Transcript What is an electric field?

Electrostatic Electricity, Electric Charge (CH 15)
EXPERIMENT NO. 1 (Electrostatics)
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Benjamin Franklin’s Experiment (1706----1790)
About Electrostatic Electricity
Electric charge and Electric Field
Conductors and Insulators
Static Electricity” - Fundamentals
Experiments and demonstrations
Electric Charge Detection and Measurement
Electrostatic Charging
Charging by Polarization
What is a Capacitor?
Coulomb’s Law, Electric Field Force and Exercises
What is an electric field? Exercises
New Terminology
Quiz No. 1
Homework No. 1
Lightning Bolt
Las Vegas, Nevada 2013
On Earth, the lightning frequency is approximately 40–50 times a second.
An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA)
Lightning strike
Barstow airport 2015
Aircraft operating in U.S. airspace have been required to be equipped with static discharge
wicks. A plane is designed to conduct the excess electricity through its skin and structure to
the wicks to be safely discharged back into the atmosphere.
Benjamin Franklin’s Experiment
(1706----1790)
KITE
After failing to exploit the energy of lightning Benjamin
Franklin devised the lightning rod, an iron rod attached to the
highest point of a structure and connected to wires leading to the
ground.
About Electrostatic Electricity
 Most electric charge is carried by the electrons and protons within an atom.
Electrons carry negative charge and protons carry positive charge. They attract
each other. Two protons repel each other, as so do two electrons.
 Protons and electrons create electric fields which exert a force called Coulomb
force, which radiates outward in all directions. The electric field radiates outward
from a charged particle and it decreases in strength as the square of the distance
from the source (1/r2).
 Because protons are generally confined to the nuclei imbedded inside atoms,
they are not nearly as free to move as are electrons. Therefore, we nearly always
mean a surplus or deficit of electrons. When an imbalance exists, electrons are
able to flow, an electric current is created,
In its broadest sense electricity describes phenomena associated with the
interaction between electrically charged objects. The simplest one is the
electrostatics where the charged objects (particles) are at rest.
Electric Charge and Electric Field
The electric charge q, is a fundamental property of matter. It is associated
with particles that make up the atom: electron and proton. Electron charge
-e = -1.60 x 10-19 C. Proton +e = 1.60 x 10-19 C. Neutron has 0 C. The SI unit of
an electric charge is the Coulomb C; [C = As}. It is equal to the charge of
approximately 6.241×1018 electrons. Proton’s mass is 104 times larger than
the mass of an electron.
q = ne– [C or As]
An electric field E, is a space radiated by electric charges q, and acted upon
by an electric force F.
E = F/q [V/m]
F = Eq [(V/m) As = Ws/m = N]; (Note: Nm = Ws)
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Conductors and Insulators
What distinguishes broad group of
substances is whether they can
transmit or conduct electric charge.
Metals are good conductors.
Rubber, glass, most plastics are
insulators.
In conductors, the electrons in the
outermost orbit are loosely bound.
They can easily leave.
In insulators, most of the electrons
are tightly bound. The charge does
not move easily.
“Static Electricity” - Fundamentals
What is “Static Electricity”?
Static electricity is the imbalance of positive and negative charges.
Positive and negative charges attract.
In an atom, the nucleus is made up of Protons ( Positive ) and orbiting
Electrons (negative). The electrostatic forces between the protons and electrons
are what hold the electrons in orbit. They are about 36 orders of magnitude
stronger than the gravitational forces between them.
How can we produce “Static Electricity”?
By placing two different materials (insulators) in contact with each other,
charges are built up at the surface of the materials. This comes from two effects: 1)
The material with more electrons in the outer orbit “pushes” electrons in the other
material away from the surface (like charges repel); and 2) some of the electrons
will move between the materials. When the materials are separated, the charges
remain until they bleed off.
Why do the materials need to be insulators?
In insulators the outer electrons are tightly held, so the electrons making up
the charge cannot move easily through the material and dissipate, they remain
“static” at the surface of the material.
