click on the picture to play electrical hokey

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click on the picture to play electrical hokey
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Electric Field
Let's take a single electric charge, Q, and put
it somewhere. The space around it is different
from the space without charge. We have created a
situation in which we could have an electric
force. All we have to do is bring in a second
charge, q, to feel the force. Without q, there is
no force ....but we still have the condition that
we could have a force. We say that the space
around charge contains ELECTRIC FIELD.
How to measure/find the strength (magnitude and
direction) of electric field at particular
location P due to charge Q?
A test charge, q, placed at P will experience an
electric force, F - either attractive or
repulsive.
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Definition of electric field, E, at a point P
distance r away from Q.
The magnitude of the electric field is defined as
the force per unit charge.
As F contains q, E DOESNT depend on q at all,
only on Q.
Electric field at any point P in space is always
in the direction of the force on a positive test
charge if it were placed at the point P.
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The other way around If you know electric field
E at a point where you place a charge q, that
charge will experience the force F
F q E
q
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Electric field of a charged particle/point charge
A charged particle Q creates an electric field.
E Field independent
of test charge
? magnitude the same value on the
sphere of radius r around ? direction
radially outward or inward
example
Q1.6x10-19 C

q positive test charge
(to the right)
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Question
Say the electric field from an isolated
point charge has a certain value at a distance of
1m. How will the electric field strength compare
at a distance of 2 m from the charge?
It will be ¼ as much inverse square
law for force between two charges carries over to
the electric field from a point charge.
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We use Electric Field Lines to visualize el.
field.
Convention / agreement
Direction indicates direction in which a positive
test charge would be pushed direction of the
force!!!.
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Electric Field of a Point Charge
0.8?1011 N/C
32?1011 N/C
25?1011 N/C
2.9?1011 N/C

E
This is becoming a mess!!!
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Electric Field Lines

• Density gives strength
• lines proportional to Q
• lines never cross!
• Arrow gives direction
• Start on , end on -

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negative charge
positive charge
So always point away from charges, towards
charges
Denser lines - stronger field el. field
decreases with distance
more lines revels stronger field due to greater
charge
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Electric field lines can never cross. If they
crossed, that would mean that a charge placed at
the intersection, would be accelerated in TWO
directions at once! This is impossible! If two
sources are creating electric fields in the same
place, we have to add the two vectors and get a
resultant vector representing the NET ELECTRIC
FIELD.
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Question?

• What is the direction of the electric field at
  point C?
• Left
• Right
• Zero

Away from positive charge (right)
Towards negative charge (right)
y
Net E field is to right.
C
x
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Question?

• What is the direction of the electric field at
  point A?
• Up
• Down
• Left
• Right
• Zero

A
x
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Question?

• What is the direction of the electric field at
  point B?
• Up
• Down
• Left
• Right
• Zero

y
B
x
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Question?

• What is the direction of the electric field at
  point A, if the two positive charges have equal
  magnitude?
• Up
• Down
• Left
• Right
• Zero

A
x
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Electric field of a capacitor
Uniform electric field (the one that has constant
magnitude and direction is generated between two
oppositely charged parallel plates. Edge effect
is minimazed when the length is long compared to
their separation.
FOR MORE CLICK
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Electric field of a charged conducting sphere in
electrostatic equilibrium
Electric field outside a charged sphere (evenly
distributed charge q over surface) at distance r
from its center is the same as if the charge q
is concentrated at the center of the sphere
r
q
E k
R
at the surface
What is electric field inside the sphere?
OK. Lets start. In general if you have many
charges you have to find el. field of each of
them and then add them up as vectors to get net
el. field at certain point. So imagine a
solid!!!!!!!!! Good luck.
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Conductor is in electrostatic equilibrium when
there is no net motion (flow ) of charge within
a conductor or on its surface.
Different approach

• Conductor ? electrons free to move
• charges in electric field feel the force F Eq
• only free electrons can move in the conductor so
  they will move until
• E 0 inside a conductor

if not then electrons would respond to its
presence and be accelerated within the conductor.
And that is not conductor in electrostatic
equilibrium.
Q.E.D

• If such conductor has excess charge, it resides
  entirely on the conductors outer surface running
  away from each other as far as possible. El.
  field is still zero everywhere inside the
  conductor.

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Therefore, electric field of charged conducting
sphere in electrostatic equilibrium is
r
E0
q
R
at the surface
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El. field just outside a charged conductor is
perpendicular to the conductors surface.
if not
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• Charge tends to accumulate where the curvature
  is the greatest (sharp points).

