Exam Review I

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Physics 208 Exam 1 Review
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Exam covers Ch. 21.5-7, 22-23,25-26,Lecture,
Discussion, HW, Lab
Exam 1 is Wed. Feb. 20, 530-7 pm, 2103 Ch
Adam(301,310), Eli(302,311), Stephen(303,306),
180 Science Hall Amanda(305,307),
Mike(304,309), Ye(308)

• Chapter 21.5-7, 22
• Waves, interference, and diffraction
• Chapter 23
• Reflection, refraction, and image formation
• Chapter 25
• Electric charges and forces
• Chapters 26
• Electric fields

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Quick Quiz

• An electric dipole is in a uniform electric field
  as shown. The dipole

Dipole

• accelerates left
• accelerates right
• stays fixed
• accelerates up
• none of the above

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Electric torque on dipoles
Remember torque?
Here there are two torques, both into page

• Total torque is sum of these

• Torque on dipole in uniform field

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Dipole in non-uniform field

• A permanent dipole is near a positive point
  charge in a viscous fluid. The dipole will

• rotate CW move toward charge
• rotate CW move away
• rotate CCW move toward
• rotate CCW move away
• none of the above

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Properties of waves

• Wavelength, frequency, propagation speed related
  as
• Phase relation
• In-phase crests line up
• 180 Out-of-phase crests line up with trough
• Time-delay leads to phase difference
• Path-length difference leads to phase difference

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Chapter 22 Waves interference

• Path length difference and phase
• different path length -gt phase difference.
• Two slit interference
• Alternating max and min due to path-length
  difference
• Phase change on reflection
• p phase change when reflecting from medium with
  higher index of refraction
• Interference in thin films
• Different path lengths reflection phase change

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Path length difference

• Path length difference d
• Phase difference (d/?)2p radians
• Constructive for 2pn phase difference

L
Light beam
Recording plate
Foil with two narrow slits
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Question

• You are listening to your favorite radio station,
  WOLX 94.9 FM (94.9x106 Hz) while jogging away
  from a reflecting wall, when the signal fades
  out. About how far must you jog to have the
  signal full strength again? (assume no phase
  change when the signal reflects from the wall)

?3.16 m
Hint wavelength (3x108 m/s)/94.9x106 Hz

• 3 m
• 1.6 m
• 0.8 m
• 0.5 m

path length diff (dx)-(d-x) 2x
Destructive ?2x?/2?x?/4
Constructive ?make 2x??x?/2
x increases by ?/4 3.16m/40.79m
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Two-slit interference
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Two-slit interference path length
?
Constructive int
Phase diff Path length diff
Destructive int.
Phase diff Path length diff
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Reflection phase shift

• Possible additional phase shift on reflection.
• Start in medium with n1, reflect from medium
  with n2
• n2gtn1, 1/2 wavelength phase shift
• n2ltn1, no phase shift
• Difference in phase shift between different paths
  is important.

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Thin film interference
?air
1/2 wavelength phase shiftfrom top surface
reflection
air n11
Reflecting from n2
?air/n
No phase shift frombottom interface
n2gt1
t
Reflecting from n1
air n11
Extra phase shift needed for constructive
interference is
Extra path length
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Diffraction from a slit

• Each point inside slit acts as a source
• Net result is series of minima and maxima
• Similar to two-slit interference.

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Overlapping diffraction patterns

• Two independent point sources will produce two
  diffraction patterns.
• If diffraction patterns overlap too much,
  resolution is lost.
• Image to right shows two sources clearly resolved.

