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Electrostatics
Electrostatics is electricity at rest. It involves electric charges, the forces
between them and their behavior in materials.
Electrical Forces and Charges
Electrical Forces are acting on us all the time. There is both an attractiveforce and a repulsive force. These
forces arise from the particles that make up atoms.
A Bit of History
� Ancient Greeks
– Observed electric and magnetic phenomena as early as 700 BC
� Found that amber, when rubbed, became electrified and attracted pieces of straw or feathers
� Magnetic forces were discovered by observing magnetite attracting iron
A Bit More History
� William Gilbert, M.D.
– 1600
– Found that electrification was not limited to amber
– Wrote De Magnete and distinguished electrical from magnetic phenomena.
� Charles Coulomb
– 1785
– Confirmed the inverse square relationship of electrical forces
History Final
� Hans Oersted– 1820
– Compass needle deflects when placed near an electrical current
� Michael Faraday– A wire moved near a magnet, an electric current
is observed in the wire
Properties of Electric Charges
� Two types of charges exist
– They are called positive and negative
– Named by Benjamin Franklin
Like charges repel and unlike
charges attract one another
More Properties of Charge
� Nature’s basic carrier of positive charge is
the proton
– Protons do not move from one material to another because they are held firmly in the nucleus
� Nature’s basic carrier of negative charge is
the electron
– Gaining or losing electrons is how an object becomes charged
Conservation of Charge
� The outer electrons of most atoms are not tightly bound to the nucleus. The amount of energy need to remove electrons varies for each element. For example, rubber keeps electrons, but fur gives them up.
� Electrons cannot be created or destroyed, only transferred.– For electricity we call this principle conservation
of charge.
� Electrons always are given and taken in whole units
Question #1
The charge on sphere 2 is three times the
charge on sphere 1. Which force diagram is
correct? (e) is none of the others.
Answer #1: (d)
The charge on sphere 2 is three times the
charge on sphere 1. Which force diagram is
correct? (e) is none of the others.
More Properties of Charge
� Electric charge is always conserved
– Charge is not created, only exchanged
– Objects become charged because
negative charge is transferred from one
object to another
Properties of Charge
� Charge is quantized
– All charge is a multiple of a fundamental unit of charge, symbolized by e
– Electrons have a charge of –e
– Protons have a charge of +e
– The SI unit of charge is the Coulomb (C)
� e = 1.602 x 10-19 C
Conductors
� Conductors are materials in which the
electric charges move freely
– Copper, aluminum and silver are good conductors
– When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material
Insulators
� Insulators are materials in which electric
charges do not move freely
– Glass and rubber are examples of insulators
– When insulators are charged by rubbing, only the rubbed area becomes charged
� There is no tendency for the charge to move into other regions of the material
Semiconductors
� The characteristics of semiconductors are
between those of insulators and conductors
� Silicon and germanium are examples of
semiconductors
Charging by Conduction
� A charged object (the rod) is physically touches the other uncharged, object (the sphere)
� The same type of charge is CONDUCTED from the rod to the sphere
Charging by Induction
1. NO physical contact between between
charged & uncharged
object
2. OPPOSITE charged
is INDUCED
Charging by Polarization
o When induction occurs in an insulator, the
insulator molecule will change orientation.
� This is seen with a comb attracting an
uncharged piece of paper or a balloon
attaching itself to an uncharged wall.
� Molecules that have a more positive side and
a more negative side are said to be electric
dipoles.
Charging by Polarization
••A charged object like a comb is brought near a neutral A charged object like a comb is brought near a neutral object like paper, which repels the like charges on the object like paper, which repels the like charges on the surface of the papersurface of the paper
Question #2
An alpha particle with two positive charges and
a less-massive electron with a single negative
charge are attracted to each other.
The force on the electron is:a) Greater than that on the alpha particleb) Less than that on the alpha particlec) Same as that on the alpha particled) I haven’t a clue…
Answer #2: (c) Same
The force on the electron the same as that on the alpha particle -Newton’s Third Law.
Question #3
An alpha particle with two positive charges and
a less-massive electron with a single negative
charge are attracted to each other.