The fact that pointed objects create strong
electric fields if charged is the reason for the
shape of lightning rod. Youll see.
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Summary Conductors in electrostatic equilibrium

• no net motion (flow) of charge within a
  conductor . Then

• El. field is zero everywhere inside the
  conductor
• If such conductor has excess charge, it resides
  entirely on the conductors outer surface. El.
  field is still zero everywhere inside the
  conductor.
• El. field just outside a charged conductor is
  perpendicular to the conductors surface.
• Charge tends to accumulate where the radius of
  curvature is smallest (sharp points).

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Electrical Shielding

• Electric field is zero inside conductor,
  regardles how strong the electric field is
  outside.
• Even more electric field inside metal cavities
  is zero, regardles whats going outside - so put
  electrical equipment in metal boxes. Outside may
  be very strong fields and high charges, but the
  charges on the metal surface rearrange to give 0
  inside.
• This is why if lightning hits a car and youre
  inside, youre safe, at least if it is not too
  strong ot make everything melt. Electrons from
  the lightening bolt mutually repel and spread
  over the outer surface. Although the electric
  field they set up maybe great outside the car,
  the overall electric field inside the car
  practically cancels to zero.
• Radio signals cannot penetrate through a metal
  enclosure
• (mobile in an elevator)
• The metal bars (rebar) that reinforce the
  concrete in walls can interfere also

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Electrostatic shielding

• The effect of the high voltage on the van de
  Graff generator stops on the outside of the metal
  cage ? Homer is SAFE!

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Lightning great weapon of the Gods

• causes 80 million dollars in damage each year in
  the US
• kills 85 people a year in the US
• is all over in a thousandth of a second
• carries up to 200,000 Amperes (currant)
• temperature of a lightning bolt can reach
  30,000C 5x hotter than the surface of the
  sun
• causes the thunder!

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development of a lightning bolt
1. step Positive and negative charges separate.
A cloud becomes positively charged at the top and
negatively charged at the bottom. Reasons for
this charge distribution are complex and only
partially understood.
2. step As the negative charges collect at the
bottom of the cloud it forces the negative
charges in the ground to be forced away from the
surface.  This leaves the ground positive. If
attractive force is large enough electrons will
leap from negative region downward from the cloud
to the ground.
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http//regentsprep.org/Regents/physics/ phys03/ali
ghtnin/default.htm
3. step As the electrons approach the earth, el.
field just above the surface becomes so intense
that it ionizes atoms and molecules in the air.
An intense flash of light is produced.
4. step When first breakdown occurs, negative
charge in the column farther above the earth can
rush down through the region of ionized air.
Now that flow of electrons originated farther
above the earth causes a flash of light. That way
flash appears to move up.
The rapidly moving electrons excite the air along
the path so much that it emits light. It also
heats the air so intensely that it rapidly
expands creating thunder.
One thing to notice is that the positive charges
that make up both the cloud and the ground do not
move.  Even the positive streamer launched by the
ground (in the animation) is really only made up
of positively charged air particles because the
electrons left the particle.
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Electrons in atoms orbit the nucleus well defined
orbits called stationary states. By absorbing
energy atom can be raised to higher energy level
called excited state. When excited electron jumps
back to a lower level it emits photon.
At ordinary temperatures and when not in strong
electric filed most atom are in the ground state.
But if the temperature is high and/or electric
field very strong the air will turn into plasma.
The light is generated when the stripped
electrons in the plasma recombine with the
ionized ions. The probability of recombination
process is pretty high, so the time for the
recombination to occur is short. The streaks of
light from the lightening fade away almost
immediately. When a meteor shower enters the
very high regions of the atmosphere, streaks of
light last for as long as 2 3 seconds before
fading. Because the probability of freed
electrons finding an ionized atom with which to
recombine is very low (pressure and density are
low - atoms are relatively far apart).
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Lightening Rods
Place rod above a building, and connect it to
ground. Then the point of the rod picks up
electrons from the air (leakage), so prevents
large build up of on the building, hence
decreasing chance of a lightning strike.
But even if there is a lightning strike (if
leakage not enough), the electricity goes through
rod to ground, rather than through building.
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While enjoying the Sequoia National Park from a
lookout platform, this woman found her hair
rising from her head. Amused, her brother took
her photograph. Why should she run for cover?
Five minutes after they left, lightning struck
the platform, killing one person and injuring
seven.
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