Download Electrostatics Summary

ELECTROSTATICS
I. Overview
•  ELECTROSTATICS is the study of STATIC ELECTRICITY.
•  Static electricity is an electric charge carried on an insulated object. The object
DISCHARGES (transfers) it upon contact with another object.
•  A static charge can be placed on an object with FRICTION (most common).
•  To understand static electricity, you need to review the characteristics of the atom.
•  Static electricity results from a temporary imbalance of the number of protons
and electrons in an object. Electrons are gained or lost to create charge, NOT protons.
•  Opposite charges attract. Like charges repel. This is an important property of
objects carrying static electricity.
II. The atom and charges
The PROTONS (+ charge) and neutrons are
in the nucleus
The electrons (- charge) surround it.
Outermost electrons are “loosely held” and can
be made to transfer with friction-rubbing two
objects together
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the atom, continued
Materials that hold their electrons tightly are INSULATORS. They resist the
flow of electric charge.
Materials that hold their electrons loosely are CONDUCTORS. These electrons
are willing to “jump” or be displaced toward + charged atoms, including being
transferred between objects
Atoms on the surface of objects will pick up or lose electrons when two objects
are contacted by rubbing.
One object loses electrons, the gains them. Both objects have an IMBALANCE
of + and – charge. The + charged object has more protons than electrons.
The – charged object has more electrons than protons.
WE CALL THE CHARGES ATOMS IONS.
STATIC ELECTRICITY RESULTS ON BOTH OBJECTS. IT IS THE IMBALANCE
OF + AND - CHARGE ON AN OBJECT DUE TO THE GAIN OR LOSS OF ELECTRONS.
AN ATOM OR OJECT WITH EQUAL NUMBERS OF PROTONS AND ELECTRONS
IS ELECTRICALLY NEUTRAL
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III. The nature of charged objects
A. How charged objects interact
OPPOSITE CHARGES ATTRACT
LIKE CHARGES REPEL
EITHER CHARGE ATTRACTS NEUTRAL OBJECT DUE TO POLARIZATION
The ELECTRIC FORCE (a vector) CAUSES THE
ATTRACTION OR REPULSION
pith ball is polarized when
the – charged rod
approaches it
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B. FORCES on a charged particle
If the charged object is suspended, then there is a tension force.
Gravity is always downward. The repulsive electric field is perpendicular to the gravity force.
In this case, the electric field
is parallel (opposite direction)
to the gravity force. For the
paper to lift, the electric force
must exceed the gravity force
charged rod

F electr

F grav
polarized paper
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nature of charged objects, continued
C. Conducting ability
image from
the physics
classroom on-line
hold electrons
tightly
hold electrons
loosely
D. How objects can get a static charge
When one object obtains electrons from another object, the NET charge is zero.
The + charge on the electron poor object is equal in magnitude to the – charge
on the electron rich object.
1. FRICTION-One object obtains electrons from another object by rubbing them
together. Some materials like to get electrons, some like to give them.
The triboelelectric series is a listing of materials in order of their ability to gain
electrons.
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TRIBOELECTRIC SERIES (partial list)
+ POSITIVE END OF SERIES
asbestos
glass
nylon
wool
lead
silk
aluminum
paper
cotton
steel
hard rubber
nickel & copper
brass & silver
synthetic rubber
orlon
saran
polyethylene
teflon
silicone rubber
- NEGATIVE END OF SERIES
+ charged
hairs
- charged sphere
they stand out
because they repel
each other.
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2. INDUCTION
Remember the concepts of polarization, electronegativity, dipoles van der Waals force, etc
from Chemistry? INDUCTION is related to these concepts.
neutral molecule
but with
charge separation
(dipole)
neutral
molecule with
even charge
distribution (non-polar)
+ end of the dipole
approached non-polar
molecule
electrons are
attracted to the
+ end of the dipole
This phenomenon is why charged objects attract neutral objects
Charging by induction works in the same way.
INDUCTION INVOLVES PUTTING A CHARGE ON AN OBJECT WITHOUT CONTACT
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induction, continued
- charged rod approaches
neutral object
induced + charge and – charge
form on object as electrons
move away from the rod
The electrons are “shunted”
off of the object by transferring
them away (“grounded”)
The object now has on it
a net + charge
A negative charge can be put on the object by using a + charged rod
instead. In this case, the ground would give electrons instead of taking them.
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3. CONDUCTION
CHARGING AN OBJECT WITH CONTACT FROM ANOTHER CHARGED
OBJECT
the rod actually
touches the object
to be charged
charge on the object
is the same as the rod
this is a thin meal strip. Notice how
the ends push away due to repulsion
of like charges.
This sort of device is called an ELECTROSCOPE
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IV. COLOUMB’S LAW
Coulomb's law states that the electrical force between two charged objects is
directly proportional to the product of the quantity of charge on the objects and
inversely proportional to the square of the separation distance between the two
objects.
The mathematical form of Coulomb’s Law ought to remind you of Newton’s Law
of Universal Gravitation

Q1Q2
F electr = k 2
r
Q is the charge, in Coulomb units on each objects
r is the distance between the charged objects
k is Coulomb’s constant. It’s value depends upon the
medium the objects are surrounded by (e.g., air)
RECOGNIZE THAT COLULOMB’S LAW IS ANOTHER EXAMPLE WHERE THE
STRENGTH OF A FORCE FOLLOWS THE INVERSE SQUARE WITH DISTANCE…
IT’S STRENGTH RAPIDLY DIMENISHES WITH DISTANCE
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V. ELECTRIC FIELD LINES
The electric field generated by a particle is a vector.
The electric field lines are always drawn
pointing AWAY from a + charge. The
more arrows that radiate from a particle,
the stronger the field being represented.
These are drawn by computing the
resultant vector after adding the
interacting vectors head to tail at a given
point in space.
This is like the diagram above, lower left
but in this case the resultant vectors are
shown. The particle on the right is
+ and the one on left is -
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V. Strength of the electric field
The strength of the electric field of an object is usually determined by placing
a + TEST charge in the electric field of the source particle and measuring the
force required to move the test charge. This is called the Force/Charge Ratio
Combining Coulomb’s Law with the Force/Charge Ratio

 F electr
E=
q
E is the field strength
F is the net force the
test charge feels
q is the test charge
 Felectr
E=
q

qQ
F electr = k 2
r

F electr
Q
=k 2
q
r

Q
E=k 2
r
source charge
test charge
+Q
q
r

F electr
Coulomb’s Law with the test charge as one of the particles
This equation is the electric field strength of the particle
with charge Q
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