CHAPTER
1 Electric Charges and Fields
Electric Charge Q1 r
Charge is a property of matter due to which two bodies interact x=
Q1 ± Q2
with each other electromagnetically. There are two kinds of
charges- positive and negative. SI unit is coulomb. Charge is x → distance of null point from Q1 charge
quantized and additive. (+) for like charges [null point will be in between Q1 & Q2]
(–) for unlike charges [null point will be outside Q1 & Q2 and
Coulomb’s Law near weaker charge]
Force between two charges
Equilibrium of Three Point Charges
1 q1q2
F= r X
4π ∈0 ∈r r 2
∈r = dielectric constant of the medium, ∈0 = permittivity of free Q1 q Q2
space r
q1 q2 (i) Q1 & Q2 must be of like nature.
r (ii) q should be of unlike nature.
Coulomb’s Law is applicable only for static point charges. Q1 −Q1Q2
= x = r and q
( )
2
Q1 + Q2 Q1 + Q2
Principle of Superposition
Force on a point charge due to many charges is given by Electric Dipole
F = F 1 + F 2 + F 3 + ........
Electric dipole moment p = qd , where d is distance from
The force due to one charge is not affected by the presence negative to positive charge.
of other charges. Torque on dipole placed in a uniform electric field τ= p × E
Electric field at a general point due to a dipole.
Electric Field or Electric Field Intensity y E
Er
F
E=
q E
Unit is N/C or V/m.
P
Electric Field due to Point Charge Q r
F 1 Q
=E L= im r
q0 → 0 q
0 4 π ∈0 r 2 x
–q +q
Null Point for Two Charges 1 p 1 + 3cos 2 θ
Electric field: E =
r If Q1 > Q2
4π ∈0 r3
Q1 Q2
Eθ 1
⇒ Null point is near Q2 Direction: tan =
α = tan θ
Er 2
1 Electric Charges and Fields
Electric Charge Q1 r
Charge is a property of matter due to which two bodies interact x=
Q1 ± Q2
with each other electromagnetically. There are two kinds of
charges- positive and negative. SI unit is coulomb. Charge is x → distance of null point from Q1 charge
quantized and additive. (+) for like charges [null point will be in between Q1 & Q2]
(–) for unlike charges [null point will be outside Q1 & Q2 and
Coulomb’s Law near weaker charge]
Force between two charges
Equilibrium of Three Point Charges
1 q1q2
F= r X
4π ∈0 ∈r r 2
∈r = dielectric constant of the medium, ∈0 = permittivity of free Q1 q Q2
space r
q1 q2 (i) Q1 & Q2 must be of like nature.
r (ii) q should be of unlike nature.
Coulomb’s Law is applicable only for static point charges. Q1 −Q1Q2
= x = r and q
( )
2
Q1 + Q2 Q1 + Q2
Principle of Superposition
Force on a point charge due to many charges is given by Electric Dipole
F = F 1 + F 2 + F 3 + ........
Electric dipole moment p = qd , where d is distance from
The force due to one charge is not affected by the presence negative to positive charge.
of other charges. Torque on dipole placed in a uniform electric field τ= p × E
Electric field at a general point due to a dipole.
Electric Field or Electric Field Intensity y E
Er
F
E=
q E
Unit is N/C or V/m.
P
Electric Field due to Point Charge Q r
F 1 Q
=E L= im r
q0 → 0 q
0 4 π ∈0 r 2 x
–q +q
Null Point for Two Charges 1 p 1 + 3cos 2 θ
Electric field: E =
r If Q1 > Q2
4π ∈0 r3
Q1 Q2
Eθ 1
⇒ Null point is near Q2 Direction: tan =
α = tan θ
Er 2
