cause a
lightbulb to light (but only if the
magnet is attached!)
• This is an example of a generator.
• 5h=“Students know changing magnetic
fields produce electric fields, thereby
inducing currents in nearby conductors.”
Energy conversions:
• Generators are a device that converts
_____ energy into _____ energy.
• Batteries are devices that convert _____
energy into _____ energy.
• Motors are a device that converts _____
energy into _____ energy.
(this is the new one!)
Temporary vs permanent magnets:
• Do the domains remain aligned for a long time
after you remove the thing that was doing the
aligning? (ie: remove the current, and/or
remove the other magnet)
• If YES – its a “permanent” magnet
• If NO – its only a “temporary” magnet
• In the picture, the needle/nail hanging from the
magnet is only a temporary magnet, as it will
lose its magnetism once it is
removed from the bar magnet.
Earth’s Magnetic Field:
• Earth has a
magnetic field!
• Over the history
of the earth –
its magnetic
poles have
reversed
multiple times!
compass pointing to north pole
More on these reversals:
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There is evidence in rock samples that the Earth’s
magnetic field has diminished to zero in the past and
even reversed itself!
More than 20 reversals have taken place in the past 5
million years, with the most recent being 70,000 years
ago. The magnetic field was switched off entirely for
10,000-20,000 years when modern humans emerged.
We can not really predict when the next reversal will
come. But recent evidence shows a decrease of 5% in
the last 100 years, which may mean another magnetic
field reversal within 2000 years.
Since the Earth’s magnetic field shields us from most
of the harmful radiation coming from outer space, what
will happen to humans when it is gone??????
The field lines around the Earth look
just like those around a bar magnet
Earth’s Magnetic Field cont…
• Compasses are just floating magnets attracted by the Earth’s
magnetic field
• Recall: The true “geographic” north pole, or “spin-axis north pole” is
NOT the same an magnetic North. They are off by what is called the
“magnetic declination”
• Geese & other animals have small magnets in their brains to
navigate by (“animal magnetism”)
• Big steel things like the Eiffel tower, make compasses point right at
them! 
• Thus, on a big steel ship, you need a “binnacle” so the ship’s
compass does NOT align with the bow & stern. (note – steel ships
only, not wood boats!)
• Other astronomical objects:
– planets (except Venus, Mars, Pluto) have magnetic fields.
– Gas giants have BIG magnetic fields.
– Little rocky planets rarely have enough iron & nickel in their cores, or
their cores don’t spin fast enough to align the electron-spins.
– The moon does not have a magnetic field; but the sun does!
– Our galaxy has a very small magnetic field over-all
Northern & Southern Lights:
1.
2.
3.
4.
5.
6.
The earth’s magnetic field extends way into outer space.
The earth’s magnetic field is strongest at the poles
Charged particles coming from the sun are called the
“solar wind”.
The charge particles get trapped in
our magnetic field, and those
“belts” of charged particles floating
out there are called the “Van Allen
belts” (after the famous American
physicist who discovered them!)
Since the earths magnetic field is
strongest at the poles, a lot of
charged particles get SUCKED in
to the poles, and when they collide
you get ….
“Aurora Borealis” and “Aurora
Australis” (which are actually plasma’s!)
• END
The 2nd Right-hand rule:
• Curl fingers of right hand
in direction of
conventional current
• Conventional current is
defined to be “opposite
direction of electron-flow
current” (so from + to - )
• Thumb points towards
North pole!
Practice 2nd right hand rule
1. Take pen/pencil, lay it east/west, POINTING west.
2. Wrap wire around pencil starting at eraser, going
CLOCKWISE if looking at eraser.
3. Pretend to “plug in” eraser end of wire into a POSITIVE
lead of a battery.
4. Wrap the fingers of your right hand in the direction of
the conventional current flow.
5. The North pole is which of these 6 directions:
6. Up, down, left, right, “into the page”, “out of the page”
7. Lay the same pencil north/south facing north, and now
pretend to plug in the north end of the wire.
8. Wrap the wire the other way.
9. Answer the question below:
• END
The 1st Right-hand rule:
• Let’s “see” the 3 dimensions (pink hand).
• Thumb goes in direction of conventional current.
• Palm/compass to thumb/wire: palm must be
positioned next to thumb as compass is positioned
next to wire
• Finger tips point in direction of magnetic field
(compass alignment)
Forms concentric
circles with
magnetic field
tangential to circle
at all points
1. ABOVE paper at ___
Practice:
2. BELOW paper at ___
3. To the LEFT/West of wire ___
a
4. To the RIGHT/East of wire ___
e
5. To the north of wire ___
c
f
d
6. To the south of wire ___
g
b
Possible answers:
•Left/west
•Right/east
•North/up
•South/down
•“into the paper”
•“out of the paper”
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Conceptual Questions:
That’s different!
A. Oddly enough, when two wires have current
going in the SAME direction lie parallel to each
other, they attract.
B. Similarly, when two wires have current going in
the OPPOSITE directions lie parallel to each
other, they repel.