Transcript q 1 - Proportions

PSE Chapter 15 pg. 197
Textbook Chapter 32
ELECTROSTATICS – the study of electric
charges, forces and fields
Two types of charges
exists, arbitrarily named
POSITIVE and NEGATIVE
By Benjamin Franklin
So what is positive and what is negative?
We know that charged particles
exist in atoms
Electrons are responsible for
negative charge
and Protons for positive charge
Benjamin Franklin did not know about the
existence of these particles but, he did
investigate the behavior of static discharge and
lightning.
Ben knew that if certain electrically neutral objects
are rubbed, they can become charged.
For example; when rubber is
rubbed with a wool cloth, both
become charged.
or a comb through hair
The rubber scrapes electrons
from fur atoms. So the rubber is
negatively charged and the cloth
is positively charged.
Ben also knew that a charge separation occurs
when a glass rod is rubbed with a silk cloth
In the case of the glass and silk, the glass rod
loses negative charge and becomes positively
charged while the silk cloth gains negative
charge and therefore becomes negatively
charged.
Ben experimented with the interactions between
the charge objects. He suspended one and
brought other charged objects near…
repel
attract
Ben observed that
like charged object repel
and unlike charges attract
repel
CONSERVATION OF ELECTRIC CHARGE
In the process of rubbing two solid objects together,
electrical charges are not created. Instead, both objects
contain both positive and negative charges. During the
rubbing process, the negative charge is transferred from
one object to the other leaving one object with an excess
of positive charge and the other with an excess of negative
charge. The quantity of excess charge on each object is
exactly the same.
A NEGATIVE charge is an EXCESS of electrons
A POSITIVE charge is a SHORTAGE of electrons
An electroscope is a device
that detects static charge.
Negatively charged
Positively charged
The metal leaves of the electroscope move apart
if a charged object is brought near the knob.
Benjamin Franklin used a similar device when he
investigated charges.
The SI unit of charge is the coulomb (C).
1 C = 6.25 x 1018 electrons or protons
The charge carried by the electron is represented by the symbol
-e, and the charge carried by the proton is +e. A third particle,
which carries no electrical charge, is the neutron.
The charge of a single
electron = -1.6 x 10-19 C
The charge of a single
proton = +1.6 x 10-19 C
A material can be an electric …
insulator
An insulator is a material in which the electrons are
tightly held by the nucleus and are not free to move through the
material. There is no such thing as a perfect insulator, however
examples of good insulators are: glass, rubber, plastic and dry wood.
conductor A conductor is a material through which electrons are
free to move through the material. Just as in the case of the insulators,
there is no perfect conductor. Examples of good conductors include
metals, such as silver, copper, gold and mercury.
semiconductor
A semiconductor is a poor conductor of electricity
at normal temperatures. As the temperature rises, electrons break
free and move through the material. As a result, the ability of a
semiconductor to conduct improves with temperature.
Objects become charged by…
Friction
Induction
Electrons are rubbed off one insulator
onto another insulator
Charging an object WITHOUT
touching a charged object
Conduction Charging by CONTACT with a
charged object
Charging by Contact
Some electrons leave rod
and spread over sphere.
Charging by conduction results in an object
with the same charge
To understand the process of charging by Induction
Neutral objects can be temporarily attracted to
charged objects by a process called
POLARIZATON.
Polarization occurs because the electrons are
attracted or repelled by the charged object.
This results in a polarization or temporary
separation of the charge, and attraction results.
Electrons are free to
move in metals.
Nuclei remain in place;
electrons move to bottom
Charging by Induction
polarization
grounding
permanent charge
The rod does not touch the sphere. It pushes electrons out
of the back side of the sphere and down the wire to
ground. The ground wire is disconnected to prevent the
return of the electrons from ground, then the rod is
removed.
The charge on the object is opposite
Grounding is allowing charges to move freely
along a connection between a conductor and
the ground.
The Earth (the ground) is a practically
infinite reservoir of electric charge.