Angularseparation
?
Circular aperture diffraction limited
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Diffraction gratings

• Diffraction grating is pattern of multiple slits.
• Very narrow, very closely spaced.
• Same physics as two-slit interference

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Chap. 23 Refraction Ray optics

• Refraction
• Ray tracing
• Can locate image by following specific rays
• Types of images
• Real image project onto screen
• Virtual image image with another lens
• Lens equation
• Relates image distance, object distance, focal
  length
• Magnification
• Ratio of images size to object size

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Refraction

• Occurs when light moves into medium with
  different index of refraction.
• Light direction bends according to

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Lenses focusing by refraction
F
P.A.
Object
F
1) Rays parallel to principal axis pass through
focal point.
2) Rays through center of lens are not refracted.
3) Rays through F emerge parallel to principal
axis.
Here image is real, inverted, enlarged
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Different object positions
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Question

• You have a focused image on the screen, but you
  want the image to be bigger. Relative to the
  lens, you should

• Move screen away, move object away
• Move screen closer, move object away
• Move screen away, move object closer
• Move screen closer, move object closer

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Equations
Image and object different sizes
p
q
Relation between image distance object
distance focal length

• Magnification M

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Question

• You want an image on a screen to be ten times
  larger than your object, and the screen is 2 m
  away. About what focal length lens do you need?

• f0.1m
• f0.2m
• f0.5m
• f1.0m

q2 m mag10 -gt q10p-gtp0.2m
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Chapter 25 Electric Charges Forces

• Triboelectric effect transfer charge
• Total charge is conserved
• Vector forces between charges
• Add by superposition
• Drops off with distance as 1/r2
• Insulators and conductors
• Polarization of insulators, conductors

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Charges conductors insulators

• Two types of charges, and -
• Like charges repel
• Unlike charges attract
• Conductor
• Charge free to move
• Distributed over surface of conductor
• Insulator
• Charges stuck in place where they are put

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Electric force magnitude direction

• Electrical force between two stationary charged
  particles
• The SI unit of charge is the coulomb (C ), µC
  10-6 C
• 1 C corresponds to 6.24 x 1018 electrons or
  protons
• ke Coulomb constant 9 x 109 N.m2/C2
  1/(4peo)
• eo ? permittivity of free space 8.854 x 10-12
  C2 / N.m2
• Directed along line joining particles.

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Forces add by superposition
Equal but opposite charges are placed near a
negative charge as shown. What direction is the
net force on the negative charge?

• Left
• Right
• Up
• Down
• Zero

-

-
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Chapter 26 The Electric Field

• Defined as force per unit charge (N/C)
• Calculated as superposition of contributions from
  different charges
• Examples
• Single charge
• Electric dipole
• Line charge, sheet of charge
• Electric field lines
• Force on charged particles

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Electric field

• Fe qE
• If q is positive, F and E are in the same
  direction

Example electric field from point charge
E
E (9?109)(1.6?10-19)/(10-10)2 N 2.9?1011 N/C
(to the right)
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Pictorial representation of E Electric Field
Lines
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Electric field lines

• Local electric field tangent to field line
• Density of lines proportional to electric field
  strength
• Fields lines can only start on charge
• Can only end on - charge (but some dont end!).
• Electric field lines can never cross

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Electric dipole
q
-q

• Electric field magnitude drops off as 1/r3

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Quick quiz continuous charge dist.

• Electric field from a uniform ring of charge.
  The magnitude of the electric field on the x-axis

y

• Has a maximum at x0
• Has a maximum at x?
• Has a maximum at finite nonzero x
• Has a minimum at finite nonzero x
• Has neither max nor min

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Force on charged particle

• Electric field produces force qE on charged
  particle
• Force produces an acceleration a FE / m
• Uniform E-field (directionmagnitude) produces
  constant acceleration if no other forces
• Positive charge accelerates in same direction as
  field
• Negative charge accelerates in direction opposite
  to electric field

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Force on a dipole

• Dipole made of equal and - charges
• Force exerted on each charge
• Uniform fieldcauses rotation

Dipole
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Dipole in non-uniform field

• A dipole is near a positive point charge in a
  viscous fluid. The dipole will

• rotate CW move toward charge
• rotate CW move away
• rotate CCW move toward
• rotate CCW move away
• none of the above