# Electric Charges and Fields

Document Sample

```					Electric Charges and Fields

Pages 628 - 653
Introduction

   Electricity in one form or another
underlies just about everything around
you from the lightning in the sky to the
spark when you touch someone to
holding atoms together to form
molecules.
Atoms

 Basic units of matter
 Made up of a nucleus surrounded by the
shells or clouds containing electrons
Atomic Particles

 Electrons are located in the shells or
clouds and have a negative charge.
 Protons are located in the nucleus and
have a positive charge.
 Neutrons are located in the nucleus and
have no charge.
Types of Materials

   There are 3 types of materials:
Conductors, Insulators, and
Semiconductors.
Conductors

   Conductors have electrons that are free
to move from one atom to another within
the material. Therefore, they DO have
the ability to conduct electricity.
Insulators

   Insulators have electrons that are tightly
bound to the nucleus and are not free to
migrate. Therefore, they DO NOT have
the ability to conduct electricity.
Semiconductors

   Semiconductors are poor conductors of
electricity under most conditions, but
under very specific conditions /
environments have the ability to become
much better conductors.
Electroscopes

 Electroscopes are devices that detect
the presence of charge.
 Electroscopes come in 3 types: pith-ball,
gold leaf, and moving vane.
Gold Leaf Electroscopes

 Gold leaf electroscopes show they are
charged when their leaves diverge. This
happens because both leaves have the
same charge and therefore repel each
other.
 Gold leaf electroscopes show that they
are neutral when their leaves hang
vertically.
Induction of a temporary charge
in an Electroscope
 Bring a positively charged rod or object
NEAR the knob of a neutral
electroscope.
 The excess positive charges in the rod
attract the electrons in the electroscope.
Cont’d

 The electrons in the electroscope
migrate up to the knob which is closest
to the positively charged rod. The knob
now has an excess of electrons and it is
temporarily negatively charged.
 This migration of electrons causes only
protons to remain in the leaves.
 Both of the leaves now have an excess
of positive charges and repel each other.
Cont’d

 Once the charged rod is removed, the
electrons will flow back to their original
positions and the leaves of the electroscope
will once again fall back to their original
vertical positions.
 Bring a negatively charged rod or object NEAR
the knob of a neutral electroscope.
 The excess negative charges in the rod repel
the electrons in the knob of the electroscope.
Cont’d

 The electrons in the knob of the
electroscope migrate down to the leaves
which are furthest from the negatively
charged rod. The leaves now have an
excess of electrons and are temporarily
negatively charged.
 Both of the leaves now have an excess
of negative charges and repel each
other.
Cont’d

 This migration of electrons causes only
protons to remain in the knob which now
has a temporary positive charge.
 Once the charged rod is removed, the
electrons will flow back to their original
positions and the leaves of the
electroscope will once again fall back to
their original vertical positions.
Polarization of a Conductor

 Bring a charged rod NEAR a conductor.
 Electrons in the conductor will migrate
toward a positively charged rod and
away from a negatively charged rod.
Cont’d

 This migration of electrons will leave one
end of the conductor with a net positive
charge and the other end with a net
negative charge creating 2 poles, one
positive and the other negative.
 Once the charged rod is removed, the
electrons will drift back to their original
positions and the whole conductor will no
longer have poles.
Polarization of an Insulator

 Bring a charged rod NEAR an insulator.
 In an insulator the electrons are NOT
free to migrate so the whole atom rotates
to form an orderly arrangement that
creates poles within the insulator.
   Polarization of Conductors and
Insulators
Charging by Induction

 Bring a charged object NEAR a neutral
conductor.
 “Ground” the object by connecting it with
a conductor to a reservoir of charge such
as the Earth. This allows electrons to
flow either into or out of the conductor.
Cont’d

 Remove the “ground” connection.
 Remove the charged rod from the
vicinity.
 The conductor now has a “permanent”
charge that is opposite to the charge of
the object brought near it.
Charging by Conduction

 TOUCH a charged object to a conductor
/ electroscope.
 If it was a negatively charged object,
some of the excess electrons will spread
throughout the conductor / electroscope,
causing both objects to have the same
negative charge.
Cont’d

   If it was a positively charged object,
electrons from the conductor /
electroscope will be transferred to the
positively charged until both objects
become equally positively charged.
Detecting Charge with a
Charged Electroscope
   When a positively charged rod is brought
near a positively charged electroscope
the leaves will diverge further because
the positively charged rod causes the
few remaining electrons to flow up to the
rod, causing the leaves to become even
more positively charged.
Cont’d
 When a negatively charged rod is
brought near a negatively charged
electroscope the leaves will diverge
further because the negatively charged
rod causes the electrons in the knob to
flow down into the leaves, causing the
leaves to become even more negatively
charged.
 When the electroscope and charged
object have the same charge, the leaves
diverge further.
Cont’d

   When a positively charged rod is brought
near a negatively charged electroscope
the leaves will collapse back toward the
vertical because some of the excess
electrons in the leaves will be drawn
back up into the knob causing the leaves
to be less negatively charged.
