Transcript electrical field

Static Electricity
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“Static”- not moving. Electric
charges that can be collected and
held in one place
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–
–
Examples: sparks on carpet, balloon
against hair, lightning, photocopier
History: ancient Greeks made little
sparks when rubbing amber with fur
(Greek word for amber: “elektron”)
Electric charge, “q”, is measured in
Coulombs, C. One Coulomb is charge
is a dangerously high charge. An
average lightning bolt has about 10
Coulombs of charge.
Atomic View
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Proton: in nucleus
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Positive charge
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q = + 1.6 x 10-19 C
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Electron: outside nucleus
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Negative charge
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q = - 1.6 x 10-19 C
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Protons and Electrons have the same amount of
charge but a proton has much more mass!
Neutron: in nucleus, has no charge
Molecules
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–
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2 or more atoms bonded together
usually atoms and molecules are neutral,
but if they have a net charge, they are called
IONS
Hmmm..
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Ben Franklin was the
first to use the terms
“positive” and
“negative” to describe
electrical charge.
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Behavior of charges
– Unlike charges attract
– Like charges repel
– A neutral object will attract both positive
and negative charges
The Earth is able to absorb much electrical
charge.
Touching a charged object to the Earth in
order to discharge it is called
GROUNDING
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Materials
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Conductors
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Allow charges to move
easily because they
have “free electrons”
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Examples: metals
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Insulators
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Charges cannot move
easily
•
Examples: plastic,
wood, glass
Semiconductor: used in
computers
Conducts under the right
conditions
Superconductor: NO
resistance to the flow of
electrons. So far, no
material is a superconductor
except at extremely low
temperatures.
–
Water: insulator or conductor?
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•
•
–
PURE water does NOT conduct
electricity, therefore it is an insulator
Any impurities in water will make
it a conductor
(the conduction of electricity is called
ELECTROLYTIC behavior- )
Air: insulator or conductor?
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•
Usually an insulator, thankfully
When strong forces are present, electron’s can be
pulled from air molecules, creating ions
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ions conduct (putting NaCl into water, for example)
example: lightning
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Static devices
– Electroscope: the
separation of metal
leaves indicates the
presence of static
charge
– Van de Graaff
generator: a charge is
delivered by a rubber
belt to a metal dome
Lightning
An electrical discharge between the clouds
and the ground or between two clouds.
As the electrons flow through the ionized
air, they generate so much heat that a
PLASMA is produced. We see that plasma
and call it LIGHTNING!
The air around the lightning expands so
rapidly from the heat that it creates a
strong pressure wave of air molecules
(that’s sound!)
We call that THUNDER!
How much electrical charge is flowing
through a lightning bolt?
Typically around 10 Coulombs.
How many electrons, each with a negative
charge of 1.6 x 10-19 C, does it take to
have 10 C of charge?
10 C / 1.6 x 10-19 C =
6.25 x 1019 electrons !
How many electrons are flowing in a 12 C
lightning bolt?
7.5 x 1019 electrons
•
Methods to electrically charge an
object
– Conduction:
• Direct contact: will transfer
electrons, such as touching your
car door in the winter
• Friction: rubbing your feet against
carpet, hair against a balloon
– Induction: no direct contact
• Start with a neutral object. Then, bring an electrically charged object
near, but not in contact with, a neutral object
• The charges in the neutral object will be “induced” to separate to get
closer or farther from the charged object.
• If provided a pathway, the separated electrons will leave.
• The object is now positively charged.
•
Coulomb’s Law
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–
Calculates the magnitude of the electric
force between two charges
Each charge experiences equal but
opposite forces
q1q2
F k 2
d
where k is a constant, k = 9 x 109 N·m2/C2
Coulomb’s Law looks VERY similar to
Newton’s Universal Law of Gravitation
FG
m1m2
d
2
Fk
q1q 2
d
2
Differences:
1. Gravitational Force is based on MASS.
Coulomb’s law is based on CHARGE.
2. Gravity is ALWAYS an attractive force.
The Electric Force can attract and repel.
3. “G” is a tiny number, therefore gravity force is a small
force.
“k” is a huge number, therefore electric force is a large
force.