Here a positively charge rod
attracts electrons from the
ground into the electroscope
Here a negatively charge rod
repels electrons into the
ground from the electroscope
To review…
Induction results in an OPPOSITE CHARGE
Conduction results in the SAME CHARGE
Applications of Electrostatic Charging
Fine mist of negatively charged
gold particles adhere to
Negatively charged paint
positively charged protein on
adheres to positively
fingerprint.
charged metal.
COULOMB’S LAW
Coulomb’s Law states that two point charges
exert a force (F) on one another that is
directly proportional to the product of the
magnitudes of the charges (q) and inversely
proportional to the square of the distance
(r) or (d) between their centers. The
equation is:
q1q2
Fk 2
r
F = electrostatic force (N)
q = charge (C)
k = 9x109 N m2/C2
r or d = separation between charges (m)
Two negatively charged balloons are 0.70m apart.
If the charge of each is 2.0 x 10-6C, What is
the electric force between the two balloons?
q1 = q2 = 2.0 x 10-6 C
d = r = 0.70 m
q1q2
Fk 2
r
F = 9.0 x 10
F = 0.073 N
9
N m2/C2 (2.0 x 10-6 C)2
(0.70m)2
An attracting or repelling force?
Two equally charged balloons repel each other
with a force of 4.0 x 10-3 N. If they are
0.015 m apart, what is the charge of the
each balloon?
F = 4.0 x 10-3 N
d = 0.015 m
q1q2
Fk 2
r
q2 = Fd2
k
q2 = (4x10-3N)(0.015m)2
(9x109Nm2/C2)
q1 = q2 = 1.0 x 10-8C
WORK on PSE problems # 1-5
for next 15 mins.
Due EOC today!!
Chapter 33 in Textbook
PSE Chapter 15 page 200
ELECTRIC FIELD
An electric field is said to exist in a region of
space in which an electric charge will experience
an electric force. The magnitude of the electric
field intensity is given by:
F
E
q
Units: N/C
The direction of the electric field intensity at a point
in space is the same as the direction in which a
positive charge would move if it were placed at that
point. The electric field lines or lines of force indicate
the direction.
+
-Q
Electric field line flow Out of positive charges
and into Negative charges.
The electric field is strongest in regions where
the lines are close together and weak when the
lines are further apart.
These fields can be detected in lab…
Threads floating on oil bath become polarized
and align themselves with the electric field.
The electric field intensity E at a distance r from
a single charge q can be found as follows:
kq
E 2
r
Units: N/C
Example What is the electric field intensity at a distance
of 2 m from a charge of -12 μC?
r = 2 m
q = -12 μC
kq
E 2
r
q = -12μC
9 x109 (12 x106 )

22
= 2.7x104 N/C, towards q
or to the left
When more than one charge contributes to the field, the
resultant field is the vector sum of the contributions from each
charge.
kq
E 2
r
Units: N/C
Note we will look at direction of the field to know
whether fields add or subtract at a point.
Two charges q1=-8 μC and q2=+12 μC are placed 120 mm apart
in the air. What is the electric field at the midpoint between
them?
The fields have the same direction
q1 = -8 μC
q2 = +12 μC
r = 0. 120m
ET
E1
q1
kq
E 2
r
X
so they add
E2
+
q2
= kq1 + kq2
r2
r2
= (9 x 109)(8 x 10-6) + (9 x 109)(12 x 10-6)
(0.06)2
(0.06)2
E= 2.0 x 107 + 3 x 107 = 5.0 x 107 N/C
to the left
Two charges q1=+8 μC and q2=+12 μC are placed 120 mm apart
in the air. What is the electric field at the midpoint between
them?
The fields are in opposite
q1 = + 8 μC
q2 = +12 μC
r = 0. 120m
ET
+
q1
kq
E 2
r
E1
X
directions so they subtract
E2
+
q2
= kq1 - kq2
r2
r2
= (9 x 109)(8 x 10-6) - (9 x 109)(12 x 10-6)
(0.06)2
(0.06)2
E= 2.0 x 107 - 3 x 107 = -1.0 x 107 N/C
E = 1.0 x 107 N/C to the left
Work on the PSE Problems # 6-9
for the remainder of class.
Due BOC W.26/R.27!!