Download Static Electricity

Static Electricity
What are the fundamental
particles and how do they
interact?
Atomic Structure
Lithium
charges
3 electrons
3 protons
3 neutrons
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(-3)
(+3)
(0)
Electrons are lighter and are much easier
to remove from an atom.
Atoms are electrically neutral
Ions: atoms have lost one or more e-
3 Big Rules of Electrostatics
•Opposites Attract
•Likes Repel
•Only Electrons Move
How do objects pick up static
charge?
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Friction causes electrons to transfer to a material
with more “affinity”.
Conservation of charge: Total charge remains
the same.
Electron Affinity
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Cellophane tape
Rubber
Copper, brass
Amber
Wood
Cotton
Silk
Wool
Nylon
Glass
Rabbit fur
Becomes more
Negative
Electrostatic Forces
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Experiments with pith balls
Does a charged object attract a
neutral object?
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A neutral stream of
water bends toward
the negatively charged
comb
Polarization of Charges
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Charges inside
an object
separate and
attraction is
then possible.
Charge Transfer
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Conductors: materials that allow charges move
easily. Static charges spread out evenly on the
surface and transfer easily.
ex: Metals, Ion solutions
Insulators: Materials that do not allow charge
to flow easily. ex: air, wood, plastic
If an object is charged will it remain
charged forever?
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Static shocks (lightning)
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Water molecules (polar molecules) touch and
remove charges.
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Large round surfaces can hold more charge.
Charge Transfer
Two identical conducting spheres are charged and then
allowed to touch.
before
contact
What is the net charge for both?
Which way will the charge flow?
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After contact they share the charge.
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Describe the charge transfer.
Metal sphere A has a charge of +12 elementary
charges and identical sphere B has a charge of
+16 elementary charges. After the two spheres are
brought into contact, the charge on sphere A is
+12
A
a)+28 elem. charges
b)-2 elem. charges
+16
B
c)+2 elem. Charges
d)+14 elem. charges
What if the spheres are not the same
size?
What if one sphere was the earth?
Grounding
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when an object is connected to the earth it
becomes neutral.
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Earth can absorb any amount of excess e- or
supply any amount of e- to make an object
neutral.
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Symbol for ground:
Electric Force
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Electric forces must be strong because they can
easily produce accelerations larger than the
acceleration caused by gravity.
These forces can be either repulsive or attractive
and so are either positive or negative
(respectively)
Charges exert forces on other charges at a
distance
The force is stronger when the charges are
closer together
Like charges repel; opposite charges attract
Forces on Charged Bodies
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An electroscope is a device consisting of a
metal knob connected by a metal stem to two
thin, lightweight pieces of metal foil, called
leaves
Charging by Conduction
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When a negatively charged rod is touched to the
knob of an electroscope, electrons are added to
the knob and then spread all over the metal
surfaces.
Thus the two leaves are negatively charged and repel
each other
 The electroscope has been given a net charge
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The same thing happens if the electroscope is
charged positively
Charging a neutral body by touching it with a
charged body is called charging by conduction
Charging by Conduction
Separation of Charge on Neutral
Objects
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If you bring a charged rod near a neutral
electroscope, you can make the electroscope
behave as if it were charged while it is still
electrically neutral by causing the interior
charges to separate
Charging by Induction
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However, if after you cause the interior charges
to separate, you give the repelled charges a way
out of the object, then you can permanently
charge the object
This process of charging an object without
touching it is called charging by induction
Charging by Induction
Coulomb’s Law
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Coulomb’s Law, also known as the Electrostatic Law,
states that the force between two charged objects can be
found by:
q1q2
F k 2
r
K is Coulomb’s constant (also called the Electrostatic Constant)
and has a value of 8.99x109 Nm2/C2
q1 is the charge of one charged object
q2 is the charge of the other charged object
r is the distance between the two charges
Units for the charges are in coulombs (C)
Unit for the distance is meters (m)
Unit for the electrostatic force are in newtons (N)
Elementary Charge
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An elementary charge is the magnitude of the
charge of an electron
The charge on a single electron is 1.60x10-19C
One coulomb is the charge of 6.24x1018
electrons or protons
A typical lightning bold can carry 5C to 25C of
charge
Find the Electrostatic Force between these two
point charges.
Electric Field
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An electric field is a region in space where a
force is exerted on a positive test charge
Electric lines of force (Electric Field Lines)
represent the direction that a positive test charge
would move in an electric field
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By convention, they start at positively charged
objects and end at negatively charged objects
Electric Field Lines
Top: 2 positive charges repel
from one another
Bottom: 1 positive charge
emits an electric field force
towards a negative charge
3-Dimensional
Image of repulsive
electronic forces
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A charge in an electric field experiences an
electric force
The equation to find the strength of this electric
force upon a test charge within the electric field
is:
E = F/q
This states that the electric field strength (in
units of N/C) is a measure of the force upon a
test charge divided by the charge value of the
test charge
Electric Field Strengths are vector quantities,
which means that they have both a magnitude as
well as a direction
Question
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What is the electric field strength of a 5.8µC test
charge that is experiencing a repulsive force of
138N?
Remember that 1µC = 1x10-6C
 Note that the force upon the test charge is repulsive…
What sign convention should it follow?
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Question
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At which point is the electric field strength the
greatest? (A, B, or C)
Is it possible to store charge?
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Ans: give an e- what it wants. To be near +
charges.
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The capacitor: Two plates separated but very
close. Each has an opposite charge.
Capacitor
This is why you shouldn’t use your
cell phone while driving.
Your cell phone gives off
electrons in its signal
This gives the phone a
positive charge which
attracts the negative
charge of the lighning
bolt
Electric Potential Difference
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Work is done by the electric field if the electric
force acting on the charge causes it to move
from one point to another
These two points differ in their electric potential
The magnitude of the work done on the charge
by the electric field is a measure of the
difference in potential
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The Electric Potential Difference is the work
done per unit charge as a charge is moved
between two points in an electric field
This follows the mathematical equation:
V = W/q = (Fd)/q
Where V is the electric potential difference, W is
the amount of work done, and q is the charge
that is moved
 Unit for the electric potential difference is the
volt (V)
Question
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What is the electric potential difference if a
100N force moves a 6µC charge a distance of
5cm?