Do Now (1/23/14):

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Do Now (1/23/14)

• Do not touch the materials on your desk until
  instructed!
• What is a magnet?
• What are some properties of magnets?
• How do you use magnets in your life?
• Are some magnets stronger than others?

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Investigate

• Work with your group to complete the activity.
  You have ten minutes!!!

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Like repels like
Opposites attract!
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Magnetism
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Magnetism Resources

• http//coe.kean.edu/afonarev/Physics/Magnetism/Ma
  gnetic20Fields20and20Forces-eL.htm

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• Magnets have been known for centuries.
• The ancient Greeks used a stone substance called
  magnetite. They discovered that the stone
  always pointed in the same direction.
• Later, stones of magnetite called lodestones
  were used in navigation.

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What is Magnetism?
force of attraction or repulsion of a magnet due
to the arrangement of its atoms, particularly
its electrons.
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• Magnetic effect is strongest at the poles (ends)
• Each magnet has 2 poles 1 north, 1 south.

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Poles of a magnet always come in pairs!
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If you cut a magnet in half,
you get 2 magnets!
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Magnetic Fields
The region where the magnetic forces act is
called the magnetic field
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• Atoms themselves have magnetic properties due
• to the spin of the atoms electrons.

• Groups of atoms join so that their magnetic
  fields are all going in the same direction

• These areas of atoms are called domains

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When an unmagnetized substance is placed in a
magnetic field, the substance can become
magnetized. This happens when the spinning
electrons line up in the same direction.
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An unmagnetized substance looks like this
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While a magnetized substance looks like this
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How to break a magnet
1. Drop it
2. Heat it
This causes the domains to become random again!
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The Earth is a magnet
surrounded by a magnetic field that is strongest
near the North and South magnetic poles
Geographic North Pole
Magnetic South Pole
Magnetic North Pole
Geographic South Pole
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Sometimes, the Earths magnetic poles flip. This
happens every half-million years or so.
Magnetic North Pole
Magnetic South Pole
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Use the Earths magnetic field to find direction.
The needle of a compass always points toward the
magnetic south pole.
We call this direction North (remember,
opposites attract)
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Do Now (1/24/14)

• (Do not touch the materials on your desk until
  instructed!)
• Write down three things you learned yesterday
  about magnets

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Magnetic Field
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1) Review

• a) Natural permanent magnets
• Like poles repel, unlike attract
• come in pairs (no monopoles)
• Interact with earth
• define N (or north-seeking) pole as pole
  attracted to North pole of earth

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• b) Magnetic field direction
• - direction of force on N pole

B
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Review

• What is an electric dipole?

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Review

• (in your notes) Draw the electric field of an
  electric dipole

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Hypothesize (2 min)

• What do you think the magnetic field of a bar
  magnet would look like? Draw it in your notes.
• Discuss with your elbow partner

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Investigation

• Work with your table to complete the
  Investigation. You have twenty minutes to
  complete the activity.

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Results

• Go to the board and draw the field lines that you
  discovered. If yours looks like another groups,
  put a check mark next to it.

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• Field of a magnetic dipole

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• d) Magnetostatics for poles
• (identical to electrostatics for charges)
• 2 types N, S vs ,-
• Unlike attract, like repel
• Inverse square law
• Force along joining line
• Magnetic Field

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• Why study magnetism?
• No monopoles (yet)
• Poles (dipoles) produced by moving charges
• Charges affected by magnetic field
• The fundamental unit is still charge
• magnetic fields can be created by charge
• The force on a charge can be due to magnetic
  field
• Magnetic fields do not interact with stationary
  charges!!!!!

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Magnetic Field

• Represented by B
• Units Tesla
• Brainstorm how big do you think Earths magnetic
  field is?

10-5 T!!!!!
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Force on a moving charge in a magnetic field
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Proportional to component of v perp to B
(Alternative definition of B)
Perpendicular to B
Perpendicular to v
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Force on a moving chargewhat would result in a
force of zero?
charge traveling parallel to B
charge at rest
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Force on a moving chargewhat would result in a
maximum force?
charge traveling _at_ 90 to B
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Example

• What is the force on an electron moving at 43 m/s
  in a magnetic field of 0.5 T?

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Practice

• Work on the Magnetic Force Worksheet

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Do Now (1/27/14)

• (On your Do Now sheet from last week)
• How fast is a proton moving in a magnetic field
  of 0.8 T if the magnetic force exerted on it is
  0.9 N?

