Assignment physics
Electric charge
Electric
charge, basic property of matter carried by some elementary
particles. Electric charge, which can be positive or negative, occurs in
discrete natural units and is neither created nor destroyed.
Many fundamental, or subatomic,
particles of matter have the property of electric charge. For example,
Electrons have negative charge
Protons have positive charge,
but Neutrons have zero charge
History of charge:
In the 18th century,
Ø Benjamin Franklin
in America
Ø Franklin
was
an advocate of a ‘single fluid’ model of electric charge. An object with
an excess of fluid would have one charge;
an
object with a deficit of fluid would have the opposite charge.
Other
scientists
had advocated a ‘two fluid’ theory, with separate positive and negative
fluids moving around. It took over a century for the debate to come down on Franklin
Ø Ernest Rutherford's
The nucleus was discovered in
1911, as a result of Ernest Rutherford's efforts to test Thomson's "plum
pudding model" of the atom.
Ø J.J. Thomson
The electron had
already been discovered earlier by J.J. Thomson himself. Knowing that atoms are electrically neutral,
Ø Thomson postulated
That there must be
a positive charge as well.
Types of
charge:
Electric charges are of two general types:
Ø Positive
Ø Negative
Two objects that have an excess of one type of charge exert a
force of repulsion on each other when relatively close together. Two objects
that have excess opposite charges, one positively charged and the other
negatively charged, attract each other when relatively near.
Examples:
Many fundamental, or subatomic, particles of matter have the
property of electric charge For example,
Electrons have negative charge and protons have positive charge, but neutrons have zero charge. The negative
charge of each electron is found by experiment to have the
same magnitude, which is also equal to that of the positive charge of
each proton.
Charge thus exists in natural units equal to the charge of an
electron or a proton, a fundamental physical
constant.
A direct and convincing measurement of an electron’s
charge, as a natural unit
of electric charge, was first made (1909) in the
Millikan
oil-drop experiment. Atoms of matter are electrically neutral
because their nuclei contain the there are same number of protons as electrons
surrounding the nuclei.
Electric
current and charged
objects involve the separation of some of the negative charge of neutral atoms.
Current in metal wires consists of a drift of
electrons of which one or two from each atom are more loosely bound than the
rest. Some of the atoms in the surface layer of a glass rod positively charged by rubbing it
with a silk cloth have lost electrons, leaving a
net positive charge because of the unneutralized protons
of their nuclei. A negatively charged object has an excess of electrons on its
surface.
Unit of
electric charge:
Ø The unit
of electric charge in the metre–kilogram
Ø second
and SI systems is the coulomb, equivalent to the net amount of electric
charge that flows through a cross
section of a
conductor in an electric
circuit during each
second when the current has a value of one ampere.
Ø One coulomb consists of 6.24 × 1018 natural units of electric charge, such as
individual electrons or protons. One electron itself has a negative charge of
1.6021766208 × 10−19 coulomb.
Ø An
electrochemical unit of charge, the faraday, is useful in describing electrolysis reactions, such as in metallic electroplating. One faraday equals 9.648533289 × 104 coulombs, the charge of a mole of electrons (that is, an Avogadro’s
number, 6.022140857 × 1023, of electrons).
What is the origin of Electric charge?
Nucleus is present in
the center of Atom. It contains protons and neutrons. Protons carry
positive charge and neutrons carry negative charge.
The protons and neutrons in an atom are equal.
Methods
to produce charge:
Electric charge, which can be positive or
negative, occurs in discrete natural units and is neither created nor
destroyed. Electric charges are of two general types: positive
and negative. Two objects that have an excess of one type of charge exert
a force of repulsion on each other when relatively close together.
Formula
of charge:
Ø Depending
on the nature, charge density formula can be
given by,Linear charge density;
λ=ql
where q is the charge and lis
the length over which it is distributed.
Ø The SI
unit is Cm–1.
Ø Surface charge density;
σ=qA, where, q is the charge and A is the area of the surface.
