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STATIC ELECTRICITY
Physicists have identified four fundamental forces that account for all phenomena in the universe.
Force
Strength
Distance of action
Description
Strong nuclear
Very strong!
Very, very Short
Holds the nucleus together
Weak nuclear
Very weak
Short
Arise during radioactive decay
Electromagnetic
Very weak
Infinite – but decreases with
the square of distance
Attraction / repulsion of charged
particles
Gravitational
Very, very,
very weak
Infinite – but decreases with
the square of distance
Attraction between masses
With the exception of gravity, all the forces we studied up to now are due, on a molecular level, to interactions
between the electrons of objects – that is, they are caused by the electromagnetic force
normal force
tension
friction force
spring force …
 Charge it is quantized and conserved
ratom ≈ 100000 x rnucleus
mnucleon ≈ 2000 x melectron
Electricity has origin within the atom itself.
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charge is measured in Coulombs [C]
Electricity has its origin within the atom itself.
• Protons have positive charge (+1e)
• Electrons have negative charge (-1e)
• Neutrons have no charge (0)
e =1.6 x 10-19 C
• Everyday objects - electronically neutral – balance of charge – no net charge.
• Objects can be charged – there can be net charge on an object. How?
The only type of charge that can move around is the negative charge, or electrons. The positive charge stays in the
nuclei. So, we can put a NET CHARGE on different objects in two ways
◊ Add electrons and make the object negatively charged.
◊ Remove electrons and make the object positively charged.
Some materials have atoms that have outer electrons (farthest from nucleus) loosely bound. They can be attracted
and can actually move into an outer orbit of another type of atom. The atom that has lost an electron has a net charge
+e (positive ion). An atom that gains an extra electron has a net charge of – e (negative ion).
This type of charge transfer often occurs when two different materials (different types of atoms) come into contact.
 Electrical conductors, insulators, semiconductors and superconductors
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distinction based on their ability to conduct (transfer between materials) electric charge.
▪ Conductors
A conductor allows electric charge to travel through it easily.
Tap water, human body and metals are generally good conductors.
What makes metals conduct?
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Metals have loosely bound electrons (valence electrons – the electrons in the outermost orbits) –In metal,
atoms are close to each other and valence electrons from each atom get confused and forget which atom
they belong to.
They now belong to the metal as the whole. Positive ions which are tightly bound and in fixed positions can
only oscillate around their equilibrium positions, form a positive background.
All the homeless electrons are called “free electrons” or “see of electrons”
They wander around, keeping ions from falling apart – metallic bond!!
▪ Insulators
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Insulators are materials that impede the flow of electrons from atom to atom.
Insulators have tightly bound electrons – their electrons are not free to move from atom to atom.
This makes insulators poor conductors of heat and electricity.
Examples: air, pure water, plastic, glass, rubber, wood
Only if a very strong electric field is applied, the breakthrough (molecules become ionized resulting in a flow
of freed electrons) could result in destruction of the material.
▪ Semiconductors
Materials that can be made to behave sometimes as insulators, sometimes as conductors.
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Eg. Silicon, germanium.
▪ ADVANTAGE :As the conductivity of semiconductors can be adjusted by adding certain types of atomic
Impurities in varying concentrations, you can control how much resistance the product will have.
▪ Superconductors
Have zero resistance, infinite conductivity
• Not common! Have to cool to very, very low temperatures.
• Current passes without losing energy, no heat loss.
 Polarization
Charged objects can make neutral objects become polarized. When an object is polarized
• One side has + charge
• The other side has - charge
• The overall charge of the object is zero
▪ Insulators
When bring a charged object near an insulator, electrons are not free to migrate throughout material.
Instead, they redistribute within the atoms/molecules themselves: their “centers of charge” move
When a positively charged object is brought near a neutral insulator, the outermost electrons at each atom will be
drawn to the side facing the positively charged object. As the result, in each atom the
outermost electrons are slightly closer to the positively charged object (and feel an attractive
force), while the positively charged nuclei are slightly farther away (and feel a repulsive
force). But since the outermost electrons are slightly closer to the positively charged object,
and the positively charged nuclei are slightly farther away from the positively charged object,
then the attractive forces will be slightly greater than the repulsive forces, such that there is a
net attraction between the neutral insulator and positively charged object
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▪ Conductors
In a conductor outer electrons are free to move around within the conductor ("sea of
electrons“).