Electric Charge Characteristics of Materials
Scientists have ranked materials in order of their ability to hold or give up
electrons. This ranking is called the Triboelectric series.
TRIBOELECTRIC SERIES
your hand
glass
nylon
wool
fur
MORE POSITIVE
silk
paper
cotton
hard rubber
polyester
polyvinylchloride plastic
If these materials are rubbed together, the one higher on the list gives up electrons. Thus
becomes positively charged.
Electric Charge Detection and Measurement
How can we detect and measure Static Charges?
There are three types of Electroscopes: The Pith Ball Electroscope; the Foil
Electroscope and; the swinging needle Electroscope. Two types will be
demonstrated in class:
The Pith Ball Electroscope:
This instrument, invented in 1754 by a British apprentice John Canton, consists of
two balls of lightweight nonconductive material, originally pith, suspended by a silk
thread from an insulated stand.
Typical Pith
Ball
Electroscope
Pith Ball
Electroscope
showing
charge effects
Electric Charge Detection and Measurement
The Gold Foil Electroscope:
This instrument, invented in 1787 by a British physicist Abraham Bennet, consists of
a vertical metal rod from witch hang two parallel strips of thin flexible conductive material
(originally gold). When the metal rod is touched with a charged object, the strips spread
apart in a “V”. This is because some charge on the object is conducted through the rod to
the strips. Since they receive the same signed charge, they repel each other.
Original gold
foil
Electroscope
Modern
Electroscope
What causes the rods to attract or repel?
Like electric charges repel each other, and unlike electric charges attract
each other. The repulsive and attractive forces are equal and opposite.
Example:
 Glass Rod (positive charge)
 Rubber Rod (negative charge)
How can an object become electrically charged?
Electrostatic Charging
Charging by Friction
If a hard rubber rod is rubbed by fur it acquires a negative charge;
Rubbing a glass rod with silk will give it a positive charge;
Charging by Conduction (Contact)
“Conduction” refers to the flow of a charge during the short period of
time the electrons are transferred.
Charging by induction
A negatively charged rubber rod (charged by friction) can affect the
charge of another object without being in contact of that object.
Charging by Polarization
Charging by contact and induction involves removal of charge from an
object and creating an electric field around an object to be polarized.
Examples of Electrostatic Applications
 Charged by Friction
Balloon and Wall
Comb and Paper
 Electrostatic Air Cleaners
 Electrostatic Copier
 Capacitors used in electronics, camera flash, defibrillator
Charging by Polarization
Some materials are more susceptible to become polarized than others.
A dielectric is an electrical insulator that can be polarized by the action of an externally
applied electric field.
When a dielectric material is placed in an electric
field, electric charges do not flow through the
material as in a conductor, but only slightly shift
from their average equilibrium positions causing
the dielectric polarization inside.
The positive charges are displaced along the
direction of the field while the negative charges
shift in the opposite direction. This creates another
electric field.
A polarized electric object stores energy that can be released on demand. When designed for
that purpose it is called capacitor
which is commonly used in electronic circuits.
What is a Capacitor?
Capacitance
C = k ε0 x A/d
ε0 = 8.85 x 10-12 [C2/(Nm)2]
C = q /V [As)/V]
Energy stored
U = ½ qV = ½ CV2 [VAs]
To maximize the energy stored a capacitor should have: (1) large plates, (2)
a close distance, and (3) high polarizability material between them.
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Coulomb’s Law and Electric
Force
Following Benjamin Franklin’s work, electrical research advanced by leaps.
Quantitative measurements were carried out in 1785 by the French
physicist Charles-Augustin de Coulomb (1786----1806).
He showed that attraction (or repulsion) between given (electric) charges, q varied inversely as the
square of their distance, r from each other. It became known as the Coulomb’s Law where Electric
force, F between two electric charges between two points is:
F = k(q1 x q2)/r2 ; k = 9.00 x109 Nm2/C2
F = [(Nm2/C2)x C2/m2] = [N]
W= Fxm [Nm]
Example: Two point charges q1 = -1 nC and q2 = +2 nC; distance, r between them is 0.30 m. What is the electric force,
F on each particle?
