Transcript Interactions b/w hail and smaller ice crystals transfer e

E2 - Electric Field
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Electrostatic & gravitational force
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act at a distance, no direct push or pull
Electric Field
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conceived by Michael Faraday
extends out from every charged object
other charges interact with field
direction of field determined by motion induced on
a positive test charge
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E2 - Electric Field
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Coulomb’s Law
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Action-at-a-distance model
Charges act directly on one each other
charge  charge
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Electric Field
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
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Field model
Field acts on charge
charge  field  charge
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Electric Field
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Electric field is a vector expressed in terms of
the electric force and the charge
r
r Fe
E
q
r
r
Fe  qE
r
1 q
ˆ
E
r
PC
2
40 rPC

Insert fig. E2.1
 • If Q > 0, E points away from Q
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If Q > 0, E points away from Q
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Magnitude of Test Charge, q0
• The test charge is a conceptual tool to help understand the
magnitude and direction of the electric field.
• The magnitude of the test charge is VERY small.
• The test charge does not affect existing charge distribution
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Electric Field:
A proton is released in a
uniform electric field, and it experiences an
electric force toward the south. What is the
direction of the electric field?
1.
2.
3.
4.
North
South
East
West
Electric Field:
An electron is released in a
uniform electric field, and it experiences an
electric force toward the west. What is the
direction of the electric field?
1.
2.
3.
4.
North
South
East
West
Electric Field:
What are the magnitude and direction of the
electric force on an electron (e=1.6x10-19 C) in
a uniform electric field of strength 2000 N/C
that points due east?
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2.
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3.2x10-16, east
3.2x10-16, west
8.0x10-23, east
8.0x10-23, west
Think-Pair-Share
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Calculate the electric field 0.1 m from a
10-6 C charge.
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Analogy
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Electric field is electric force per unit
charge. What is the analogous quantity
for gravitational force? Derive a
mathematical expression for
gravitational field. Have you seen this
quantity before?
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Electric Field Lines
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Field lines show the magnitude (in a
relative way) and the direction of the
force felt by a positive test charge
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Insert E2.2
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Electric Field Lines (L)
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Draw field lines for the following charge
distributions
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Electric Field
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Electric Field Lines (L)
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Draw field lines for the following charge
distributions
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Superposition
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How do you calculate the electric field
due to multiple charges?
• Calculate the field due to each
charge and add them up!
r r
r
r
E  E1  E 2  E 3  ...
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Think-Pair-Share (L)
Q1 = -1x10-6 C
Q2 = 10 x 10-6 C
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Two charges, Q1 and Q2, are
separated by 100 nm. What is the
electric field midway between the
charges?
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Think-Pair-Share (L)
Q1 =
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1x10-6
Q2 = 10 x 10-6 C
C
Two charges, Q1 and Q2, are
separated by 100 nm. Where on the
line joining Q1 and Q2 is the electric
field equal to zero?
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Electric Field is a Vector
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What is the net Electric field at point A?
What is the net force on a charge Q that
is placed at A?
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Electric Field of a dipole
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A dipole is a pair of particles separated
by a small distance, equal magnitude
and opposite signs
The field of a dipole decreases with
distance more rapidly than a single
charge
Insert E2.4
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A dipole has an electric field
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A dipole is electrically neutral, so why
does it have a field?
The electric field of the near charge is
longer than that of the far charge
Insert E2.5
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Field of the dipole
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Insert E2.6 and E2.7, E2.8
E2.9, E2.10
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Torque of a dipole
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The torque of a dipole in an external
electric field
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d is the distance displacement vector from
“-” to “+”
r
r
v
v r
v
v
dp  d  qE  qd  E  pe  E
r
r
pe  qd
Insert E2.9
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Water is a dipole
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If water is put in an electric field, the
water molecules will experience a
torque that attempts to twist them
Insert E2.8 and E2.10
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Lightning
Interactions b/w hail and smaller ice crystals transfer e- to
hail. They are heavier and sink to base of cloud.
Ice
Crystals
Hail
Thunder:
shock wave
created by
rapid
heating of air
Upward streamers of + ions
travels up from ground.
Lightning occurs if it meets
stepped leader from cloud.
e- run away from cloud
base, leaving net + charge
on ground under cloud
Light
due to e-’s
recombining
w/ions
Path of
lightning
is about
1 cm in
diameter
Lethal Strike
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Physiological effects of
a direct strike:
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Heart may restart, but breathing won’t
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stops heart
paralyzes muscles
internal burns
need mouth-to-mouth
Other types of strikes:
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Side flash or touching struck object
Ground current
Dead-end upward streamer
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Lightning Rods
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Provide path for current
to pass directly to
ground without passing
through building
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path of least resistance
Diffuse charge build-up
Photo from lightning
research center in FL
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lightning rods may not
provide enough
protection to homes built
on sandy soil
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Lightning Safety
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Lightning kills ~100 people per year in US
Stay inside if possible. Avoid:
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showers
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much lower resistance when wet
phones (cell phones are ok)
any electrical appliances
If outside,
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avoid tall objects
seek low-lying area
duck head, crouch
pull feet together, run, hop on one foot (ground current)
stay in car - Faraday cage (Note: Saturns are not safe!)
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Lightning: Physical Characteristics
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Voltage
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Current
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several hundred million volts
1020 electrons in a fraction of a second
~10 kiloamps
So what is a Volt? what is an amp?
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Good question. We’ll get to that soon.
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Electric Field: The electric field just
before a lightning strike points:
1. toward the ground
2. toward the cloud
3. along the ground
Group Problems
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E2B.4, E2B.5, E2S.2
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