Exam I, Monday, Sept. 29

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Exam I, Monday, Sept. 29
Physics 102 Lecture 08

• What will exam cover?
• Lectures through 7
• Homework through 3
• Discussions through 3
• Text Chapters 16, 17, 18
• Labs through 3
• Review, Sunday, Sept. 28, 3 PM, 141 Loomis
• I will go over the Spring 2008 exam

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Todays Lecture MagnetismThis material is NOT
on exam 1

• Magnetic fields
• Magnetic force on charged particles

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Magnets are Cool!

• North Pole and South Pole
• Opposites Attract
• Likes Repel

• Magnetic Field Lines
• Arrows give direction
• Density gives strength
• Looks like dipole!

Lets Break it!
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4
Field Lines of Bar Magnet
S N
Magnetic field lines dont start or stop. There
are no magnetic charges (monopoles)
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Preflight 8.1
Which drawing shows the correct field lines for a
bar magnet?
(1) 10 (2) 30 (3) 60

• Magnetic field lines are continuous
• Arrows go from N to S outside the magnet
  (S to N inside).

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The Earth is a Magnet
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Electric Field Lines vs. Magnetic Field Lines

• Similarities
• Density gives strength
• Arrow gives direction
• Leave , North
• Enter -, South
• Differences
• Electric (E) Start/Stop on electric charge
• Magnetic (B) No Magnetic Charge, lines are
  continuous!
• FYI
• x x x x x x x INTO Page
• OUT of Page

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No Magnetic Charges

• Magnetic Fields are created by moving electric
  charge!
• Where is the moving charge?

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Magnetic Fields and Forces

• Magnetic fields exert forces on moving charge
• We will discuss the magnitude of the force
  shortly
• Direction of force is given by a right-hand
  rule

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Direction of Magnetic Force on Moving Charges

• Magnetic force is perpendicular to both B and v
• Right-hand rule
• Point fingers of right hand in v direction
• Curl the fingers so that they rotate into the B
  direction
• Then your thumb points
• in the direction of F for charge
• opposite to F for charge
• Note
• the text gives a different version
• I like my way better

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Direction of Magnetic Force on Moving Charges

• Velocity B Force
• out of page right up
• out of page left down

out of page up
ACT 1) Up 2) Down 3) Right 4) Left
5) Zero
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Direction of Magnetic Force on Moving Charges

• Velocity B Force
• out of page right up
• out of page left down
• out of page up
• out of page down

left
right
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Preflight 8.3
Each chamber has a unique magnetic field. A
positively charged particle enters chamber 1 with
velocity 75 m/s up, and follows the dashed
trajectory.
2
1
v 75 m/s q 25 mC
What is the direction of the force on the
particle just as it enters region 1? 1) up 2)
down 3) left 4) right 5) into page 6) out of page
21 7 9 30 24 9
Particle is moving straight upwards then veers to
the right.
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Preflight 8.4
Each chamber has a unique magnetic field. A
positively charged particle enters chamber 1 with
velocity 75 m/s up, and follows the dashed
trajectory.
2
1
v 75 m/s q 25 mC

• What is the direction of the magnetic field in
  region 1?
• 1) up
• 2) down
• 3) left
• 4) right
• 5) into page
• 6) out of page

17 6 8 22 20 26
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ACT 2 Chambers
Each chamber has a unique magnetic field. A
positively charged particle enters chamber 1 with
velocity 75 m/s up, and follows the dashed
trajectory.
2
1
v 75 m/s q 25 mC

• What is the direction of the magnetic field in
  region 2?
• A) down
• B) left
• C) right
• D) into page
• E) out of page

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Magnitude of Magnetic Force on Moving Charges

• The magnetic force on a charge depends on the
  magnitude of the charge, its velocity, and the
  magnetic field.
• F q v B sin(q)
• Direction from RHR
• Note if v is parallel to B then F0

V
q
B
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ACT Moving Charges

• The three charges below have equal charge and
  speed, but are traveling in different directions
  in a uniform magnetic field.
• 1) Which particle experiences the greatest
  magnetic force?
• A) 1 B) 2 C) 3 D) All Same
• 2) The force on particle 3 is in the same
  direction as the force on particle 1.
• A) True B) False

F q v B sin(q)
B
3
2
1
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ComparisonElectric vs. Magnetic

• Electric Magnetic
• Source Charges Moving Charges
• Act on Charges Moving Charges
• Magnitude F Eq F q v B sin(q)
• Direction Parallel to E Perpendicular to
  v,B

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Velocity Selector
Example

• Determine magnitude and direction of magnetic
  field such that a positively charged particle
  with initial velocity v travels straight through
  and exits the other side.

E
v
Electric force is down, so need magnetic force
up. By RHR, B must be into page
For straight line, need FE FB
q E q v B sin(90) B E/v
What direction should B point if you want to
select negative charges? A) Into Page B) Out of
page C) Left D) Right
FE would be up so FB must be down.
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Motion of Q in uniform B field

• Force is perpendicular to B,v
• B does no work! (WF d cos q )
• Speed is constant (WD K.E. )
• Circular motion

• Force is perpendicular to B,v

• Solve for R

Uniform B into page
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Motion of Q in uniform B field

• Force is perpendicular to B,v
• B does no work! (WF d cos q )
• Speed is constant (WD K.E. )
• Circular motion

• Force is perpendicular to B,v

• Solve for R

Uniform B into page
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Preflight 8.6
Each chamber has a unique magnetic field. A
positively charged particle enters chamber 1 with
velocity v1 75 m/s up, and follows the dashed
trajectory.
2
1
v 75 m/s q 25 mC

• What is the speed of the particle when it leaves
  chamber 2?
• 1) v2 lt v1
• 2) v2 v1
• 3) v2 gt v1

17 46 36
Magnetic force is always perpendicular to
velocity, so it changes direction, not speed of
particle.
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Preflight 8.8
Each chamber has a unique magnetic field. A
positively charged particle enters chamber 1 with
velocity v1 75 m/s up, and follows the dashed
trajectory.
2
1
v 75 m/s q 25 mC

• Compare the magnitude of the magnetic field in
  chambers 1 and 2
• 1) B1 gt B2
• 2) B1 B2.
• 3) B1 lt B2

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Larger B, greater force, smaller R
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ACT
Each chamber has a unique magnetic field. A
positively charged particle enters chamber 1 with
velocity v1 75 m/s up, and follows the dashed
trajectory.
2
1
v 75 m/s q ?? mC

• A second particle with mass 2m enters the chamber
  and follows the same path as the particle with
  mass m and charge q25 mC. What is its charge?
• 1) Q 12.5 mC
• 2) Q 25 mC
• 3) Q 50 mC

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extra questions to try
B

• Which angle do you use to determine the magnitude
  of the force on the charged particle? (q1, q2 ,
  either one).
• Below I have drawn the trajectory for two charged
  particles traveling through a magnetic field. Is
  particle 1 positive or negatively charged?
• If particles 1 and 2 have the same mass and
  velocity, which has the largest charge?

V
q1
q2
Either one sin(q) sin(180-q)
1
Positive
Particle 1
2
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Summary

• We learned about magnetic fields B
• We learned about magnetic forces on moving
  charged particles

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