ELECTRIC CHARGE AND FIELDS
CHAPTER-1
1. ELECTROSTATICS-
It deals with the study of forces , fields and potentials arising from static charges.
2. ELECTRIC CHARGE –
Electric charge is the property associated with matter due to which it
produce and experience electric and magnetic effect .
There is two types of charges –
1. Positive charge –
Lack of electron in a matter is called positive charge .
2. Negative charge –
Excess of electron in a matter is called negative charge .
# like charges repels and unlike charges attracts each other . (Fundamental law of electrostatics )
# SI unit of charge =coulomb (C)
3. TYPES OF MATTER -
According to flow of charges , there is three types of matters .
1. Conductor –
The substances through which electric charges can flow easily are called conductor
. like iron , aluminium , copper , silver etc.
2. Insulator –
The substance through which electric charges can not flows , called insulators . like –
plastic , wood etc.
3. Semiconductor –
The substances which are behave as insulator at low temperature and behave as a
conductor at high temperature . called semiconductor . like germanium , silicon etc.
#when some charges is given to surface of a conductor then due to property of conduction that
charges distributed uniformly at the surface of that conductor . But if we give some charges to
insulators then due to its property the charges does not distributed to its surface . it remain at the
same point where we give that charges .
4. TYPES OF CHARGING -
1. BY RUBBING METHOD –
When we rub two substances then due to friction heat is produced . then one of
substance absorb that heat and emit electron , and second one absorb that electron . The one which
emit electron due to lack of electron it become positive charged substance and second one due to
excess of electron become negative charged substance .
Following table represents negative and positive charge body when it is rubbed -
Positive charge Negative charge
Glass rod Silk cloth
Flannel or cat skin Ebonite rod
Woollen cloth Amber rod
Woollen coat Plastic seat
2. BY CONTACT METHOD –
When two identical shape and size conductor touches each other . where one of conductor
is charged and second one is neutral . then by touching charge will transfer form charged
conductor to neutral conductor . and by this both will have same charge .
3. BY INDUCTION METHOD –
When charged body brings nearer to a conductor . then due to attraction of unlike charges ,
unlike charges accumulated at closer surface of conductor and due to repulsion like charges
accumulated at rear surface . this temporary electrification of a conductor is called electrostatic
induction .
4. BASIC PROPERTIES OF ELECTRIC CHARGES –
1. ADDITIVITY OF ELECTRIC CHARGES –
, According to this property if a system contain n charges Q1, Q2 ,Q3 ,Q4 ,Q5 ……..then total
charges of system is
Q= Q1 +Q2 +Q3 +Q4 +Q5 ……
# Charge has magnitude but no direction .
# Proper sign have to be used while adding the charges in a system
2. CHARGES ARE CONSERVED –
According to this property no new charges either created or destroyed . Total
magnitude of charges in initial and final condition is equal .
3.QUANTISATION OF CHARGES –
It is established that all free charges are integral multiple of a basic unit of charge
denoted by e , thus charge Q on a body always given by
Q=ne n=integer number
e=1.6 x 10-19 C
This fact is called quantisation of charges.
5. COULOMB’S LAW –
According to coulomb , if two charges q1 and q2 are kept at r distance . then forces
between these two charges is –
1. Directly proportional to product of two charges . it means
F ∝ q1 x q2
2. reciprocal to square of distance between these charges . it means
1
F ∝ 𝑟2
By combining above –
q1 x q2
F ∝ 𝑟2 where k= Coulomb constant .
k x q1 x q2 𝑁𝑚2
F= k= 9 x 109
𝑟2 𝐶2
1
Case (1) when both charges situated in vacuum , then k = 4𝜋Ԑₒ , Ԑₒ = permittivity of free space
𝐶2
= 8.85 x 10-12
𝑁𝑚2
1
Case (2) when both charges situated in medium ,then k = 4𝜋Ԑ , Ԑ = permittivity of medium .
Relation between Ԑₒ and Ԑ Ԑ = Ԑₒ x Ԑr , Ԑr = relative permittivity
Some values of Ԑr
1. vacuum =1
2. air = 1.00054
3. water = 80
4. metal = infinite
6. VECTOR NOTION OF COULOMB LAW –
Let two charges q1 and q2 are located at r1 and r2 position vector with reference to point O.
then joining vector along direction from r1 to r2 is
r12 = r1 – r2 = -r21
𝑘𝑞1 𝑞2
F12 = 𝑟 2 𝒓̂𝟏𝟐 , 𝒓̂
𝟏𝟐 = unit vector along r12 vector
𝑘𝑞1 𝑞2
F21 = 𝒓̂
𝟐𝟏 , 𝒓̂
𝟐𝟏 = unit vector along r21 vector
𝑟2
CHAPTER-1
1. ELECTROSTATICS-
It deals with the study of forces , fields and potentials arising from static charges.
2. ELECTRIC CHARGE –
Electric charge is the property associated with matter due to which it
produce and experience electric and magnetic effect .
There is two types of charges –
1. Positive charge –
Lack of electron in a matter is called positive charge .
2. Negative charge –
Excess of electron in a matter is called negative charge .
# like charges repels and unlike charges attracts each other . (Fundamental law of electrostatics )
# SI unit of charge =coulomb (C)
3. TYPES OF MATTER -
According to flow of charges , there is three types of matters .
1. Conductor –
The substances through which electric charges can flow easily are called conductor
. like iron , aluminium , copper , silver etc.
2. Insulator –
The substance through which electric charges can not flows , called insulators . like –
plastic , wood etc.
3. Semiconductor –
The substances which are behave as insulator at low temperature and behave as a
conductor at high temperature . called semiconductor . like germanium , silicon etc.
#when some charges is given to surface of a conductor then due to property of conduction that
charges distributed uniformly at the surface of that conductor . But if we give some charges to
insulators then due to its property the charges does not distributed to its surface . it remain at the
same point where we give that charges .
4. TYPES OF CHARGING -
1. BY RUBBING METHOD –
When we rub two substances then due to friction heat is produced . then one of
substance absorb that heat and emit electron , and second one absorb that electron . The one which
emit electron due to lack of electron it become positive charged substance and second one due to
excess of electron become negative charged substance .
Following table represents negative and positive charge body when it is rubbed -
Positive charge Negative charge
Glass rod Silk cloth
Flannel or cat skin Ebonite rod
Woollen cloth Amber rod
Woollen coat Plastic seat
2. BY CONTACT METHOD –
When two identical shape and size conductor touches each other . where one of conductor
is charged and second one is neutral . then by touching charge will transfer form charged
conductor to neutral conductor . and by this both will have same charge .
3. BY INDUCTION METHOD –
When charged body brings nearer to a conductor . then due to attraction of unlike charges ,
unlike charges accumulated at closer surface of conductor and due to repulsion like charges
accumulated at rear surface . this temporary electrification of a conductor is called electrostatic
induction .
4. BASIC PROPERTIES OF ELECTRIC CHARGES –
1. ADDITIVITY OF ELECTRIC CHARGES –
, According to this property if a system contain n charges Q1, Q2 ,Q3 ,Q4 ,Q5 ……..then total
charges of system is
Q= Q1 +Q2 +Q3 +Q4 +Q5 ……
# Charge has magnitude but no direction .
# Proper sign have to be used while adding the charges in a system
2. CHARGES ARE CONSERVED –
According to this property no new charges either created or destroyed . Total
magnitude of charges in initial and final condition is equal .
3.QUANTISATION OF CHARGES –
It is established that all free charges are integral multiple of a basic unit of charge
denoted by e , thus charge Q on a body always given by
Q=ne n=integer number
e=1.6 x 10-19 C
This fact is called quantisation of charges.
5. COULOMB’S LAW –
According to coulomb , if two charges q1 and q2 are kept at r distance . then forces
between these two charges is –
1. Directly proportional to product of two charges . it means
F ∝ q1 x q2
2. reciprocal to square of distance between these charges . it means
1
F ∝ 𝑟2
By combining above –
q1 x q2
F ∝ 𝑟2 where k= Coulomb constant .
k x q1 x q2 𝑁𝑚2
F= k= 9 x 109
𝑟2 𝐶2
1
Case (1) when both charges situated in vacuum , then k = 4𝜋Ԑₒ , Ԑₒ = permittivity of free space
𝐶2
= 8.85 x 10-12
𝑁𝑚2
1
Case (2) when both charges situated in medium ,then k = 4𝜋Ԑ , Ԑ = permittivity of medium .
Relation between Ԑₒ and Ԑ Ԑ = Ԑₒ x Ԑr , Ԑr = relative permittivity
Some values of Ԑr
1. vacuum =1
2. air = 1.00054
3. water = 80
4. metal = infinite
6. VECTOR NOTION OF COULOMB LAW –
Let two charges q1 and q2 are located at r1 and r2 position vector with reference to point O.
then joining vector along direction from r1 to r2 is
r12 = r1 – r2 = -r21
𝑘𝑞1 𝑞2
F12 = 𝑟 2 𝒓̂𝟏𝟐 , 𝒓̂
𝟏𝟐 = unit vector along r12 vector
𝑘𝑞1 𝑞2
F21 = 𝒓̂
𝟐𝟏 , 𝒓̂
𝟐𝟏 = unit vector along r21 vector
𝑟2