Both laws are INVERSE SQUARE LAWS
“The Force varies with the inverse of the
distance squared.”
At twice the distance,  22 in denominator
= ¼ the Force,
At three times the distance, 32 in denominator,
= 1/9 the Force
At half the distance,  (1/2)2 in denominator
= 4 times the Force
Now if one CHARGE doubles…. The Force doubles
since they are directly related.
Coulomb’s Law looks VERY similar to
Newton’s Universal Law of Gravitation
FG
m1m2
d
2
Fk
q1q 2
d
2
Differences:
1. Gravitational Force is based on MASS.
Coulomb’s law is based on CHARGE.
2. Gravity is ALWAYS an attractive force.
The Electric Force can attract and repel.
3. “G” is a tiny number, therefore gravity force is a small
force.
“k” is a huge number, therefore electric force is a large
force.
Electric Field
A gravitational field
surrounds all masses.
An electric field surrounds
all charges.
The stronger the electric
charge, the stronger the
electric field surrounding
it.
One way to measure the
strength of a gravitational
field is to release a mass in
the field and measure how
strength of the force exerted
on it.
One way to measure the
strength of an electrical
field is to release a charge
in the field and measure the
strength of the force exerted
on it.
So… the strength of the electric field, E, is
given by
Electric Field = Force ÷ charge
E=F÷q
For example:
A 0.5 C charge of mass 0.2 kg experiences a force
of 20 N when placed in an electric field.
What is the strength of the electric field, E?
E=F÷q=
20 N ÷ 0.5 C =
40 N/C
If the charge was released, what would be its
acceleration? (Use Newton’s 2nd Law!)
F = ma
a = F/m
a = 20 / 0.2
a = 100 m/s2
The electric field near a charged
piece of plastic or styrofoam is
around 1000 N/C.
The electric field in a television
picture tube is around 10,000 N/C.
The electric field at the location of the
electron in a Hydrogen atom is
500,000,000,000 N/C!
The further you go from an electric
charge, the weaker the field
becomes.
The electric field around a charge can be
represented by
Electric field lines
Electric fields exist, but electric field lines
don’t really exist but provide a model of
the electric field.
Electric Field
Lines
Electric field lines always point OUT of a positive
charge and INTO a negative charge
To indicate a stronger
electric field, just draw
MORE lines.
The farther apart the lines,
the weaker the field.
Since the electric field, E,
has both magnitude and
direction, it is a vector.
+2q
- 4q
• The electric field
INSIDE a hollow
conductor is ZERO
even if there are
charges on the
OUTSIDE of the
conductor!
Michael Faraday, 1791-1867
Michael Faraday
demonstrated that the
electrostatic charge only
resides on the exterior of a
charged conductor, and
exterior charge has no
influence on anything
enclosed within a conductor.
This was one of many
contributions he made to
electromagnetic theory.
Electric Shielding
There is no way to shield from
gravity, but there is a way to
shield from an electric field….
Surround yourself or whatever
you wish to shield with a
conductor (even if it is more
like a cage that a solid surface)
That’s why certain electric
components are enclosed in
metal boxes and even certain
cables, like coaxial cables have
a metal covering.
The covering shields them from
all outside electrical activity.
“Faraday Cage”
Are you safe from lightning inside
your car?
Why or why not?
• Move to a sturdy building or car.
• Do not take shelter in small sheds, under isolated
trees, or in convertible automobiles.
• Get out of boats and away from water.
• Telephones lines and metal pipes can conduct
electricity. Unplug appliances if possible and
avoid using the landline wired telephone (unless
it is an emergency) or any electrical appliances.
• Do not take a bath or shower.
• If you are caught outdoors and
unable to find shelter:
• Find a low spot away from trees,
fences and poles.
• If you are in the woods, take
shelter under the shorter trees.
• If you feel your skin tingle or your
hair stand on end, squat low to
the ground on the balls of your
feet. Place you hands on your
knees, put your head down and
try to make yourself the smallest
target possible while minimizing
your contact with the ground.
Voltage
Voltage can be thought of as a kind of
pressure- Electrical Pressure
Voltage is also called Electric Potential
Think of the water supply at your housesometimes you have high water
pressure-water flows quickly- and
sometimes low water pressure- water
flows slowly.