As the positively charged rod is brought near the conductor, the electrons are attracted
toward the charged rod. Free electrons will move to the side facing towards a positively
charged object. There is a net attraction between the neutral conductor and positively
charged object.
▪ Polar Molecules
Once again a similar effect occurs when a charged object
(positive or negative) is brought near a liquid containing polar
molecules. However, in this case the molecule will reorient
itself such that the negative end will move to the side facing
towards a positively charged object, or away from a negatively
charged object, such that net attraction will result in either
case.
ELECTROSTATIC – ELECTRIC - COULOMB FORCE
 Force between TWO POINT charges q1 and q2 at distance r from each other is proportional to the product of the
amount of the charges on each one, and inversely proportional to the square of the distance between them.
F k
q1q2
r2
Force is a vector (magnitude and direction!)
k  8.99 109 N  m 2 / C 2
. Electric field, E, at a point P distance r away from Q.
● The magnitude of the electric field is defined as the force per unit charge.
E=
F
q
E  
N
C
𝑄
Electric field due to point charge Q 𝐸 = 𝑘
𝑟2
at distance r from it is:
Electric field DOES NOT depend on
the charge placed at that point
● Direction of electric field is direction of the force on
a positive test charge at point P.
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Electric field lines
We use “Electric Field Lines” to visualize el. field.
positive charge
negative charge
El field always point away from + charges, towards – charge
Electric field lines can never cross. If they crossed, that would mean that a charge placed at the intersection, would be
accelerated in TWO directions at once! This is impossible! If two sources are creating electric fields in the same place,
we have to add the two vectors and get a resultant vector representing the NET ELECTRIC FIELD.
Electric field is a vector, so if one has to find electric field duw to more charges one has to do vector sum.
Electrical POTENTIAL Energy and Electrical Potential
In the electrical case, a charge placed in electric field will have electric potential energy. How do we know that? If left
on its own it will start accelerating due to electric force acting on it. We say it has electric potential energy which will
be converted into kinetic energy. On the other hand if external force pushes the charge against electric
field, the work done by that force will be stored as potential energy in the charge
Potential energy difference between two points (∆𝑼) is equal to the work done on a charge in order to
move it from one point to the other.
∆U = W
Potential Difference Between Two Points (ΔV = VB – VA)
is equal to the work done per unit positive charge in order to move it from one point to the other.
∆𝑉 =
∆𝑈
𝑞
1 𝑉𝑜𝑙𝑡 =
1 𝐽𝑜𝑢𝑙𝑒
1 𝐶𝑜𝑢𝑙𝑜𝑚𝑏
CHARGE IN UNIFORM FIELD E (constant in magnitude and constant direction)
▪ Work done by electric force on charge q
Electric force on charge q is 𝐹 = 𝑞𝐸.
It moves t
hrough a potential difference, ∆V.
The work done on it by electric force is equal to the decrease in its
electric potential energy which is converted into kinetic energy:
W = ∆U = ½ mv2
q ∆V = ½ mv2
W = Fd = q Ed
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▪ Work done by external force on charge q
Electric force on charge q against electric field is 𝐹𝑒𝑥𝑡 = 𝑞𝐸.
It moves through a potential difference, ∆V.
The work done on it by external force against electric field is stored in charge
as the change in electric potential energy U.
W = ∆U
Fext d = q ∆V
qEd = q ∆V ⇒ 𝐸 =
∆𝑉
𝑑
⇒ (E) = NC-1 = Vm-1
electron-Volt (eV)
How much work is done in moving one electron through a potential difference of one volt?
An electron volt is the amount of energy/work it takes to move an electron through a potential difference of 1 volt.
∆U = W = q ∆V
1 eV = 1.6x10-19 J
The electron volt is not a smaller unit for volts!!! It is a smaller unit for energy.
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relationship between uniform electric field E and potential difference between two points distance d from
each other along electric field line:
ΔV = Ed
→ E = ΔV/d
and NC-1 = Vm-1