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Finding Directions of B-Fields

• Consider an arrow

OUT OF THE PAGE
INTO THE PAGE
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The Right Hand Rule!

• Follow along on your paper
• Three different methods find the one that works
  for you!

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1 (ON YOUR PAPER)

• Direction of v
• Direction of B
• Direction of F

To the right
Out of the page
DOWN (TOWARDS THE BOTTOM OF THE PAGE)
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Practice

• Complete the Right Hand Rule Worksheet by the end
  of class.
• If you finish early, please continue working on
  your homework (Magnetic Force)

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Do Now (1/28/14)

• Draw the following on your paper
• A magnetic field pointing to the right
• A proton traveling towards the top of the page
• What is the direction of the force exerted on the
  charge?
• 2. If the proton travels at 3000 m/s and the
  magnetic field has a magnitude of 4 T, what is
  the force exerted on the proton?
• 3. What is the protons acceleration?

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RHR Electron vs. Proton

• What if an electron travels through the field
  instead of a proton?

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2) Magnetic field due to current (direction)

• Oersted (1820)

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Right Hand Rule 2
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3) Magnetic force on current
a) Orthogonal case
defines B

• Direction from RHR1 B fingers, I thumb, F palm

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Practice

• Use the rest of class to work on your HW
  (Magnetic Force and/or Force on a
  Current-Carrying Wire)

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Do Now (1/29/14)

• What is the force on a 15 cm wire carrying a 10 A
  current surrounded by a 0.2 T?

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Force on a current carrying wire

• Look on your homework paper.

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Example

• A current in the x direction and a magnetic
  field in the y direction

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Investigate!

• Work on ONE of the investigations for full
  credit.
• Work on both for extra credit!

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Do Now (1/30/14)

• Come in quietly, pass in your Do Nows and
  Homework, then wait for further instructions.

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b) General case
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4) Force between parallel wires
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Attraction or repulsion? Does it depend on
reference frame?
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v
v
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• Define Ampere as the quantity of current that
  produces a force per unit length of 2 x 10-7 N/m
  for separation of 1 m

This defines C and gives
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Permeability of free space
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5) Field due to long straight wire (magnitude)
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6) Force on a moving charge
Zero at rest
Zero parallel to B
Max perpendicular to B
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Proportional to component of v perp to B
(Alternative definition of B)
Perpendicular to B
Perpendicular to v
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7) Motion of a charge in a magnetic field

• a) Constant force

motion is parabolic
electric or gravitational field
not everywhere perp to velocity
not magnetic field
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Mass spectrometer

• Diagram

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• b) Constant magnitude perpendicular to motion

radial field (circular motion)
mass on a string
motion is circular
magnetic field produces circular motion
(initial vel. perp. to B)
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Force due to the field
For circular motion
So,
r depends on v, B
angular freq. independent of speed, radius
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Tracks in a bubble chamber
 electron-positron creation
1, 3 positive
2 negative
energy 3 gt 2 gt 1
energy decreases by collisions
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Example Find speed and radius for proton
B 0.10 T
V 2100 V
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• c) Work done by magnetic field

Work by a force F
For a magnetic field,
Work 0
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• d) Velocity selector

Force due to E (down)
Force due to B (up)
For zero deflection, FE FB
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• e) Mass Spectrometer

Ion energy
Radius of motion
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Additional Info about Magnets
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William Gilbert, an English physician, predicted
in 1600 that the Earth would be found to have
magnetic poles.
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The sun has a magnetic field, too. It extends
far above the suns surface.
Other planets in the solar system also have these
magnetic fields
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When a charged particle enters a magnetic field,
an electric force is exerted on it. If a
charged particle moves at an angle to a magnetic
field, the magnetic force acting on it will cause
it to move in a spiral around the magnetic field
lines.
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The solar wind is constantly bombarding the
Earths magnetic field. Sometimes these charged
particles penetrate that field. These particles
are found in two large regions known as the Van
Allen Belts.
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The Earths magnetic field extends far into
space. It is called the magnetosphere.
When the magnetic particles from the sun, called
solar wind, strike this magnetosphere, we see a
phenomenon called
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The Aurora Borealis in the Northern Hemisphere
And the Aurora Australis in the Southern
Hemisphere