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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
 comes in two forms + and –
 quantized (there is a smallest amount – quant)
• cannot divide up charge into smaller units than that of electron (or proton)
i.e. all objects have a charge that is a whole-number multiple of charge of the smallest amount
 law of conservation for charges – charge is conserved, charges can be separated, can not be
created or destroyed, just transferred
• Electric charge is detected by the effect (force) it produces.
• Matter (stuff) has two basic properties
◊ mass  is what gives the gravitational force
◊ charge  is what gives electric and magnetic forces There are two kinds of charge:
Electricity has origin within the atom itself.
•
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)
ratom ≈ 100000 x rnucleus
mnucleon ≈ 2000 x melectron
e =1.6 x 10-19 C
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 Electric forces
•
charges exert electric forces on other charges
• two positive charges repel each other
• two negative charges repel each other
• a positive and negative charge attract each other
• The repulsive electric force between 2 protons is 1,000,000,000,000,000,000,000,000,000,000,000,000
times stronger than the attractive gravitational force!
• Electrical force is behind all of how atoms are formed … all of chemistry… and, in fact, nearly all common,
everyday phenomena other than gravity
• 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
-
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?
• 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!!
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▪ Insulators
•
•
•
•
•
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.
Eg. Silicon, germanium.
In pure crystalline form, are insulators. But if replace even one atom in 10 million with an impurity
atom (ie a different type of atom that has a different # of electrons in their outer shell), it becomes
an excellent conductor.
• Transistors: thin layers of semiconducting materials joined together.
Used to control flow of currents, act as switches detect and amplify radio signals, act as digital
switches…An integrated circuit contains many transistors
The movement of electrons in semiconductors is impossible to describe without the aid of quantum
mechanics.
▪ 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.
ADVANTAGE – A HUGE ONE
▪ Superconductors
Have zero resistance, infinite conductivity
• Not common! Have to cool to very, very low temperatures.
• Current passes without losing energy, no heat loss.
• Discovered in 1911 in metals near absolute zero
(recall this is 0oK, -273oC)
• Discovered in 1987 in non-metallic compound (ceramic) at “high” temperature around 100 K,
(-173oC)
• Under intense research! Many useful applications eg. transmission of power without loss, magneticallylevitated trains…
 Polarization
Insulators
Conductors
Polar Molecules
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 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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When a negatively charged object is brought near a neutral insulator, the outermost
electrons at each atom will move to the side facing away from the negatively charged
object. Note that for each atom, the outermost electrons are slightly farther way from
the negatively charged object (and feel a repulsive force), while the positively charged
nuclei are slightly closer to the negatively charged object (and feel a attractive force).
But since the outermost electrons are farther away from the negatively charged object,
and the positively charged nuclei are slightly closer to the negatively charged object,
then the repulsive forces will be slightly less than the attractive forces, such that there is
a net attraction between the neutral insulator and negatively charged object.
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
▪ 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.
As the negatively charged rod is brought near the conductor, the electrons are repelled
away from the charged rod. Free electrons will towards the side facing away a
negatively charged object. There will be attractive force between the rod and the
conductor.
As a result, we can say that a charged object will always be attracted by a conductor.
● Only the electrons are free to move, the protons (ions) are fixed in place because they
make up the mass of the conductor. It is also important to notice that when no charged
object is near the conductor, the electrons evenly distribute themselves within the
conductor.
Bring a charged object near a conducting surface, electrons will move in conductor even though no physical
contact: electrons in conductor are repelled from the charged object – since some are free to move, they will!
Once separated
from each other with rod still close they’ll remain charged. Charge is conserved, so charges on spheres A and B are
equal and opposite.
Note, the charged rod never touched them, and retains its original charge.
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▪ 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
k  8.99 109 N  m 2 / C 2
Force is a vector, therefore it must always
have a direction.
Question:
SHE accumulates charge q1 = 2.0 x 10-5 C sliding out of the seat of a car.
HE accumulates charge q2 = -8.0 x 10-5 C while waiting in the wind.
What is the force between them a) when she opens the door 6.0 m from him and b) when their separation is
reduced by half to 3 m?
a) They exert equal forces on each other only in opposite direction
F k
q1q2
 0.40 N
r2
(“-“ = attractive force)
b) r = 3 m
F k
q1q2
 1.6 N  4 F
r2
How many electrons is 2.0 x 10
At very small separation - spark
-5
C?
2.0 10 5 C
 10 14 electrons
1.6 10 19 C
Example: Three point charges :
q1= +8.00 mC; q2= -5.00 mC and q3= +5.00 mC.
(a) Determine the net force (magnitude and direction) exerted on q1 by the other two charges.
(b) If q1 had a mass of 1.50 g and it were free to move, what would be its acceleration?
Force diagram
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F2 = k
q1q2
= 0.213N
r2
F3 = k
q1q3
= 0.213N
r2
x-components will cancel, because of the symmetry
F = 0.213 sin23 0 + 0.213 sin23 0 = 0.166 N
.
a  F  0166
m / s2
m 1.5 103

a = 111 m/s2
Electric Field
Let's take a single electric charge, Q, and put it somewhere. The space around it is different from the space without
charge. We have created a situation in which we could have an electric force. All we have to do is bring in a second
charge, q, to feel the force. Without q, there is no force ....but we still have the condition that we could have a
force. We say that the space around charge contains ELECTRIC FIELD.
How to find the magnitude and direction of electric field at particular location P which is r distance from charge Q?
A test charge, q, placed at P will experience an electric force, F - either attractive or repulsive.
Definition of 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
As F contains q, E DOESN’T depend on q at all, only on Q.
● Direction of electric field is direction of the force on a positive test charge at point P.
 The other way around:
If you know electric field E at a point where you place a charge q , that charge will experience the force F:
F = qE
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 Electric field of a charged particle/point charge
REMEMBER: we just imagine test charge q placed at point P. It serves as help for calculation. BUT,
there was never a charge there.
◊ A charged particle Q creates an electric field.
◊ magnitude
E=
F
Q
=k 2
q
r
El field depends only on charge Q and location of
the POINT where you want to find electric field.
DOES NOT depend on test charge.
the same value on the sphere of radius r around charge Q
◊ direction – radially outward or inward.
Example:
q (test charge)
E  9 109
1.6 1019
10 
10 2
 2.9 1011 N / C to the right
Example:
Say the electric field from an isolated point charge has a certain value at a distance of 1m. How will the electric
field strength compare at a distance of 2 m from the charge?
It will be ¼ as much – inverse square law for force between two charges carries over to the electric field from a
point charge.
Electric field lines
We use “Electric Field Lines” to visualize el. field.
Instead of drawing vector E at every point around certain charge (a huge mess) we decided to go with direction
of electric field and density of electric field lines indicating the magnitude of el. field.
positive charge
negative charge
Denser lines - stronger field → el. field decreases with distance
more lines revels stronger field due to greater charge
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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.
Example:
What is the direction of the electric field at point C if the two charges have
equal magnitude??
Example:
What is the direction of the electric field at point A if the two positive
charges have equal magnitude?
Example:
What is the direction of the electric field at point B if the two
positive charges have equal magnitude??
Example:
What is the direction of the electric field at point A, if the two positive
charges have equal magnitude?