Nature of force
A positively charged
plastic rod is suspended by nylon string. [Fig: Nature of force]
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Nature of force
|
Now a negatively charged polythene rod is brought near it. What do
you observe? The plastic rod will move towards the polythene rod. So, it is
proved that two oppositely charged object attract each other. [Fig: Nature
of force
(a)]
Now what will you observe when a positively charged plastic rod is
brought near a freely suspended positively charged plastic rod the suspended
rod will move apart quickly. That is charges of same nature repel each other.
Coulomb’s Law
We know that the charges of opposite nature attract each other and
charges of same nature repel each other. The force of attraction or repulsion
between the two charges depends on,
1. Quantity of charges.
2. Distance between two charges.
3. The nature of the medium between the two charges.
Scientist Coulomb states a law about the force of attraction or
repulsion between the two charges. This is called Coulomb’s law.
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Figure: 1
|
Law: The forces
of attraction or repulsion between two charged bodies in particular medium is directly
proportional to the product of the charges and inversely proportional to the
square of the distance between them and the force acts along the straight line
connecting them.
Suppose, two charges q1 and q2 are at a
distance d from each other [Fig: 1]. If the force of attraction or repulsion
between these two is F, then according to Coulomb’s law,
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Coulomb’s
law
|
Here C is a constant of proportionality. Its value in vacuum is 9
x 109 Nm2C-2. Sometimes it is called coulombs
constant.
Unit of charge: The unit of charge is coulomb (C). It is a
derived unit. Coulomb is defined from ampere.
If 1 ampere (1 A) current flows through a conductor for 1 second
(1 s), then the amount of charge that passes through any cross section of the
conductor is called coulomb (1 C).
Electric Field
Suppose A is a positively charged body. Now if a charge +q is
placed at point P, then due to the charge of A, the +q charge will gain
a force. We say that at point P there is an electric field, the source of which
a charged body A. That is if a charged body, in which the influence of the charged
body exists is called the electric field of the charged body.
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Electric Intensity
|
According to coulomb’s law, it is found that the nearer the
point P [Fig: Electric Intensity] to the charged body A, the more will be the
strength of electric field at that point. The strength of the electric field is
called intensity. If at any point of an electric field a unit of positive
charge is placed and the force that it acquires is called the electric
intensity at that point. If the charge at point P acquires a force F, then the
intensity of electric field at that point P,
E = F ÷ q
Electric intensity is a vector quantity
and its direction is along the force acting on a unit positive charge placed in
an electric field. The unit of electric charge is newton/coulomb (NC-1).
Electric Lines of Force
Michel Faraday introduced electric lines of force to get an idea
about electric field. If a positive charge is placed in an electric field it
would experience a force. If the charge is a free one, gaining this force
instead of remaining stationary it would move in a definite path. Electric line
of force is the path of a free positive charge that moves in an electric field.
There is no real existence of lines of force. These lines are imaginary. The
electric lines of force are used to for measuring the electric intensity and
explaining its direction at a point in an electric field. The lines of force of
an electric field are such that, the tangent drawn at a point to a line of
force indicates the direction of electric intensity at that point. The number
of lines of force passing through unit area perpendicular to the lines of force
at a point in the electric field is proportional to magnitude of the electric intensity
at that point. In a diagram of lines of force of an electric field, the gap
between the lines indicates the magnitude of intensity of electric field. In an
electric field where the lines of forces are closer magnitude of E is greater
there and where the lines of forces are away magnitude of E is less there.
For different positions of charged object, the nature of the lines
of force of an electric field varies. Lines of force of a few electric are
described below. For the simplicity of description the conductors are taken as
spherical.
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Electric
Lines of Force
|
1. For an isolated positive charge the nature of lines of
force is shown in figure Electric Lines of Force (a). In
this case the lines of force emerged uniformly from the surface of the
conductor perpendicularly. If the charge of the body increases then the number
of lines of force also increases.
2. The lines of force of an electric field produced by two
equal and opposite charges are shown in figure Electric
Lines of Force (b). In this case the line of force emerges from positive
charge and terminates at negative charge.
3. The lines of force of an electric field produced by two
equal positive charges placed nearby are shown in figure Electric Lines of Force (c). In this case the lines of force go
far away from each other; as a result there will be no lines of force in
between them. In figure this place is indicated by ‘X’ sign. If a charge is
placed at this place it will experience no force. This point is called neutral
force.
4. The lines of force of an electric field produced by two
unequal positive charges placed nearby are shown in figure Electric Lines of Force (d). In this case, the neutral point ‘N’
would not be nearer to the smaller charges.
Electric Potential
As there is intensity of an electric field, it also has electric potential.
Potential determines the direction of motion of a charge in an electric field
and also determines the direction in which the charge will flow when two charged
conductors are connected by a conductor wire. If the charge creating field is
positive, some work is done against the force of repulsion if another positive
charge is brought near it. Therefore, the more positive charge is brought from
a point at infinity nearer to the body, the more work will have to be done. So,
within the electric field of a positively charged body, the more a point is
brought nearer to the body, the more will be the quantity of potential. If any electric
field created by a positively charged body and a free positive charge is placed
and allowed to move freely, it would go away from the body. Therefore, we can
say that positive charge moves from higher potential to lower potential. On the
other hand, negative charge moves towards positively charged body. Thus,
negative charge moves from lower potential to higher potential. If the body
creating the electric field is charged negatively, then some work will be done
due to attraction of a unit positive charge bringing towards it. A positive
charge itself does work while coming from infinity towards a negatively charged
body, which creates an electric field. As a result the charge loses energy and
the potential at a point in the electric field is considered as negative.
Measurement
of potential: The work done to bring a unit positive charge from infinity to a
point in an electric field is called the potential of that point. Again from
infinity if a unit of positive charge is brought near to the conductor, the
work done by the electric force or against the electric force is called
potential of that conductor.
If a unit positive charge q is brought very near to the conductor
from a point at infinity and if the amount of work done is W, the potential ‘V’
of the conductor or of that point will be,
V = W ÷ q
Electric potential determines in which direction the flow of
electric charge takes place when two charged conductors are electrically
connected.
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Electric Potential
|
If two positively charged metallic spheres are connected by
conducting wire (fig: Electric
Potential) than any of the following phenomena may occur.
1. Some charge from the left sphere may go to the right
sphere.
2. Some charge from the right sphere may go to the left
sphere.
3. The charges may remain as it is.
The movement of charge from one sphere to another does not depend
on the quantity of charge of the spheres but it depends on electric potential.
The positive charge will flow from sphere to sphere of higher potential to that
if lower potential. This flow of charge will continue until the potential of
these two spheres become equal. So, potential is an electric condition of a
charged conductor that determines whether it takes or gives up charge when
connected to another charged conductor by a connecting wire.
Similarity between potential and temperature and free surface of liquid
The role which is played by temperature and the height of free
surface of liquid in heat and hydro-statics respectively, potential plays the
same role in electrostatics. We know, if we connect two bodies thermally, there
may be exchange of heat between them. The flow of heat does not depend on the
mass of i.e. inherent heat within it, but on the temperature.
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| Similarity between potential and temperature and free surface of liquid |
Two tubes A and B are placed at same horizontal level. They are
connected by a tube with a stop cock S (Fig: Similarity between potential and
temperature and free surface of liquid). Closing the stop cock water is poured
in to A and B tubes in such a way so the height of water column is same in two
tubes. As the diameter of B is much greater than that of A, to raise the water
level at same height much more water is requires for tube B. Now if the stop cock
is opened there would be no change in water height i.e. there is no flow of
water. Though the amount of water is different in two tubes, but as there is
height is same, so there is no flow of water. Now if closing the stop cock a
little amount water is poured into A tube, the amount of water in it will still
be less than that of B, but the height of water level will increase slightly.
After if the stop cock is opened, then water will flow from A to and the height
of the water column will w same in both A and B. It is thus understood that
flow of water does not depend on the amount of water rather the height.
Suppose two conductors are positively charged. The amount of
charge in first conductor is greater than that of second conductor but the
potential of the first one is less than that of the second. Now if, two
conductors are connected electricity then positively charge will flow from
second conductor to first conductor. Though the amount of charge is greater in
first conductor yet it will take charge because its potential is low. As a
result of flow of charges when the potential of the two conductors become equal
then the flow will stop.
Therefore it can be said, the role of temperature in heat, role of
free surface of liquid in hydro-static and the role of potential in
electrostatic are same.
Electric Potential of Earth
Earth is an electric conductor. When a charged body is connected
to the earth, it becomes electrically neutral. When a positively charged body
is grounded electrons coming from the earth neutralize the body. When a
negatively charged body is grounded electrons from the body flow to the earth
and the body becomes neutral. The earth is so big that if charge is added or
taken away from it its potential does not change at all. Likewise if water is
taken away from sea or poured in the water level does not change. The earth is
always taking charge from different bodies and simultaneously it supplies charge
to other bodies. Hence earth is considered charge less. To determine the height
of a place the height of the sea level is taken as zero, similarly to determine
the potential of a body, the potential of earth is taken as zero.
Zero, Positive and Negative Potential
The potential of an uncharged conductor is taken as zero. When a
charged conductor is connected to the earth its potential becomes zero. Because,
in the connected state, are both the conductor and the earth is considered as a
single conductor. The potential of a positively charged body is positive and
negatively charged body is negative.
Unit of potential, Volt
If the work done in bringing 1 coulomb (1C) of positive charge
from infinity to a point in the electric field is 1 joule (1J), then the
potential at that point is called 1 volt (1V).
The potential at a point in an electric field is 20V means to
bring 1 coulomb (1C) positive charge from infinity to that point 20J work is to
be done.
Potential difference
Let, in an electric field A and B are two points and the
potentials of the points are VA and VB respectively (fig:
Potential difference). The work done in bringing a unit positive charge from infinity
to point A is VA and to point B is VB. Therefore the work
done in bringing a unit positive charge from point B to point A is VA
–VB i.e. the potential difference between these two points.
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Potential
difference
|
The work done in transferring a unit positive charge from one
point to another point in an electric field is called potential difference
between two points.
Electric capacitor
The capability of storing energy
as electric charge is called capacitor. Capacitor is the mechanical device
designed to sustain the capacitance. Capacitor stores energy from a source such
as electric cell and again uses it. A capacitor is made by placing an insulating
material such as air, glass, plastic etc. Therefore, the mechanical process of
storing energy as electric charges by placing an insulating medium in between two
nearby conductors is called capacitor.
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| Electric capacitor |
A simple capacitor is made by placing two insulating metal plates
parallel to each other. When a battery is connected to its two plates (Fig:
Electric
capacitor) then electrons may flow to a plate from its negative rod and is
charged negatively. Electrons flow to the positive rod of battery from the
other plate of the capacitor. As a result that plate is charged positively. The
amount of charge deposited in the plates depends on the voltage of the battery.
Capacitors are used in radio, television, record player and the
circuits of other electronic devices widely.
End
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