Methods
of produce charge
The
process of supplying the electric charge (electrons)
to an object or losing the electric charge (electrons) from an object is called
charging. An uncharged object can be charged in different ways.
v Charging by friction
v Charging by conduction
v Charging by induction
Charging by friction:
When
an object is rubbed over another object, the electrons get transferred from one
object to another. This transfer of electrons takes place due to friction between
the two objects. The object that transfers electrons loses negative charge
(electrons) and the object that accepts electrons gains negative charge
(electrons).
Hence, the
object that gains extra electrons becomes negatively charged and the object
that loses electrons becomes positively charged. Thus, the two objects get
charged by friction. The charge obtained on the two objects is called friction
charge. This method of charging an object is called electrification by
friction.
Charging by conduction:
The process of charging
the uncharged object by bringing it in contact with another charged object is
called charging by conduction.
A charged object has unequal number
of negative (electrons) and positive charges (protons). Hence, when a charged
object is brought in contact with the uncharged conductor, the electrons get
transferred from charged object to the conductor.
Charging by induction:
The process of charging
the uncharged object by bringing another charged object near to it, but not
touching it, is called charging by induction.
Consider an uncharged
metal sphere and negatively charged plastic rod as shown in below figure (1).
If we bring the negatively charged plastic rod near to uncharged sphere as
shown in below fig (2), charge separation occurs.
The positive charges in
the sphere get attracted towards the plastic rod and move to one end of the
sphere that is closer to the plastic rod. Similarly, negative charges get
repelled from the plastic rod and move to another end of the sphere that is
farther away from the plastic rod. Thus, the charges in the sphere rearrange
themselves in a way that all the positive charges are nearer to the plastic rod
and all the negative charges are farther away from it.
If this sphere is
connected to a ground through the wire as shown in fig (3), free electrons of
the sphere at farther end flow to the ground. Thus, the sphere becomes
positively charged by induction. If the plastic rod is removed as shown in fig
(4) all the positive charges spread uniformly in the sphere.
METHODS OF CHARGING A
BODY
Making a body to acquire property of attracting small objects is called charging(or electrification). A body can by charged by the following ways:
Ø
By rubbing: When a body is rubbed
with another body, both of them charged. One of the bodies acquires positive
charge and the other acquires negative charge.
For example, when a glass rod is rubbed with a silk, the glass rod acquires
positive charges,and at the same time, the silk,the silk acquires negative
charges.
Ø
By conduction:When an uncharged body
is made in contact with a charged body flow into the non charged body and the
body is charged. For example ,if an uncharged sphere A is made in contact with
a charged sphere B, the sphere A will be charged sphere B, the sphere A will be
charged with the same charge as in the sphere B.
Ø
By induction:When a charged particles or body is brought
near non charged body without touching, charges are developed in the uncharged body.This
method of charging a body is called charging by induction.
Applications of Electro-statics
Physics that deals with phenomena
due to attractions or repulsions of electric charges but not dependent upon
their motion
Photocopier
An electrostatic copier works by arranging positive charges in a pattern to be copied on the surface of a non-conducting drum, and then gently sprinkling negatively charged dry toner particles onto the drum. The toner particles
An electrostatic copier works by arranging positive charges in a pattern to be copied on the surface of a non-conducting drum, and then gently sprinkling negatively charged dry toner particles onto the drum. The toner particles
temporarily
stick to the pattern on the drum and are later transferred to the paper and ‘melted’
to produce the copy.
Spray Painting
In spray painting, particles of paint are give positive charge as they leave the nozzle of a spray gun. The object to be painted is earthed so that there is an electric field between the nozzle and the object. The charged paint droplets follow the field lines are are deposited evenly over the surface of the object.
In spray painting, particles of paint are give positive charge as they leave the nozzle of a spray gun. The object to be painted is earthed so that there is an electric field between the nozzle and the object. The charged paint droplets follow the field lines are are deposited evenly over the surface of the object.
Electrostatic Precipitator
Tiny particles of soot, ash, and dust are major components of the airborne emissions from fossil fuel-burning power plants and from many industrial processing plants. Electrostatic precipitators can remove nearly all of these particles from the emissions.
Tiny particles of soot, ash, and dust are major components of the airborne emissions from fossil fuel-burning power plants and from many industrial processing plants. Electrostatic precipitators can remove nearly all of these particles from the emissions.