The particle with the most acceleration is thea) Alpha particleb) Electronc) Neither - they have the same accelerationd) I haven’t a clue…
Answer #3: (b) Electron
The particle with the most acceleration is the ELECTRON. Newton’s Second Law (F=ma)
Question #4
An alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other. As the particles get closer to each other, each experiences an increase in:
a) forceb) speedc) accelerationd) All of thesee) None of these
Answer #4: (d) ALL
As the particles get closer, the FORCE
⇑ and thus the ACCELERATION ⇑ and also the SPEED ⇑
Coulomb’s Law
� Governs forces and charges,
� ke is called the Coulomb Constant
– ke = 8.99 x 109 N m2/C2
� Typical charges can be in the µC range
� Remember that force is a vector quantity
F = k e
q1 q2
r2
Question #5
If q1 = +20 µC and q2 = +10 µC and the two charges are 3 meters apart, what is the
MAGNITUDE of the force between them?
F = k e
q1 q2
r2
a) 0.2 Nb) 0.6 Nc) 22.22 N d) 2.0 x 10 11 Ne) I don’t have a clue
Vector Nature of Electric Forces
� Two point charges are separated by a distance r
� The like charges produce a repulsive force between them
� The force on q1 is equal in magnitude and opposite in direction to the force on q2
Vector Nature of Forces, cont.
� Two point charges are separated by a distance r
� The unlike charges produce a attractive force between them
� The force on q1 is equal in magnitude and opposite in direction to the force on q2
Question #6
If q1 = +20 µC and q2 = +10 µC and the two charges are 3 meters apart, what is the
DIRECTION of the force between them?
F = k e
q1 q2
r2
a) Away from each otherb) Towards each otherc) One chases the other d) Nothing - they don’t move at alle) I don’t have a clue
Answer #6: (a) Away
If q1 = +20 µC and q2 = +10 µC and the two charges are 3 meters apart, what is the
DIRECTION of the force between them?
Like charges repel
Electrical Field
� An electric field is said to exist in the region
of space around a charged object
– When another charged object enters this electric field, the field exerts a force on the second charged object
Electric Field, cont.
� A charged particle, with charge Q, produces an electric field in the region of space around it
� A small test charge, qo, placed in the field, will experience a force
Question #8
� What is the magnitude of the electric field
0.50 meters away from a -3µC point charge?
F = q0E
E pt chg = kq
r2
a) 1.08 x 105 N/C
b) -1.08 x 105 N/C
c) 5.4 x 104 N/C
d) -5.4 x 104 N/C
e) I don’t have a clue…
Answer #8: (a) 1.08x105 N/C
� What is the magnitude of the electric field
0.50 meters away from a -3µC point charge?
F = q0E
E pt chg = kq
r2
a) 1.08 x 105 N/C
b) -1.08 x 105 N/C
c) 5.4 x 104 N/C
d) -5.4 x 104 N/C
e) I don’t have a clue…
Question #9
� What is the magnitude of the electric field
0.50 meters away from a -3µC point charge?
F = q0E
E pt chg = kq
r2
a) 1.08 x 105 N/C
b) -1.08 x 105 N/C
c) 5.4 x 104 N/C
d) -5.4 x 104 N/C
e) I don’t have a clue…
Direction of Electric Field
� The electric field produced by a negative charge is directed toward the charge
– A positive test charge would be attracted to the negative source charge
Direction of Electric Field, cont
� The electric field produced by a positive charge is directed away from the charge
– A positive test charge would be repelled from the positive source charge
Question #9
� What is the electrostatic force acting on a 2 nCcharge placed in a 335 N/C electric field?
a) 0 N
b) 6.7 x 10-4 N
c) 6.7 x 10-7 N
d) 6.7 N
e) I don’t have a clue…
F = q0E
E pt chg = kq
r2
Answer #9: (c) 6.7 x 10-7 N
� What is the electrostatic force acting on a 2 nCcharge placed in a 335 N/C electric field?
F = qE
F = 2x10−9C( ) 335
N
C
F = 6.7x10−7N
Electric Field Lines
� A convenient aid for visualizing electric field
patterns is to draw lines pointing in the
direction of the field vector at any point
� These are called electric field lines and were
introduced by Michael Faraday
Electric Field Lines, cont.
� The field lines are related to the field by
– The electric field vector, E, is tangent to the electric field lines at each point
– The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region
Electric Field Line Patterns
� Point charge
� The lines radiate equally in all directions
� For a positive source charge, the lines will radiate outward
Electric Field Line Patterns
� An electric dipoleconsists of two equal and opposite charges
� The high density of lines between the charges indicates the strong electric field in this region
Electric Field Line Patterns
� Two equal but like point charges
� At a great distance from the charges, the field would be approximately that of a single charge of 2q
� The bulging out of the field lines between the charges indicates the repulsion between the charges
� The low field lines between the charges indicates a weak field in this region
Electric Field Patterns
� Unequal and unlike charges
� Note that two lines leave the +2q charge for each line that terminates on -q
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