Magnetism: Intro

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Magnetism Intro

• Physics 12

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Clip of the day

• Minutephysics..
• How far is a second?
• http//www.youtube.com/watch?vWp20Sc8qPeolistPL
  ED25F943F8D6081C

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Review Question

• A soccer ball is kicked horizontally off a
  22.0-meter high hill and lands a distance of 35.0
  meters from the edge of the hill. Determine the
  initial horizontal velocity of the soccer ball.
• ANSWER 16.5 m/s

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Magnet Brainstorm.and play time!

• Are all magnets alike?
• What kind of things do magnets attract?
• Which part of the magnets are strongest?
• How can you tell where the poles are?
• How can you tell which is which?
• Do magnets work through other materials?
• What does it depend on?
• Can you make a magnet out of a nail?

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Shall we start with Bill Nye on magnets?

• http//www.youtube.com/watch?vak8Bh9Zka50

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Magnetism

• Magnets have 2 poles (north and south)
• Like poles repel
• Unlike poles attract
• Magnets create a MAGNETIC FIELD around them

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Magnetic Force

• Magnetic forces have properties that are similar
  to electric and gravitational force in that
• The force varies following an inverse square
  relationship due to the separation of poles
• The force varies directly with the strength of
  the poles
• However, since we dont have magnetic monopoles
  there is no equation for magnetic force

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Magnetic Domains

• Within a magnetic compound, there are many
  different magnetic domains
• If these are randomly arranged, the material is
  not magnetic
• If they are aligned, the material will be magnetic

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Temporary vs Permanent Magnets

• Some materials will easily align the domains in
  the presence of a magnetic field however they
  will usually return to a random arrangement after
  the field is removed these are called temporary
  magnets (like iron)
• Other materials will not easily align the
  domains, however once aligned they will remain
  aligned these are called permanent magnets (like
  steel)

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Magnetic Field Lines

• It is possible to map a magnetic field in a
  manner similar to that of electric or
  gravitational fields
• Convention says field lines go from north to
  south

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Magnetic Field

• Symbol B
• SI units tesla (T)
• T N__
• A m

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Electric Charges and Magnetic Poles

• In 1819, Oersted noticed that a current carrying
  wire would create a magnetic field
• This was the first step into the realm of
  electromagnetism
• Oersteds experiment led to Right Hand Rule 1

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Oersteds experiment

• When there is no current the needle of the
  compass points north
• When the circuit is complete, electricity flows
  and the compass needles deflected
• There must be some forces causing the
  deflectionmagnetic force!

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Why does the needle move?
THE WIRE ITSELF MUST BE MAGNETIC!!! In other
words the wire has its own INTERNAL MAGNETIC
FIELD that is attracted or repulsed by the
EXTERNAL FIELD.
As it turns out, the wires OWN internal magnetic
field makes concentric circles round the wire.
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Electromagnetism

• Moving electrons produced a magnetics field
• A changing magnetic field causes electrons to move

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Right Hand Rule 1

• The direction of magnetic field lines around a
  current carrying wire can be determined using a
  right hand rule
• Thumb direction of current
• Fingers curl around in direction of magnetic
  field

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Amperes experiment

• Ampere discovered that when compasses were placed
  close to a current carrying wire the compass
  would be deflected based on the direction of the
  current

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• A the needles are moving counter clock wise
• B the needles are moving clock wise

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• Ampere theorized that the magnetic fields between
  two charge carrying wires would either result in
  attractive or repulsive forces
• Based on this,
• Ampere was able to
• develop an equation
• to describe the
• magnetic field

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Magnetic Field
Force (N)
Magnetic field (T)
Length of wire (m)
Current (A)
Number of turns
For a solenoid
Length of each turn (m)
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Sample problem
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Try it?

• Page 767
• 1 a, b, c and 2
• Page 778
• 1-4