Download Magnetism

Magnetism
Chapter 21 (pg 765)
Lets Review
• Force equals mass times acceleration
– This is always true, even with magnetic force
• Centripetal force is caused by something
– In the next couple of chapters the force will be
electromagnetism
• Torque equals force times lever arm
• Electric fields are created by charged
particles
– Electrons & protons
• Current equals amount of charge per unit
time
The History of Magnetism
• In the 4th century a Chinese
writing “Book of the Devil
Valley Master” makes the first
mention of magnetism
• “The lodestone makes iron
come or it attracts it.“
• Around 201 used spoon
shaped lodestones to show
direction
• About 700 AD the Chinese
magnetized a needle after
noticing the lodestone could
tranfer it’s magnetism to other
materials
More History
• In 1819 Danish physicist Hans Christian
Orsted (1777-1851) discovered by mere
accident that a current carrying wire changed
the deflection of a near by compass (watch
for your self)
• This leads British physicist Michael Faraday
(1791-1867) to demonstrate the process of
induction in 1831 by producing a current in a
wire by rotating a disc through a magnetic
field
• This invention is called the Faraday Disc,
which lead to today's generators
Magnetic Principles
• Like “poles” repel like
– Magnetic poles are labeled as “North” and
“South”
– This originates from the earths magnetic
poles
– The Magnetic north pole of the earth is on
Antarctica and the Magnetic South pole is in
the northern part of Canada
– Since opposite poles attract the “North pole”
of the compass points to the magnetic south
pole – which is geographically North
– Confused?
Angle of declination
• This is the difference
between true north and
magnetic north.
• It changes through out the
years, but still remains
relatively close to an
average value
• In New York the present
declination angle is about
12o west, meaning the
compass needle points
12o west of geographic
north
It’s all about the pOles
• Unlike electric field
lines that can start or
end at infinity, magnetic
field lines, up to this
point in history, always
form loops.
• There are always a
North and South poles
no matter how small
you cut the magnet!
• Even cut all the way to
a single electron.
So why aren’t all things magnets
• All materials are
magnetic, but not all
materials are magnets
• Inside materials exist
magnetic domains
• When the domains
align in the same
direction the magnetic
fields add together
• When the domains are
random, some add and
some subtract, causing
a net filed on zero
Click here for a video of a magnetized frog.
Click here to learn about the types of magnetism.
The latest data
• This picture shows the
direction of the earths
magnetic field in 2006
• This shows the Earth s
magnetic field will
eventually flip, so
compasses will point
South, not North
• It turns out this flip
occurs about every
million years
Click here for the link to article
Those lines?
• The lines shown on the last slide are
magnetic lines of force, also know as
Magnetic Field lines
• These are essentially the same as
gravitational or electric field lines but show
the direction a magnetized particle would
move
• So place a piece of magnetized iron on a
field line and it will or orient itself with its
North pole facing the Blue line, or the
magnetic South pole
Take a look
Take a look
Describing Direction
• For electric flied we
mostly focused on 2-D
representations
• Magnetic fields are
perpendicular to
electric fields, often we
need to look at a three
dimensional problem
• Look at table 21-1 on
page 767 or the chart
to the right
In the plane of the page
Out of the page (Up)
Into the page (Down)
Models of B – Fields
Magnetic force
• Remember, all force
can be expressed by
Newton second law
• Charge must be
moving to experience a
magnetic force
• The velocity of the
moving charge must be
perpendicular to the
B(magnetic) – field
• The direction of the
force is given by the
“right-hand rule”
Right hand rule (cross product)
• This is and easy way to find the direction in
space for either the B- Field, Force, or
velocity direction.
• The magnetic force on a particle is a vector
equal to:

 
F  qv  B
• q is charge, v is velocity, and B is magnetic
field strength
• However, as v and B become more parallel v
x B goes to zero
Consider the following
• Watch what happens as q goes to zero
F
v
q
B
Parallel vectors
• If v and B are parallel, the force on a
charged particle is zero
• Keep in mind, the right hand rule is for a
positive (conventional) charge carrier
• Since the force goes to zero when q goes to
zero it’s said v x B equals vBsin q
• Rearranging to solve for B the result is:
F
B
qo v sin q
Example 21-1
Magnetic force does no work
• In this case the magnetic force simply
changes the direction of the particle
• It does not change the particle energy
• This means the speed remains constant
• Does a changing direction with constant
speed sound familiar?
mv
F
r
2
Example 21-3
Cathode Ray Tube
• By changing the current in the steering coils,
the electron beam turns to strike different
parts of the screen
• Click here to learn more about America’s
favorite past time
Works Cited
• Reference
– http://en.wikipedia.org/wiki/Magnetism
– http://concise.britannica.com
– http://inventors.about.com/od/cstartinvention
s/a/Compass.htm
• Images
– http://www.physics.ubc.ca/~outreach/phys420
/p420_05/dean/lodestone.jpg