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Physics – Unit 9 – Electrostatics – NOTES
Kowinsky
Electrostatic Unit Outline:
1. Nature of Charge
a) Electron intro
Flow of charge
Quantization of charge
2. Types of Materials
b) Conductors
c) Insulators
d) Semiconductors
e) Superconductors
2. Charging objects
a) Friction
b) Conduction
c) Induction
3. Lightening
a) Franklin vs the kite
b) Lightening
4.Electric Force
Similarities to gravity
equation
5.Electric Field
Similarities to gravity
equation
6.Field Lines
Drawing field lines to determine how charges will be pulled
Rules for drawing field lines
7.Superposition
Determining how multiple forces and/or fields will interact with multiple charges.
Steps for solving superposition problems
Physics – Unit 9 – Electrostatics – NOTES
Kowinsky
1.Electrons Intro:
a) the atom & valence shells
Bohr atom: Contains a TINY nucleus with P and N. e-‘s are orbiting in shells around the nucleus. 1st shell 2 e‘s, 2nd shell 8 e-‘s, etc…
The outer shell is called the valence shell.
Since P are tightly bound to the nucleus, they cannot escape.
e-‘s however are in orbitals, the outer ones can occasionally escape.
THEREFORE, when dealing with charges, e-‘s transfer the charge
A (+) object has a deficit of electrons (e-‘s left the object)
A (-) object has an excess of electrons (extra e-‘s went into the object
b) electron charge and quantization
All electrons are identical. An e- in plutonium is identical to an e- in hydrogen.
Therefore, all e-‘s carry the EXACT same amount of charge.
q for e- = -1.6 x10-19 c
An electron cannot be broken up, like P and N. Therefore, one cannot have ½ e-, ¾
e-, etc…
Therefore charge can only come in integers of the electron charge.
1.6
3.2
4.8
etc…
One cannot have 1.8, 3.5, etc… must be in integers of the e- charge.
Quantization of charge – charge can only come in integers of the e- charge.
Physics – Unit 9 – Electrostatics – NOTES
Kowinsky
2. Types of Materials
Conductors – materials in which valence electrons are loosely bound to the atom.
-Therefore charge can flow freely through conductors.
Electrons in the outer valence shell are loosely bound. An electron from each atom can easily escape the atom. These
electrons freely float in the substance and go from atom to atom. Since there are so many atoms in a substance
(Avogadro’s number), one can think of this as a “SEA of Electrons” which flow throughout the material.

ALL electrons do not flow freely, only approx. 1 per atom.
So, if a (+) charge is introduced, the free flowing e-‘s can move into that substance.
Examples: most metals like gold, silver, copper, aluminum, iron,
Insulators - materials in which valence electrons are tightly bound to the atom.
-Therefore charge cannot flow freely through conductors.
Examples: most non-metals: glass, rubber, plastic, wood.
Pure water is an insulator – however, impurities make regular water a good conductor.
c) Semiconductors
Some electrons can move freely. Therefore it conducts some electricity.
Very important in circuitry where one needs to control the flow of charges. Most ciruit elements are made from
semiconductors – resistors, diodes, transistors, chips, etc…
Ex: silicon, germanium, lead
d) Superconductors
Electrons can move freely with NO LOSS of energy. (conductors, there is a small amount of energy lost)
Ex: semiconductors at very low temperatures
4. Charging objects
a) Friction
Physically tearing electrons from a substance. – this is the only way to charge insulators. Since the electrons are
tightly bound, one must rip them off.
Ex: rubbing plastic with fur – electrons are torn off the fur onto the plastic.
Ex: rubbing feet on carpet, you become charged
b) Conduction
Charging with contact – the object physically touches the charged one
Contact – object acquires same charge as charged object.
Example: touching van de graff
Physics – Unit 9 – Electrostatics – NOTES
Kowinsky
c) Induction
Charging with NO contact – the object is grounded out.
Induction – object acquires opposite charge as the charged object
Ex: Pith ball charging to get pith ball attracted to plastic rod
3. Lightening
a) Franklin vs the kite
Franklin vs kite reading
Key notes:
Franklin tied a thin wire from the kite to the end of string. Tied a key to the end of wire. Then
tied a string from the wire to his hand (so he wouldn’t get zapped).