A: F = 9.00 x 109 Nm2/C2 x (1x10-9 C x 2x10-9 C)/0.09 m2
= 18.0 x 10-9 N/0.09
F12
0.30m
F21
-9
-2
= 18.0 x 10 N/9x 10
= 0.20 x 10-6 N;
F = 0.20 μN
Coulomb’s Law and Electric Force Exercises
Exercise 21, Ch. 15
Compared to electric force the gravitational force between two protons is a, about the same, b,
somewhat larger, c, very much larger, d, very much smaller. A.: 21 d
What is an electric field?
The electric force, like the gravitational
force, is an “action at a distance” force. Its
range is infinite. The electric field is a domain where the presence of electric
charges exert an electric force. Charges in one field can interact with charges in
another one. The electric field is a vector field that enables us to determine the
force exerted on a charge in a particular location.
An electric field E [V/m], is the region of space surrounding electrically charged
particles, q [As] where electric charges are acted upon by an electric force F
E = Fq/qo
E = k(qox q)/qor2 ; k = 9.00 x109 [Nm2/C2]
E = kq/r2 [Nm2/C2/m2] = [N/C]
Exercise:
1.The electric field due to positive charge (a) varies as 1/r; (b) points toward the charge, (c ) points
away from the charge or (d) has a finite charge.
41c
What is an electric field? Exercise No. 55 Ch. 15
(a) What would be the magnitude and the
direction of a vertical electric field that
would just support the weight of a proton on
the surface of the Earth?
(b) of an electron?
A: 55 1.0 x 10-7 N/C; upward; 5.6 x 10-11 N/C downward
What is an electric field?
Exercises
2. An electric force acts vertically downwards on an electron. The direction of the electric field at that point
is: (a) up, (b) down, (c ) zero, (d) undetermined. A: 43b
3.
A positive charge is inside an isolated metal sphere. Describe the situation of
+ electric field and the charge on the sphere. Also, how would it change if the
charge were negative.
A: 47 Yes, by induction the inner surface of the sphere would be negatively charged
while the outer surface would become positive and would continue radially outward as if emanating from
the center point of charge. If the charge were negative the field lines would reverse direction.
4. Could the electric field due to two charges ever be zero at some location nearby? If, yes, describe and
sketch the situation. A: 49 Yes! When the electric fields are equal in magnitude and opposite in direction at a
location.
q1
F2
F1
q2
The electric force and work can
be expressed as a function of an
electric field
F = Eq [N] or [(N/C)C]
= Eq [(V/m) As] = [VAs/m] = [Ws/m] = [N]
W= FxL [Nm = Ws]
The electric force and work can be expressed as a function of electric field:
F = Eq [N] or [(N/C)C]
F = Eq [(V/m) As] = [VAs/m] = [Ws/m] = [N]
W= FxL [Nm = Ws]
Exercise:
1.The magnitude of electric force between two point charges is given by a) the charge-force law, b)
conservation of charge, c) Coulomb’s Law or d) both a) and b)
c
New Terminology
Electricity is the set of physical phenomena associated with the presence or flow of an electric
charge.
Electrostatics:---The electrostatics is an electricity phenomenon associated with the interaction between
electrically charged objects while they remain at rest.
Electric Field, E: It is the region of space surrounding electrically charged particles q, acted upon by an electric
force F. E = F/q [V/m]
Electric charge:-- The electric charge q, is a fundamental property of matter associated with particles that make
up the atom: electron and proton. Electron, proton, neutron: Electron charge e-- = -1.60 x 10-19 [C]. Proton +e =
1.60 x 10-19 [C]. Neutron has 0 C. (C=As)
Electric current--- is a flow of electrically charged particles. I = q/t, [A]
Conductors:--------In conductors, the electrons in the outermost orbit are loosely bound. They can easily leave.
Insulators: In insulators, most of the electrons are tightly bound. The charge does not move easily.
Capacitor:----------An electronic component that can store and release energy. A capacitor passes AC, but will
not pass DC once charged.
