Magnetism and Magnetic Force

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Magnetism and Magnetic Force

• Unit 10 Lecture 1
• AP Physics

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Magnets
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A bit of history

• The ancient Greeks knew about "lodestones (rare
  natural magnets) with the power to attract iron.
• One site where such stones were found was near
  the city of Magnesia in Asia Minor (now Turkey),
  and from that (perhaps) came the term
  "magnetism."
• But they were not the first to understand how to
  use it.

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A bit of history

• Somewhere around 500 ACE, the Chinese built a
  primitive compass by placing a lodestone on a
  boat in a bowl of water.
• The boat always rotated to point in the same
  direction, no matter where or how the experiment
  was performed.
• The boat was never pulled in any direction it
  only rotated.

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William Gilbert

• Physician to Elizabeth I
• Published On the Magnet and Magnetic Bodies in
  1600.
• First understood that the earth was itself a
  magnet.
• Summarized all of the knowledge of magnetism up
  to his day.

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Magnets Overview

• Has two ends or poles, called N and S
• Opposite poles attract like poles repel

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Magnets Overview

• However, if you cut a magnet in half, you dont
  get a north pole and a south pole you get two
  smaller magnets.

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Sources of Magnetism

• A strongly magnetized bar consists of aligned
  domains.

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Sources of Magnetism

• A broken magnet

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Ferromagnetic material

• The domains persist when the external magnetic
  field is removed
• Also called hard magnets or permanent

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Paramagnetic material

• Domains can align with an external magnetic field
• The domains do not persist when the external
  magnetic field is removed
• Also called soft magnets

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Nonmagnetic material

• Atom has no net magnetic field so there can be no
  domain
• Exhibits no magnetic effects
• Example copper, gold and silver

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Magnets Overview

• Magnetism can be destroyed by heat and restored
  again by cooling.

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Magnetic Earth

• The earth acts as a giant magnet.
• Geographic north is a south magnetic pole
• Freely suspended magnets will align with this
  earth magnet.
• A compass will point downwards near the north
  pole.
• The magnetic pole is not located at the
  geographic pole. There is a difference!

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The earth as a magnet
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Magnetic Declination
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20.1 Magnets and Magnetic Fields
Magnetic fields can be visualized using magnetic
field lines, which are always closed loops.
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20.1 Magnets and Magnetic Fields
A uniform magnetic field is constant in magnitude
and direction.
The field between these two wide poles is nearly
uniform.
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Magnetic Forces

• Hans Christian Ørsted is credited with
  discovering the link between current and magnetic
  forces. (1820)
• He was giving a demonstration of the battery when
  he noticed that a compass always reacted when he
  turned the electricity off and on.

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Force of a Charged Particle

• A charged particle (or current) experiences a
  magnetic force when it is moving through, but not
  parallel with, a magnetic field.

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Magnetic Force

• The vector force is given by a vector equation
• The magnitude of the force is given by
• The direction of the force is given by the right
  hand rule

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Right Hand Rule

• Point your fingers in direction of velocity
• Rotate wrist so you can sweep your fingers in the
  direction of the magnetic field without moving
  your palm
• Thumb points in the direction of the Force.

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Your turn

• A positive charge is moving south in a magnetic
  field that points west. What is the direction of
  the force on the charge?
• North
• South
• East
• West
• Up
• Down
• There is no force

Correct Answer 6
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Your turn

• A positive charge is moving upwards in a magnetic
  field that points south. What is the direction
  of the force on the charge?
• North
• South
• East
• West
• Up
• Down
• There is no force

Correct Answer 3
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Your turn

• A negative charge is moving west in a magnetic
  field that points north. What is the direction
  of the force on the charge?
• North
• South
• East
• West
• Up
• Down
• There is no force

Correct Answer 5
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Your turn

• A positive charge is moving east in a magnetic
  field and the force is downwards. What is the
  direction of the field?
• North
• South
• East
• West
• Up
• Down
• There is no force

Correct Answer 2
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Your turn

• A positive charge is moving along the x axis in
  a magnetic field that points in the x direction.
  What is the direction of the force on the
  charge?
• z
• -z
• x
• -x
• y
• -y
• There is no force

Correct Answer 7
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Your turn

• An electron is moving along the x axis in a
  magnetic field that points in the z direction.
  What is the direction of the force on the charge?
• z
• -z
• x
• -x
• y
• -y
• There is no force

Correct Answer 6
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Magnetic Field Defined

• The magnetic field strength is defined in terms
  of the magnitude of the force that is produced on
  a charged object.

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Magnetic Field Defined
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Magnetic Fields
Source Field (Gauss)
Earth 0.5
Appliance 10
Bar magnet 100
Human limit 2000
electro-magnet 50,000
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Example 1

• A proton moves at 8.0E6 m/s along the x-axis. It
  enters a magnetic field of B2.5 T, directed at
  an angle of 60 from the x-axis in the xy plane.
• What is the magnetic force on the proton?
• What is the acceleration of the proton?

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Forces on Conductors

• A current-carrying conductor is a stream of
  moving charges.
• There will be a force on the conductor if it is
  placed in a magnetic field.

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Representing B

• Arrows are used to represent the magnetic field.

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Force on a wire

• If the current is not perpendicular to the field
  B, then the force will be

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Aurora Borealis
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Application

• The speaker

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Motion of Charged Particle

• Since F is always perpendicular to v, the motion
  is circular

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Application

• Mass Spectrometer

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Applications

• Animals and Magnetism http//videos.howstuffworks
  .com/hsw/9511-understanding-magnetism-animals-and-
  magnetism-video.htm
• Other topics
• http//videos.howstuffworks.com/search.php?termsm
  agnetismx0y0mediavideo