ELECTROSTATICS
Some Important Things
About Atoms
1. Every atom has a positively charged nucleus
surrounded by negatively charged electrons.
2. All electrons are identical; that is, each has the same
mass and the same quantity of negative charge as every other electron.
3. The nucleus is composed of protons and neutrons.
(The common form of hydrogen, which has no neutrons, is the only exception.) All protons are
identical; similarly, all neutrons are identical. A proton has nearly 2000 times the mass of an electron, but its positive charge is equal in magnitude to the negative charges of the electron. A neutron has slightly greater mass than a proton and has no charge.
4. Atoms usually have as many electrons as protons, so
The Fundamental Rule
Like charges
Everyday Examples
Clothes tumble in the dryer and CLING together You walk across the carpeting to exit a room
and receive a doorknob SHOCK
You pull a wool sweater off at the end of the
day and see SPARKS of electricity
You pet your cat’s fur and observe it STANDING
ON its end
Bolts of LIGHTNING dash across the evening sky
during a spring thunderstorm
And most tragic of all, you have a BAD HAIR
Electrostatics
Electrostatic forces hold the world of
atoms and molecules together in perfect balance.
Without this ELECTRIC force, material
things would not exist.
ELECTROSTATIC forces are foundational
The Structure of Matter
All material objects are composed of
ATOMS. There are different kinds of atoms known as ELEMENTS that can combine to form COMPOUNDS with distinctly different PROPERTIES.
An atom consists of a NUCLEUS and a vast
Electrostatics
Electrons are often removed from and
added to an atom by normal everyday occurrences.
These occurrences are the focus of this Unit
The nucleus of the atom contains
PROTONS and NEUTRONS. They are not removable or disturbable by usual
everyday methods. Electrostatic
Summary of Subatomic
Particles
Proton Neutron Electron
In nucleus In nucleus OUTSIDE NUCLEUS Tightly bound Tightly bound WEAKLY
BOUND Positive
Charge
No charge NEGATIVE CHARGE
Massive Massive NOT VERY
Neutral vs. Charged Objects
An electrically NEUTRAL object is an
object which has a BALANCE of protons and electrons.
A CHARGED object has an IMBALANCE of
protons and electrons.
The type of charge (positive or negative)
Charge (Q)
The fundamental property of matter that
causes attracting and repelling
ELECTRICAL force behavior. The two elementary carriers of charge are the ELECTRON, (e-) which carries the
“NEGATIVE” charge, and the PROTON (p+) which carries the “POSITIVE”
Unit of Charge
COULOMB (C)
1 Coulomb = 6.3 x 1018 elementary
charges
Law of Conservation of
Charge
When all objects involved are considered
prior to and after a given process, the total amount of charge amidst the
objects is the SAME BEFORE the process starts as it is AFTER the process ends.
Charge can neither be created nor
destroyed.
Charge Interactions
The two fundamental charge interactions
can be stated as follows: OPPOSITELY charged objects ATTRACT and
Charged and Neutral Objects
Attract
What type of interaction is observed
between a charged object and a neutral object?
Any CHARGED object – whether
Conductors and Insulators
The behavior of an object which has
been charged is dependent upon whether the object is made of a
Conductors and Insulators
Conductors are materials which PERMIT
electrons to flow FREELY from atom to atom. An object made of a conducting material will permit charge to be
TRANSFERRED across the entire outer SURFACE of the object.
Insulators are materials which IMPEDE the
free flow of electrons from atom to atom. If charge is transferred to an insulator at a
Conductors and Insulators
The division of materials into the
categories of conductors and insulators is a somewhat artificial division. It is
more appropriate to think of materials as being placed somewhere along a long
Methods of Charging
Charging by Friction
The presence of different atoms in objects
provide different objects with different electrical properties.
ELECTRON AFFINITY refers to the LOVE
Charging by Friction
Frictional charging results in a transfer of
ELECTRONS between two objects which are RUBBED together.
The two objects become charged with
Triboelectric Series
An ORDERING of substances
according to their ELECTRON
AFFINITY when charged by friction.
When any two materials in the
table are rubbed together, the one which is higher pulls ELECTRONS from the material which is lower.
The materials highest on the table
will have the greatest tendency to acquire the NEGATIVE charge.
Those below it become POSITIVELY
Charging by Induction
Induction charging is a method used to
charge an object WITHOUT actually TOUCHING the object to any other charged object.
The neutral object will gain a CHARGE
Electroscope
A device which is capable of DETECTING
the presence of a charged object. It is often used in electrostatic experiments and demonstrations in order to TEST for CHARGE and to deduce the type of
Charging by Conduction
Charging by conduction involves the
CONTACT of a charged object to a neutral object.
The neutral object will gain a CHARGE
Grounding-the Removal of
Charge
Objects with an excess of charge – either
positive or negative – can have this charge “removed” by a process known as
GROUNDING.
Grounding: the process of REMOVING the
excess CHARGE on an object by means of the transfer of ELECTRONS between it and
another object of substantial size.
A GROUND is simple an object which serves
as a seemingly infinite RESERVOIR of ELECTRONS; the ground is capable of
transferring electrons to or receiving electrons from a charged object in order to neutralize
Electric Force – Coulomb’s
Law
The electrical FORCE, like all forces, is
typically expressed in units of NEWTONS. Being a force, the strength of the electrical interaction is a VECTOR quantity which
has both MAGNITUDE and DIRECTION.
The direction is dependent upon whether
the charged objects are charged with like charge or opposite charge.
The best way to determine the direction of it
Coulomb’s Law
The magnitude can be calculated using
COULOMB’S LAW.
Coulomb’s Law: 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
Coulomb’s Law
Q
1 = charge on object 1 measured in Coulombs
Q
2 = charge on object 2 measured in Coulombs
d = distance between the two charges k = 9.0 x 109 Nm2/C2 k is called the
Coulomb’s Law
The pattern between electrostatic force
and distance can be characterized as an INVERSE SQUARE relationship.
Example:
Suppose that two point charges, each with a
charge of +1.00 Coulomb are separated by a distance of 1.00 meter. Determine the
In Summary
1. By friction, when electrons are transferred by friction from one object to another.
2. By contact, when electrons are transferred from one object to another by direct
contact without rubbing.
3. By induction, when electrons are caused to gather or disperse by the presence of
nearby charge (even without contact). a. If the object is an insulator, then a