Coulomb’s Law:-- It states that attraction (or repulsion) between given (electric) charges, q varied inversely as
the square of their distance, r from each other.
Electric force:-----It is the force, F exerted by the charge in an electric field. (F=Eq [N])
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Quiz No. 1
Name:_____________________________
1. What causes the rubber rods to attract or repel? Explain.
__________________________________________________________________
__________________________________________________________________
2. Being an “insulator” how come an object made of rubber can be electrically
charged?__________________________________________________________
3. What is the main feature of an object that has been charged by
polarization?________________________________________________________
__________
Electrostatic Electricity, Electric Charge (CH 15
Homework No. 1 (Electrostatics)
Due date: October 4 or 5 October 2016
Name: _____________________
1)
If you rub a glass rod with a silk cloth: a) what charge polarity will the glass rod acquire?); (b) What
method of charge transfer is being used? __________________________________________________
2)
If you charge a glass rod and balloon each with a silk cloth, will the rod and balloon attract or repel each
other? ______________________________________________________________________________
3)
No. 23 In calculating planetary orbits around the sun, why can astronomers safely ignore the electric
force? ______________________________________________________________________________
4)
No. 26 Coulomb’s Law is also called an inverse square law. What does it tell you about the relationship
between force and distance? ____________________________________________________________
5)
An electron and a proton are separated by 2.0 nm. Using the Coulomb’s Law we can calculate the
magnitude of the force on the electron, F = k(q1 x q2)/r2.= 5.8x 10-11 N; What is the net force on the
system? _____________________________________________________________________________
6)
The electric field due to negative charge (a) varies as 1/r; (b) points toward the charge, (c ) points away
from the charge or (d) has a finite charge. __________________________________________________
7)
Could the electric field due to two charges ever be zero at some location nearby? If yes,describe and
sketch the situation.
____________________________________________________________________
Positive and negative lightning
On Earth, the lightning frequency is approximately 40–50 times a second or nearly 1.4 billion flashes per year and the average duration is
30 microseconds.
CG lightning can occur with both positive and negative polarity. The polarity is that of the charge in the region that originated the lightning
leaders. An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA), and transfers 15 coulombs of electric
charge and 500 megajoules of energy. Large bolts of lightning can carry up to 120 kA and 350 coulombs.
Unlike the far more common "negative" lightning, positive lightning originates from the positively charged top of the clouds (generally
anvil clouds) rather than the lower portion of the storm. Leaders form in the anvil of the cumulonimbus and may travel horizontally for
several miles before veering towards the ground. A positive lightning bolt can strike anywhere within several miles of the anvil of the
thunderstorm, often in areas experiencing clear or only slightly cloudy skies; they are also known as "bolts from the blue" for this reason.
Positive lightning typically makes up less than 5% of all lightning strikes.
Because of the much greater distance to ground, the positively charged region can develop considerably larger levels of charge and
voltages than the negative charge regions in the lower part of the cloud. Positive lightning bolts are considerably hotter and longer than
negative lightning. They can develop six to ten times the amount of charge and voltage of a negative bolt and the discharge current may
last ten times longer. A bolt of positive lightning may carry an electric current of 300 kA and the potential at the top of the cloud may
exceed a billion volts — about 10 times that of negative lightning. During a positive lightning strike, huge quantities of extremely low
frequency (ELF) and very low frequency (VLF) radio waves are generated.
As a result of their greater power, as well as lack of warning, positive lightning strikes are considerably more dangerous. At the present
time, aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the
dangers unappreciated until the destruction of a glider in 1999. The standard in force at the time of the crash, Advisory Circular AC 2053A, was replaced by Advisory Circular AC 20-53B in 2006, however it is unclear whether adequate protection against positive lightning
was incorporated.
Aircraft operating in U.S. airspace have been required to be equipped with static discharge wicks. Although their primary function is to
mitigate radio interference due to static buildup through friction with the air, in the event of a lightning strike, a plane is designed to
conduct the excess electricity through its skin and structure to the wicks to be safely discharged back into the atmosphere. These
measures, however, may be insufficient for positive lightning.