Magnetism

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Magnetism

• Do Now List 5 facts you know about magnets

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Magnet Facts

• Magnets have two poles, N and S
• Opposite poles attract
• Like poles repel
• You can never have an isolated pole
• When you break a magnet in half you get two
  magnets, each with a N and S pole

N S
N S N S
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Distance Dependence

• Magnetic force is an action at a distance force
• Like gravity and the Coulomb force, magnetic
  force can be expressed as a field
• Lines are closer together where the field is
  stronger
• Field strength is greater at the poles
• Field lines point away from N pole and towards S
  pole
• Compass needle is tangent to field lines

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Magnetic Field Diagram
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Imaging a magnetic field
S
Remember the field is in three dimensions!
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Levitation

• How are the poles oriented for each of the
  magnets?
• What are the forces on each of the magnets?
• Are the distances between the magnets the same?
  Why or why not?
• What will happen if you push down on the top
  magnet?

http//www.arborsci.com/CoolStuff/P8-1129.jpg
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Aligning a Compass with the Field
Field Lines
Oppositely directed magnetic forces exert a
torque (turning force) to twist the magnet into
alignment
Magnetic Force
N S
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Electricity and Magnetism

• Different aspects of the same electromagnetic
  force
• Both are action-at-a-distance forces
• Both can be thought of as generating a field
• Moving electric charges produce magnetic forces
• Moving electric charges are affected by magnetic
  fields
• Moving magnets produce electric forces

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In the field view..

• A changing magnetic field induces an electric
  field
• A changing electric field generates a magnetic
  field

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Magnetic Field Induced by Current in a Wire

• Right Hand Rule
• Thumb points in direction of current
• Fingers wrap to show field lines

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Oersted Experiment

• When current flows through coil, compass points
  along direction of coil axis
• Proves current (moving charges) produces a
  magnetic field

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Magnetic Induction The Copper Tube Demo

• Changing magnetic fields due to falling magnetic
  induces currents in tube
• Currents create magnetic fields that oppose the
  motion
• Current above magnet is attractive, below magnet
  is repulsive
• Current induced magnetic forces on magnet are
  opposite to the gravitational force, so the
  magnet experiences less net force and acceleration

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Only Certain Materials Are Magnetic

• Fe, Co, Ni are the main magnetic elements
• In full atomic orbitals, electrons are usually in
  pairs, one with a clockwise spin, one with a
  counterclockwise spin
• The spinning electron is a moving charge so it
  generates a magnetic field
• If the spins are paired they cancel each other
  out
• Fe, Co and Ni have partially filled orbitals with
  unpaired electrons with parallel spins, so the
  magnetic fields add up
• Spins in adjacent atoms feel the field and
  experience a torque that aligns them with the
  field forming domains, regions of aligned spins

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Domains The Microscopic View

• In unmagnetized iron, domains are randomly
  oriented
• To make a magnet, put iron in a strong magnetic
  field
• from a bar magnet
• from an electromagnet

http//sol.sci.uop.edu/jfalward/physics17/chapter
9/magnetizedbars.jpg
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Destroying Magnets

• The basic idea is to scramble the domains
• Drop it
• Hit it with a hammer
• Heat it above the Curie temperature
• Thermal motion destroys the alignment of the
  spins in the domains above the Curie temperature

Magnetic Strength vs. Temperature For magnetite,
a natural magnetic mineral
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Technology Implications

• Electric generators
• Generate currents by moving wires through
  magnetic fields
• Electric motors
• Electric current through coil in field of
  permanent magnet generates magnetic force that
  causes change in motion
• Hard Drives
• Change domain orientation to write
• Sense domain orientation to read
• Video screens
• steer electron beam with magnetic fields from
  current through steering coils (electromagnets)

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Galvanometers Measuring Current

• Current through the coil creates a magnetic field
• Coil rotates to align in permanent magnetic field
  until stopped by the spring
• Coil rotation depends on the direction of the
  current

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Motors Using Magnetic Force to Generate Motion

• Brush makes contact with split ring commutator,
  completing circuit
• Current flows through loop, generating magnetic
  field
• Loop aligns in field of permanent magnetic
• Inertia carries loop around, split ring
  commutator reverses direction of current
• Magnetic field of loop reverses, repelled by
  permanent field, moves the next half turn to
  align

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Electric Generators

• Reverse of motor
• Mechanical work is done to turn the coil
• Varying magnetic field experienced by the wire
  causes current to flow

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Hard Drives Use Magnetic Effects
Writing Direction of current in coil changes
added magnetic field Field orients domains in
recording medium
Reading Shields block fields except
locally Magneto-resistor changes resistance when
field changes Sense change in I or V in circuit
http//www.infomrt.com/readrite/magbasic.html
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Life on Earth Implications

• Currents in rotating liquid core generate
  magnetic field
• Magnetic field deflects fast moving charged
  particles in solar wind
• The current in the solar wind also generates a
  small magnetic field