Flew the kite while a storm was forming, not during a storm. Atmosphere became charged,
induced a charge in the kite, went down the key, and charged the key. The key would send sparks
to his hand.
* He proved lightening was a form of electricity.
* Fanklin DID NOT fly the kite during a storm. He flew it before the storm, when the atmosphere
was charging. Other people tried this but flew it during a storm and got killed.
b) Lightening
Steps for lightning:
1. warm moist air creates updrafts
2. updrafts bring the moisture high in atmosphere, turns to hail stones.
3. Hail stones get bigger, eventually start falling to lower elevation (due to weight).
As they fall they rub against air molecules. Charging by friction.
4. Hail becomes – charged, air is +, hail is heavier so at bottom of cloud.
5. Cloud is now polarized: - bottom, + top
6. The – bottom induces a + charge on the ground
7. If charge builds enough, a spark occurs (lightning)
Lightening power point
Physics – Unit 9 – Electrostatics – NOTES
Kowinsky
4.Electric Force
Opposite charges attract, like charges repel. In order for the charges to physically move (accelerate) one must have a
FORCE.
We call this the electric Force
The electric force is almost identical to the force of gravity:
Gravitational Force:
Fg  G
m1m2
r2
Electric Force:
Minor changes: Mass causes gravity. What causes electric force? CHARGE
Also, would the constant be the same? No, it’s a different constant. Called Kc
Therefore:
Fe  K c
q1q2
r2
Kc is Coulombs constant: Coulomb did allot of research with electricity. The constant is named after him.
Kc = 9 x 109 Nm2/C2
Units:
Unit for charge is the Coulumb -> C
The charge of an electron is -1.6x10-19 C
The charge of a proton is + -1.6x10-19 C
Unit for electric Force -> same as always – N (Newton)
Physics – Unit 9 – Electrostatics – NOTES
Kowinsky
5. Electric Field
To help visualize how a charge, or a collection of charges, influences the region around it, the concept of an electric field
is used.
An electric field is just a mathematical model that we use to help visualize how charges interact.
The electric field E is analogous to g, which we called the acceleration due to gravity but which is really the
gravitational field. It showed how an object would move in a gravitational field.
Just like gravitational acceleration was: a = G m /r2
For example. On earth, you know the gravitational field, a = 9.8 so if you threw an object in the air, you would know
how gravity would act on it and move it. You can calculate it’s height, velocity, etc…
The electric field a distance r away from a point charge q is given by:
Electric field from a point charge : E = k q / r2
The electric field from a positive charge points away from the charge; the electric field from a negative charge points
toward the charge. Like the electric force, the electric field E is a vector. If the electric field at a particular point is
known, the force a charge q experiences when it is placed at that point is given by :
F = qE
If q is positive, the force is in the same direction as the field; if q is negative, the force is in the opposite direction as the
field.
We can do the same thing with electric field. You can determine the objects acceleration, distance, etc…
The value of the acceleration can be found by drawing a free-body diagram (one force, F = qE) and applying Newton's
second law. This says:
qE = ma, so the acceleration is a = qE / m.
Physics – Unit 9 – Electrostatics – NOTES
Kowinsky
6. Field Lines
What does an electric field look like?
An electric field can be visualized on paper by drawing lines of force, which give an indication of both the size and the
strength of the field. Lines of force are also called field lines.
a) rules for drawing field lines
1.
2.
3.
4.
5.
Field lines emanate perpendicular from the surface
# of lines are proportional to the strength of the field
E field is a vector: Arrows on field lines point away from (-) and go toward (+)
Lines concentrate at points
Field lines NEVER cross.
When there is more than one charge in a region, the electric field lines will not be straight lines; they will curve in
response to the different charges. In every case, though, the field is highest where the field lines are close together, and
decreases as the lines get further apart.
Practice drawing field lines
Physics – Unit 9 – Electrostatics – NOTES
Kowinsky
7. Superposition
How to find F or E for multiple charges:
1.) Determine each force/field acting on the charge
2.) Break each force/field into the X and Y components
3.) Add up the Total X’s and Total Y’s from step 2.
4.) Create a right triangle from the total X and total Y. Solve for the hypotenuse (magnitude), and angle (direction).
The book goes over a nice superposition example at the end of the chapter on electrostatics.
Example Superposition problem: