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Vinodkumar M, St. Aloysius HSS, Elthuruth, Thrissur [1] Electrostatics
ELECTROSTATICS
Electrostatics deals with the behavior of electric charges at rest.
Frictional electricity
It has been found that any substance if rubbed with some other substance acquires an attractive property.
The bodies are then said to have become electrified or they are said to acquire electric charge. For eg; when
r
a glass rod is rubbed with silk or an ebonite rod is rubbed with fur, they acquire this attractive property.
s su
, T s,
The phenomenon of acquiring electric charges by friction is called frictional electrification and
the bodies are said to possess frictional electricity.
uth sic
For eg; Take a plastic ruler or comb and rub it with dry hair or a piece of wool and bring it near small
pieces of paper. Then the paper pieces will get attracted towards the ruler or comb. This is because the plastic
h ri
ruler and comb got electrified due to friction with dry hair.
# Note: 1. Good conductors like copper cannot be charged by friction because any charge produced on it can
hur Phy
easily flow through the rod through our body and to the ground.
2. Insulators like plastic, ebonite, glass etc can be easily charged by friction because the charges will
stay on them.
3. Electrostatic experiments cannot be performed in moist climate because moist air is slightly conducting.
So the static charges will get conducted away from the charged body.
E l t ST
NB: How is frictional electrification caused?
The number of protons inside the nucleus of an atom is equal to the number of electrons outside the
nucleus. When a body is rubbed with another, due to friction, some electrons from one body gets transferred to
S
the other body. The body, which loses electrons, will become positively charged and which gains electrons
becomes negatively charged. The two bodies thus acquire opposite charges and they are equal in magnitude.
HS , H
This is the reason for frictional electricity.
Electric Charge
It is found experimentally that the charges are of two types: 1.Positive charge 2.Negative charge
i us ar M
The unit of charge is coulomb (C). The names of positive and negative charges are purely conventional.
Note:
S,
Positively charged body means deficiency of electrons in the body from its neutral state and a negatively
charged body means excess of electrons.
Gold-Leaf electroscope
oys m
A simple apparatus to detect charge on a body is called a gold-leaf electroscope.
Apparatus
Al dku
It consists of a vertical metal rod placed in a box. Two thin gold leaves are attached to its bottom end as
shown in figure.
St. ino
V
Working
When a charged object touches the metal knob at the top of the rod, charge flows on to the leaves and
they diverge. The degree of divergence is an indicator of the amount of charge.
Conductors and Insulators
Conductors are those substances which allow passage of electricity through them.
Insulators are those substances which do not allow passage of electricity through them.
Earthing (or) Grounding
When a charged body is brought in contact with earth, all the excess charge pass to the earth through the
connecting conductor. This process of sharing the charges with the earth is called grounding or earthing.
Earthingprovides protection to electrical circuits and appliances.
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Vinodkumar M, St. Aloysius HSS, Elthuruth, Thrissur [2] Electrostatics
Charging a body
A body can be charged in different ways1)Charging by friction 2)Charging by conduction 3)Charging by
induction
Charging by friction
r
When two bodies are rubbed each other, electrons in one body (in which electrons are held less tightly) trans-
s su
ferred to second body (in which electrons are held more tightly)
, T s,
Explanation
uth sic
When a glass rod is rubbed with silk, some of the electrons from the glass are transferred to silk. Hence
glass rod gets +ve charge and silk gets -ve charge.
h ri
Charging by conduction
hur Phy
A
Charging a body with actual contact of another body is called charging by conduction.
Explanation: If a neutral conducting body (A) is brought in contact with positively
B
charged conducting body (B), the neutral body gets positively charged.
Charging by induction
The phenomenon by which a neutral body gets charged by the presence of neighboring charged body is
E l t ST
called electrostatic induction.
Explanation
Step I : Place two metal spheres on an insulating stand and bring in contact as shown in figure (a).
S
Step II: Bring a positively charged rod near to these spheres. The free electrons in the spheres are attracted
towards the
HS , H
rod. Hence, one side of the sphere becomes negative and second side becomes positive as shown in the
figure (b).
Step III: Separate the spheres by a small distance by keeping the rod near to sphere A. The two spheres are
i us ar M
found to be oppositely charged as shown in figure (c).
Step IV: Remove the rod, the charge on spheres rearrange themselves as shown in
figure (d).
S,
oys m
Al dku
In this process, equal and opposite charges are developed on each sphere.
How can you charge a metal sphere positively without touching it?
Place the uncharged metallic sphere on an insulating stand. Bring a negatively charged rod close to the
St. ino
metallicsphereas shown in figure (b). Due to the attraction of rod electrons are piling up at the rear end and
positive charge at farther end. Connect the sphere to the earth (earthed) when the sphere is earthed,
electrons flow from the ground to the sphere and neutralize the positive charge.
Disconnect the sphere from ground and then remove rod from it. The negative charge uniformly
V
distribute over the sphere.
NB Properties of electric charges.
1.Electric charges are of two kinds – positive and negative.
2.Like charges repel and unlike charges attract each other.
3.The two kinds of charges ie +ve and – ve are really opposite. The combination of a charge +q with –q
results in a net charge equal to zero.
**4.Charge is quantised: Millikan showed that all electric charges in nature exist either as a basic chargeor
as some integral multiple of this charge. The basic charge is equal to electronic charge e = 1.602 x 10 –19 C.
Thus charge exists in discrete packets or charge is said to be quantised. According to quantisation of
electric charge, charge of a body is an integral multiple of a basic charge, which is the electronic
charge.
ie charge on a body, q ne ; where, n is an integer and e is the electronic charge.
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Vinodkumar M, St. Aloysius HSS, Elthuruth, Thrissur [3] Electrostatics
**5.Charge is conserved: It means that total charge of an isolated system remains constant. It also
implies that electric charges can neither be created nor destroyed. If an object loses some charge, an
equal amount of charge appears somewhere else.
6.Charge is a scalar quantity.
r
7. Additivity of charge: The total charge on a surface is the algebraic sum of individual charges present on
s su
that surface. If q1, q2, q3 ............., qn are the charges on a surface, then total or net charge,
, T s,
q = q1 + q2 + q3 + .............+ qn
uth sic
Q1 A conductor has a negative charge of 4.8 x 10-17C. Determine the number of excess electrons in the
h ri
conductor. [300]
Q2. A polythylene piece rubbed with wool is found to have a negative charge of 3x10 -7C(a) Estimate
the number of electrons transferred from which to which? (b) Is there a transfer of mass from wool
hur Phy
to polethylene?
NB Point charges.
If an electric charge is confined to an extremely small volume, it is called a point charge.
Physically, all charged bodies whose dimensions are very small compared to the distance between
E l t ST
them are referred to as point charges. Any charge whose dimensions are very small compared to its
distance from a point where its effect are to be analysed is also called a point charge.
S
NB Coulomb’s law or inverse square law.
Coulomb’s law or inverse square law states that the force between two stationary electric charges is
HS , H
directly proportional to the product of the two charges and inversely proportional to the square of the
distance between them.
Consider two point charges q1 and q2 separated by a distance r, then the force between them,
i us ar M
q 1q 2
Then F
r2 q1 r q2
S,
1 q1q 2
F –12 2 2
4 0 r r 2 ; where 0 is the permittivity of free space, whose value = 8.854x10 C /Nm .r is
the permittivity of the medium in which the charges are placed, relative to the permittivity of free space
oys m
and is called relative permittivity or dielectric constant whose value is 1 for free space.
Al dku
1
Now, value of 4 = 9x109 Nm2/ C2.
0
Therefore; Force between two charges in free space = 9 109 q 1 q 2 N.
r2
St. ino
9
If q1=q2=1C and r = 1m, then F= 9x10 N. So we can define the unit of charge – coulomb as follows:
One coulomb is that charge which when placed in free space at a distance of one metre from an
equal and similar charge repels with a force of 9x109 N.
V
Note: 1.Charge on one electron, e= –1.6 x10 –19 C .1C= e / (1.6 x10 –19) = 6.2 x1018 e.
ie 1C= charge on 6.2 x1018 electrons.
2.Coulomb’s law holds good only for point charges in free space.
3.The direction of force is always along the line joining the two-point charges q1 and q2. Hence this force is called
central force.
Coulomb’s law in vector form.
If F12 is the force on q1 due to q2 and r21 is the unit vector pointing from q2 towards q1,then
1 q 1q 2
F12 r̂21 .......... ........( 1)
4 0 r 2
Vinodkumar M, St. Aloysius HSS, Elthuruth, Thrissur [1] Electrostatics
ELECTROSTATICS
Electrostatics deals with the behavior of electric charges at rest.
Frictional electricity
It has been found that any substance if rubbed with some other substance acquires an attractive property.
The bodies are then said to have become electrified or they are said to acquire electric charge. For eg; when
r
a glass rod is rubbed with silk or an ebonite rod is rubbed with fur, they acquire this attractive property.
s su
, T s,
The phenomenon of acquiring electric charges by friction is called frictional electrification and
the bodies are said to possess frictional electricity.
uth sic
For eg; Take a plastic ruler or comb and rub it with dry hair or a piece of wool and bring it near small
pieces of paper. Then the paper pieces will get attracted towards the ruler or comb. This is because the plastic
h ri
ruler and comb got electrified due to friction with dry hair.
# Note: 1. Good conductors like copper cannot be charged by friction because any charge produced on it can
hur Phy
easily flow through the rod through our body and to the ground.
2. Insulators like plastic, ebonite, glass etc can be easily charged by friction because the charges will
stay on them.
3. Electrostatic experiments cannot be performed in moist climate because moist air is slightly conducting.
So the static charges will get conducted away from the charged body.
E l t ST
NB: How is frictional electrification caused?
The number of protons inside the nucleus of an atom is equal to the number of electrons outside the
nucleus. When a body is rubbed with another, due to friction, some electrons from one body gets transferred to
S
the other body. The body, which loses electrons, will become positively charged and which gains electrons
becomes negatively charged. The two bodies thus acquire opposite charges and they are equal in magnitude.
HS , H
This is the reason for frictional electricity.
Electric Charge
It is found experimentally that the charges are of two types: 1.Positive charge 2.Negative charge
i us ar M
The unit of charge is coulomb (C). The names of positive and negative charges are purely conventional.
Note:
S,
Positively charged body means deficiency of electrons in the body from its neutral state and a negatively
charged body means excess of electrons.
Gold-Leaf electroscope
oys m
A simple apparatus to detect charge on a body is called a gold-leaf electroscope.
Apparatus
Al dku
It consists of a vertical metal rod placed in a box. Two thin gold leaves are attached to its bottom end as
shown in figure.
St. ino
V
Working
When a charged object touches the metal knob at the top of the rod, charge flows on to the leaves and
they diverge. The degree of divergence is an indicator of the amount of charge.
Conductors and Insulators
Conductors are those substances which allow passage of electricity through them.
Insulators are those substances which do not allow passage of electricity through them.
Earthing (or) Grounding
When a charged body is brought in contact with earth, all the excess charge pass to the earth through the
connecting conductor. This process of sharing the charges with the earth is called grounding or earthing.
Earthingprovides protection to electrical circuits and appliances.
, www.Hsslive.in ® Join Telegram Channel :https://t.me/hsslive
Vinodkumar M, St. Aloysius HSS, Elthuruth, Thrissur [2] Electrostatics
Charging a body
A body can be charged in different ways1)Charging by friction 2)Charging by conduction 3)Charging by
induction
Charging by friction
r
When two bodies are rubbed each other, electrons in one body (in which electrons are held less tightly) trans-
s su
ferred to second body (in which electrons are held more tightly)
, T s,
Explanation
uth sic
When a glass rod is rubbed with silk, some of the electrons from the glass are transferred to silk. Hence
glass rod gets +ve charge and silk gets -ve charge.
h ri
Charging by conduction
hur Phy
A
Charging a body with actual contact of another body is called charging by conduction.
Explanation: If a neutral conducting body (A) is brought in contact with positively
B
charged conducting body (B), the neutral body gets positively charged.
Charging by induction
The phenomenon by which a neutral body gets charged by the presence of neighboring charged body is
E l t ST
called electrostatic induction.
Explanation
Step I : Place two metal spheres on an insulating stand and bring in contact as shown in figure (a).
S
Step II: Bring a positively charged rod near to these spheres. The free electrons in the spheres are attracted
towards the
HS , H
rod. Hence, one side of the sphere becomes negative and second side becomes positive as shown in the
figure (b).
Step III: Separate the spheres by a small distance by keeping the rod near to sphere A. The two spheres are
i us ar M
found to be oppositely charged as shown in figure (c).
Step IV: Remove the rod, the charge on spheres rearrange themselves as shown in
figure (d).
S,
oys m
Al dku
In this process, equal and opposite charges are developed on each sphere.
How can you charge a metal sphere positively without touching it?
Place the uncharged metallic sphere on an insulating stand. Bring a negatively charged rod close to the
St. ino
metallicsphereas shown in figure (b). Due to the attraction of rod electrons are piling up at the rear end and
positive charge at farther end. Connect the sphere to the earth (earthed) when the sphere is earthed,
electrons flow from the ground to the sphere and neutralize the positive charge.
Disconnect the sphere from ground and then remove rod from it. The negative charge uniformly
V
distribute over the sphere.
NB Properties of electric charges.
1.Electric charges are of two kinds – positive and negative.
2.Like charges repel and unlike charges attract each other.
3.The two kinds of charges ie +ve and – ve are really opposite. The combination of a charge +q with –q
results in a net charge equal to zero.
**4.Charge is quantised: Millikan showed that all electric charges in nature exist either as a basic chargeor
as some integral multiple of this charge. The basic charge is equal to electronic charge e = 1.602 x 10 –19 C.
Thus charge exists in discrete packets or charge is said to be quantised. According to quantisation of
electric charge, charge of a body is an integral multiple of a basic charge, which is the electronic
charge.
ie charge on a body, q ne ; where, n is an integer and e is the electronic charge.
, www.Hsslive.in ® Join Telegram Channel :https://t.me/hsslive
Vinodkumar M, St. Aloysius HSS, Elthuruth, Thrissur [3] Electrostatics
**5.Charge is conserved: It means that total charge of an isolated system remains constant. It also
implies that electric charges can neither be created nor destroyed. If an object loses some charge, an
equal amount of charge appears somewhere else.
6.Charge is a scalar quantity.
r
7. Additivity of charge: The total charge on a surface is the algebraic sum of individual charges present on
s su
that surface. If q1, q2, q3 ............., qn are the charges on a surface, then total or net charge,
, T s,
q = q1 + q2 + q3 + .............+ qn
uth sic
Q1 A conductor has a negative charge of 4.8 x 10-17C. Determine the number of excess electrons in the
h ri
conductor. [300]
Q2. A polythylene piece rubbed with wool is found to have a negative charge of 3x10 -7C(a) Estimate
the number of electrons transferred from which to which? (b) Is there a transfer of mass from wool
hur Phy
to polethylene?
NB Point charges.
If an electric charge is confined to an extremely small volume, it is called a point charge.
Physically, all charged bodies whose dimensions are very small compared to the distance between
E l t ST
them are referred to as point charges. Any charge whose dimensions are very small compared to its
distance from a point where its effect are to be analysed is also called a point charge.
S
NB Coulomb’s law or inverse square law.
Coulomb’s law or inverse square law states that the force between two stationary electric charges is
HS , H
directly proportional to the product of the two charges and inversely proportional to the square of the
distance between them.
Consider two point charges q1 and q2 separated by a distance r, then the force between them,
i us ar M
q 1q 2
Then F
r2 q1 r q2
S,
1 q1q 2
F –12 2 2
4 0 r r 2 ; where 0 is the permittivity of free space, whose value = 8.854x10 C /Nm .r is
the permittivity of the medium in which the charges are placed, relative to the permittivity of free space
oys m
and is called relative permittivity or dielectric constant whose value is 1 for free space.
Al dku
1
Now, value of 4 = 9x109 Nm2/ C2.
0
Therefore; Force between two charges in free space = 9 109 q 1 q 2 N.
r2
St. ino
9
If q1=q2=1C and r = 1m, then F= 9x10 N. So we can define the unit of charge – coulomb as follows:
One coulomb is that charge which when placed in free space at a distance of one metre from an
equal and similar charge repels with a force of 9x109 N.
V
Note: 1.Charge on one electron, e= –1.6 x10 –19 C .1C= e / (1.6 x10 –19) = 6.2 x1018 e.
ie 1C= charge on 6.2 x1018 electrons.
2.Coulomb’s law holds good only for point charges in free space.
3.The direction of force is always along the line joining the two-point charges q1 and q2. Hence this force is called
central force.
Coulomb’s law in vector form.
If F12 is the force on q1 due to q2 and r21 is the unit vector pointing from q2 towards q1,then
1 q 1q 2
F12 r̂21 .......... ........( 1)
4 0 r 2