Cont’d

 When a negatively charged rod is brought
near a positively charged electroscope the
leaves will collapse back toward the vertical
because the few remaining electrons in the
knob will be forced back down into the leaves
making them less positively charged.
 When the electroscope and the charged object
have opposite charges, the leaves will
collapse back toward the vertical.
Static Electricity

 Static charges are charges at rest.
 There are 2 kinds: positive and
negative.
 Like charges repel and unlike charges
attract.
 Neutrals are attracted to all charged
objects.
 Charges exert a force through a
distance.
   Static Electricity

   Electrostatics Lab
Coulomb

 The unit of charge in the metric system is
the Coulomb (C).
 It is named after Charles Coulomb a
French physicist.
Cont’d

   A coulomb is defined as the amount of
charge found on 6.25*1018 electrons or
protons. Therefore, the magnitude of the
charge of a proton or electron is 1.6*10-
19 C. The mass of a proton is 1.67*10-27

kg and the mass of the electron is
9.1*10-31 kg.
Coulomb’s Law

kq1 q 2
F     2
d
 F - Force (N)
 d - distance between charged particles (m)
 q1 - Charge of first particle (C)
 q2 - Charge of second particle (C)
 k - constant of proportionality -
9*109 Nm2/C2
 A positive force indicates repulsion.
 A negative force indicates attraction.
   Forces in a multiple charge system
Sample Problems - Coulomb’s
Law
 Calculate the net charge on a substance
consisting of 5*1014 electrons.
 How many electrons must be removed
from a metal sphere to give the sphere a
net positive charge of 4.8 μC?
Cont’d

 Two point charges of magnitude 3 μC
and 5 μC are separated by a distance of
0.5 m. Find the electric force of
repulsion between the two.
 A sphere with a charge of 4*10-5 C is
attracted by a second sphere with a
force of 350 N when the separation is 10
cm. Calculate the charge on the second
sphere.
Cont’d

 Two identical point charges are 3 cm apart.
Find the charge on each of them if the force of
repulsion is 4*10-7 N.
 Three charged spheres are located along a
metric ruler. The 3 μC charge is located at 0
cm, the 5 μC is located at the 5 cm mark, and
the -15 μC charge is located at the 10 cm
mark. Find the net force on the 5 μC charge
and which direction will it move?
Homework Quiz 1

 1. If an object has a deficit of 500
electrons, what is the charge on the
object?
 2. Two pith balls are separated by a
distance of 20 cm. They experience an
attractive force of 2 N. If one pith ball
has a charge of 4 micro Coulombs, what
is the charge on the second pith ball?
Electric Fields

 Electric fields have both magnitude and
direction.
 Lines of force show the direction that a
positive test charge would move within the
field.
 Electric field strength or intensity is indicated
by the spacing between the lines. The field is
strong where the lines are close together and
weaker where they are further apart.
Cont’d

 Field lines always begin on a positive
charge and end on a negative charge.
 The number of field lines drawn around a
charge is proportional to the magnitude
of its charge.
 No two field lines ever cross.
Cont’d

 Field lines leave and arrive at surfaces
perpendicular to the surface since that is
the shortest path.
 Electric field surrounding a single
positive charge
 Electric Field surrounding a single
negative charge
Electric Field Intensity

F
E
q
 E - Electric Field Strength or Intensity (N/C)
 F - Force experienced by a test charge at that
location (N)
 q - magnitude of the test charge placed at that
location (C).
kq1 q 2
F    d  2    kq1
E             2
q    q2      d
 E – Electric Field strength or intensity (N/C)
 k – constant of proportionality (9*109)
 d –distance (m)
 q – magnitude of charge creating the field (C)
Sample Problems - Electric Field
Intensity
 The force on a charge of -3*10-7 C is
measured and found to be 0.24 N in a
downward direction. What are the
magnitude and direction of the electric
field at this point?
 Calculate the magnitude and direction of
the electric field at a point 50 cm directly
above a charge of -2*10-6 C.
Electric Potential

kq1 q 2
PE 
d
 PE - Electric Potential Energy (J)
 d - distance between charged particles (m)
 q1 - Charge of first particle (Coulomb, C)
 q2 - Charge of second particle (Coulomb, C)
 k - constant of proportionality 9*109 Nm2/C2
Electric Potential

 Electric potential is the ratio of electric
potential energy to charge.
 Electric potential is measured in Joules
per Coulomb otherwise known as a Volt.
 If two objects are attracted to each other
and they are further separated, their PE
increases but if they come closer
together their PE decreases.
Cont’d

   If two objects are repelled by each other
and they are further separated, their PE
will decrease but if they come closer
together their PE increases.
Potential Difference or Voltage

 As with gravitational potential energy,
your frame of reference determines the
“amount” of PE, we will only talk about
changes in potential energy.
 The change in potential energy is called
potential difference.