The
flue gas containing the particles is passed between the series of positively
charged metal plates and negatively charged wires. The strong electric field
around the wires creates negative ions in the particles. The negatively charged
particles are attracted by positively charged plates and collect on them.
Periodically, the plates are shaken so that the collected soot, ash, and dust
slide down into a collection hopper.
Xerography
Most copy machines use an electrostatic process
called xerography—a word coined from the Greek words xeros for
dry and graphos for writing.
Laser Printers
Laser printers use the xerographic
process to make high-quality images on paper, employing a laser to produce an
image on the photoconducting drum, as shown in Figure
. In its most common application, the laser printer receives output
from a computer, and it can achieve high-quality output because of the
precision with which laser light can be controlled. Many laser printers do
significant information processing, such as making sophisticated letters or
fonts, and may contain a computer more powerful than the one giving them the
raw data to be printed.
Ink Jet Printers
and Electrostatic Painting
The ink jet printer, commonly used to
print computer-generated text and graphics, also employs electrostatics. A
nozzle makes a fine spray of tiny ink droplets, which are then given an
electrostatic charge
Once
charged, the droplets can be directed, using pairs of charged plates, with
great precision to form letters and images on paper. Ink jet printers can
produce color images by using a black jet and three other jets with primary
colors, usually cyan, magenta, and yellow, much as a color television produces
color. This is more difficult with xerography, requiring multiple drums and
toners.
Electric force
In
physics, a force is an interaction between two objects that
has the ability to change the motion of one or both of the objects. One of the
fundamental forces of the universe is the electric
force. This is the force that exists between all charged
particles.
Electrical Force Examples
The examples
of electric force are as mentioned below:
- The charge in a bulb.
- Electric circuits.
- Static friction between cloth
when rubbed by a dryer.
- The shock that is felt after
touching a doorknob.
Conservation
of charge
A principle in physics: the total electric charge of an isolated
system remains constant irrespective of whatever internal changes may take
place.
Conservation of Charge Examples
Conservation
of charge means one can’t produce a net charge. Some of its examples are
provided below.
- Charges due to induction.
- During radioactive decay, a
proton decays into a positron and a neutron, but no net charge production.
Charge Transfer
From the
above image, we can say that if our system is not in the influence of any other
charges, the net internal distribution among charge will go on in such a way
that the entire net charge of the system will remain same.
In other
words, we can say that: Charge can neither be created nor be
destroyed and there is total conservation of charge.
Quantization of charge.
Charge on a body can be an integral multiple of
electronic charge.i.e
.Q=± n e. If a body gains electrons,it is said
to be negatively charged and if it loses electrons,it is said to be positively
charged. Though there are particles called “quarks” which may have charge e/3
or 2e/3.Since these are generated during the disintegration of the nucleus
(neutron, proton and so on ) these cannot be transferred. Charge on an electron
is 1.6× 10-19C.
Quantization. Charge
quantization is the principle that the charge of any
object is an integer multiple of the elementary charge. Thus, an
object's charge can be exactly 0 e, or exactly 1 e, −1 e, 2 e,
etc., but not, say, 12 e, or −3.8 e, etc.
Unit of charge
The charge on a single electron is qe=1,6×10−19 Cqe=1,6×10−19 C. All other charges in the
universe consist of an integer multiple of this charge. This is known as charge
quantisation.
Q=nqeQ=nqe
Charge is measured in units called coulombs
(C). A coulomb of charge is a very large charge. In electrostatics we
therefore often work with charge in micro-coulombs (11 μCμC=1×10−61×10−6 CC) and
nanocoulombs (11 nCnC=1×10−91×10−9 CC).
EXAMPLE
of CHARGE QUANTISATION
Question:
An object has an excess charge of −1,92×10−17−1,92×10−17 CC. How many excess electrons does it have?
Solution:
Analyse the problem and identify the principles
We are asked to determine a number of electrons based on a
total charge. We know that charge is quantised and that electrons carry the
base unit of charge which is −1,6×10−19−1,6×10−19 CC.
Apply the principle
As each electron carries
the same charge the total charge must be made up of a certain number of
electrons. To determine how many electrons we divide the total charge by the
charge on a single electron:
N=−1,92×10−17−1,6×10−19
=120 electrons