With Higher Voltage (pressure), charges are
able to flow more quickly
Voltage and Pressure
You may have more PRESSURE in a shower
nozzle than in a slow moving river, but does the
pressure alone tell you how much water is
actually moving?
No! The pressure alone does not tell you how
much water was actually flowing.
The flow of water is called the “current”.
Voltage = Pressure
Rub a balloon on your hair and it
becomes negatively charged, perhaps to
several thousand volts.
Does this mean that the balloon is
dangerous??
There’s a lot of electric pressure, but was
there a lot of charge flowing (current)?
Well, the charge transferred to the
balloon is typically less than a millionth of
a Coulomb – not much at all. No
danger!
So… High Voltage does not necessarily mean that
something is dangerous.
High Voltage is not necessarily
dangerous- a Van de Graaff
generator can have more than
400,000 V, but there’s not much
charge that is transferred to you
from the globe.
And Low Voltage is not necessarily
safe. Our houses are wired with
120V and you can be killed from
that electricity.
Voltage (potential) is not the
dangerous part of electricity. The
dangerous part is how many
charges are flowing- the “current”.
The Electric Potential (Voltage), V, changes
as you move from one place to another
in an electric field
The change in Potential (“pressure”), called
the “Potential Difference” is given by
DV = Ed
Electric Field
3 meters
For example, the potential
difference between two
locations separated by 3
meters in a 4000 N/C electric
field is given by
DV = Ed = 4000 N/C x 3 m =
12,000 V
Conversion of energy
Moving a mass or
moving a charge
takes
work energy
that is converted to
potential energy
Move a mass, m
Through a gravitational field, g
A distance, h, you produce a
Gravitational Potential Energy, mgh
Move a charge, q
Through an electrical field, E
A distance, d, you produce an
Electrical Potential Energy, qEd
The work energy required to move a charge, q,
through an electric field, E, a distance d, is given by
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W = qEd = qDV
Sometimes, a charge is said to be located “at
ground”.
The potential (voltage) at “ground” is zero.
Vground = 0 Volts
It takes 2.43 x 10-15 J of work to move an electron as
distance of 2 m in an electric field. What is the strength
of the field?
W = qEd
E = W / (qd)
E = 7600 N/C
What is the potential difference, DV?
DV = Ed
DV = 7600 x 2
DV = 15200V
If you release an object
in a gravitational field,
its gravitational potential
energy is converted to
kinetic energy.
If you RELEASE a charge in an electrical
field, its potential energy, U, is converted
to kinetic energy, K!
UE = ½ mv2
E
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It takes 2.43 x 10-15 J of work to move an
electron as distance of 2 m in an electric
field. What is the strength of the field?
W = qEd
E = 7600 N/C
The electron is then released. What is the
maximum velocity it will achieve?
2.43 x 10-15 J = W = qEd = ½ mv2
v = 7.3 x 107 m/s
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How much work is required to move a 3 C charge through
an electric field of 2000 N/C a distance of 1.5 m?
How much work is required to move 0.5 C of charge
through a potential difference of 110 V?
In a TV picture tube, an electron moves through a potential
difference of 5000 V. How much work energy is required?
It takes 2000 J of work to move a certain charge through a
400 N/C electric field a distance of 2 m. What is the
charge?
What is the potential difference between two points in a
3000 N/C electric field that are 0.3 m apart?
What is the voltage between two points in a 4500 N/C
electric field that are ½ m apart?
If the potential difference between two points in an electric
field of 500 N/C is 220 V, how far apart are the two points?
What is the strength of the electric field if there is a potential
difference of 600 V at two locations that are 0.25 m apart?
Capacitors:
Electric Energy Storage
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A device consisting of two
conductors placed near,
but not touching each
other in which electric
charge and energy can be
stored.
Capacitors are Used in
– camera flashes
– defibrillators
– Computers: tiny capacitors
store the 1’s and 0’s for the
binary code
– Many keyboards have a
capacitor beneath each key that
records every key stroke.
– Virtually every electronic device
Leyden Jar, the first
“capacitor”
Dutch physicist
Pieter van
Musschenbroek of the
University of Leyden