Cont’d

   If work is required to move a charge from
one point to another, then there is a
potential difference between the two
locations. If NO work is required to
move the charge from one place to
another, then there is no potential
difference between the two locations.
Cont’d

W
V
q
 V - voltage or potential difference (V)
 W - work done (J)
 q – magnitude of the charge (C)
Sample Problems - Potential
Difference
 What is the potential difference between
two points if 200 J of energy is required
to move 40 C from one point to another?
 How much energy is required to move an
electron between the two terminals of a
large X-ray tube whose potential
difference is 4 million Volts?
Cont’d

   An electron leaves the heated cathode of
a radio or TV vacuum tube with
negligible initial velocity, and is
accelerated through an applied potential
difference of 500V. What is the velocity
of the electron as it approaches the
electrode?
Equipotential Lines or Surfaces

 Equipotential lines or surfaces consist of
all points around a charged object that
have the same electric potential.
 No work is done moving along an
equipotential line or surface.
Cont’d

 These lines or surfaces are always
perpendicular to the lines of force at the
point where the equipotential line or
surface crosses the line of force.
 Equipotential lines / surfaces do not
cross since the same point cannot have
two different potentials.
   Equipotential Lines
Parallel Plates of Charge

 A constant electric force and field can be
made by placing two large, flat
conducting plates parallel to each other
and charging them oppositely.
 The electric field between them is
constant except at the edges of the
plates.
Cont’d

 If an electron is placed near the
negatively charged plate, it will
experience a constant force as it moves
across the open space toward the
positively charged plate.
 The electric field between the two plates
is said to be directed from the positive
plate to the negative plate.
Potential Difference between two
parallel plates

V  Ed
 V - Voltage or Potential Difference
between the plates (V)
 E - Electric Field Intensity (N/C)
 d - distance between the plates (m)
Sample Problems - Parallel
Plates
   A 12 V battery is connected between two
parallel metal plates separated by 0.3
cm. Find the strength of the electric
field.
Cont’d

   A proton is released form rest in a
uniform electric field of magnitude 8000
V/m. The proton undergoes a
displacement of 0.5 m in the direction of
the field. Find the change in the electric
potential of the proton as a result of this
displacement. Find the speed of the
proton after it has been moved 0.5 m,
starting from rest.
Electric Field Simulation Lab

   http://www.gel.ulaval.ca/~mbusque/elec/
main_e.html
Honors Homework Quiz
1.   A force of 0.01is required to move a charge of 7
microCoulombs a distance of 25 cm in an electric
field. What is the size of the potential difference
between the points?
2.   Two parallel metal plates have opposite charges and
a difference of potential of 1000 V between them.
How much work is done in moving an electron from
the negatively charged plate to the positively
charged plate?
3.   A spark will jump between two people if the electric
field exceeds 4 * 106 V/m. You shuffle across a rug
and a spark jumps when you put your finger 0.15 cm
from another person’s arm. Calculate the potential
difference between your body and the other person’s
arm.
1.   How much work is done to transfer 0.15 C of
charge through a potential difference of 9 V?
2.   A voltmeter indicates that the difference in
potential between two plates is 50 V. The
plates are 0.02 m apart. What electric field
intensity exists between them?
3.   A potential difference of 0.9 V exists from
one side to the other side of a cell membrane
that is 5 nanometers thick. What is the
electric field across the membrane?
Electrostatic Equilibrium

 All systems want to be at equilibrium,
when the energy of the system is a
minimum.
 Charges in a conductor will move until
the electric potential is the same
everywhere on the conductor.
Distribution of Charges on a Conductor in
Electrostatic Equilibrium

 The electric field is zero everywhere
inside the conductor.
 Any excess charge on an isolated
conductor will reside on its surface.
 The electric field just outside the
conductor’s surface is perpendicular to
the surface.
 On irregularly shaped conductors,
charge accumulates at sharp points.
Electrical Discharges

   For an irregularly shaped conductor,
electrical discharge is most likely to
occur from the pointed regions since
charge builds up there.
Millikan’s Experiment

 Robert Millikan was able to find the
charge of an electron by suspending a
drop of oil between two parallel plates.
 When the oil drop was suspended, the
force of weight of the object had to be
equal to the force exerted by the electric
field.
Cont’d

Vq
mg 
d
 V - potential difference between plates
(V)
 m - mass of particle (kg)
 g - acceleration due to gravity 9.8 m/s2
 q - charge on particle (C)
 d - distance between plates (m)
mg  Eq
 m= mass in kg
 g = acceleration due to gravity
 E = electric field intensity or strength
(N/C)
 q = charge in Coulombs
Sample Problems - Millikan’s
Experiment
   A negatively charged oil drop weighing
1.59*10-14 N is balanced in the electric
field between the two oppositely charged
plates in a Millikan apparatus. The
difference of potential between the plates
is 100 V and the distance between them
is 0.0050 m. What is the field strength
between the plates? What is the charge
on the oil drop? How many excess
electrons are on the drop?

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 13 posted: 8/31/2011 language: English pages: 91