CBSE Class-1 Physics Quick Revision Notes
Chapter-01: Electric Charges and Fields
• Like Charges and Unlike Charges:
Like charges repel and unlike charges attract each other.
• Conductors and Insulators:
Conductors allow movement of electric charge through them, insulators do not.
• Quantization of Electric Charge:
It means that total charge (q) of a body is always an integral multiple of a basic quantum
of charge (e)
q = ne
where n = 0, ±1, ±2, ±3, ….
• Additivity of Electric Charges:
Total charge of a system is the algebraic sum of all individual charges in the system.
• Conservation of Electric Charges:
The total charge of an isolated system remains uncharged with time.
• Superposition Principle:
It is the properties of forces with which two charges attract or repel each other are not
affected by the presence of a third (or more) additional charge(s).
• The Electric Field E at a Point due to a Charge Configuration:
It is the force on a small positive test charges q placed at the point divided by a
magnitude
|q|
4πε 0 r 2
It is radially outwards from q, if q is positive and radially
inwards if q is negative.
E at a point varies inversely as the square of its distance
from Q, the plot of E versus r will look like the figure
given below.
• Coulomb’s Law:
The mutual electrostatic force between two point charges q1 and q2 is proportional to
the product q1q2 and inversely proportional to the square of the distance r21 separating
them.
k ( q1q2 ) ^
F 21 ( force on q 2 due to q1 ) = r 21
r212
^ 1
Where r 21 is a unit vector in the direction from q1 to q2 and k = is the
4πε 0
proportionality constant.
• An Electric Field Line:
It is a curve drawn in such a way that the tangent at each point on the curve gives the
direction of electric field at that point.
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, • Important Properties of Field Lines:
These are:
(i) Field lines are continuous curves without any breaks.
(ii) Two field lines cannot cross each other.
(iii) Electrostatic field lines start at positive charges and end at negative charges – they
cannot form closed loops.
• Electric Field at a Point due to Charge q:
F
E=
q
• Electric Field due to an Electric Dipole in its Equatorial Plane at a Distance r from
the Centre:
−p 1
E=
4πε 0 2 2 32
(a + r )
−p
≅ , for r>>a
4πε 0
• Electric Field due to an Electric Dipole on the Axis at a Distance r from the Centre:
2 pr
E=
4πε 0 (r 2 − a 2 ) 2
2p
≅ , for r>>a
4πε 0 r 3
• A Dipole Placed in Uniform Electric Field E experiences:
Torque τ ,
τ = pxE
• The Electric Flux:
φ = ∫ dφ = ∫ E.d s is a ‘dot’ product, hence it is scalar.
π
∆φ is positive for all values of θ <
2
π
∆φ is negative for all values of θ >
2
• Gauss’s Law:
The flux of electric field through any closed surface S is 1/ε0 times the total charge
enclosed by S.
q
φ = ∫ E.d s =
ε0
• Electric field outside the charged shell is as though the total charge is concentrated at the
centre. The same result is true for a solid sphere of uniform volume charge density.
• The electric field is zero at all points inside a charged shell.
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Chapter-01: Electric Charges and Fields
• Like Charges and Unlike Charges:
Like charges repel and unlike charges attract each other.
• Conductors and Insulators:
Conductors allow movement of electric charge through them, insulators do not.
• Quantization of Electric Charge:
It means that total charge (q) of a body is always an integral multiple of a basic quantum
of charge (e)
q = ne
where n = 0, ±1, ±2, ±3, ….
• Additivity of Electric Charges:
Total charge of a system is the algebraic sum of all individual charges in the system.
• Conservation of Electric Charges:
The total charge of an isolated system remains uncharged with time.
• Superposition Principle:
It is the properties of forces with which two charges attract or repel each other are not
affected by the presence of a third (or more) additional charge(s).
• The Electric Field E at a Point due to a Charge Configuration:
It is the force on a small positive test charges q placed at the point divided by a
magnitude
|q|
4πε 0 r 2
It is radially outwards from q, if q is positive and radially
inwards if q is negative.
E at a point varies inversely as the square of its distance
from Q, the plot of E versus r will look like the figure
given below.
• Coulomb’s Law:
The mutual electrostatic force between two point charges q1 and q2 is proportional to
the product q1q2 and inversely proportional to the square of the distance r21 separating
them.
k ( q1q2 ) ^
F 21 ( force on q 2 due to q1 ) = r 21
r212
^ 1
Where r 21 is a unit vector in the direction from q1 to q2 and k = is the
4πε 0
proportionality constant.
• An Electric Field Line:
It is a curve drawn in such a way that the tangent at each point on the curve gives the
direction of electric field at that point.
Portal for CBSE Notes, Test Papers, Sample Papers, Tips and Tricks
, • Important Properties of Field Lines:
These are:
(i) Field lines are continuous curves without any breaks.
(ii) Two field lines cannot cross each other.
(iii) Electrostatic field lines start at positive charges and end at negative charges – they
cannot form closed loops.
• Electric Field at a Point due to Charge q:
F
E=
q
• Electric Field due to an Electric Dipole in its Equatorial Plane at a Distance r from
the Centre:
−p 1
E=
4πε 0 2 2 32
(a + r )
−p
≅ , for r>>a
4πε 0
• Electric Field due to an Electric Dipole on the Axis at a Distance r from the Centre:
2 pr
E=
4πε 0 (r 2 − a 2 ) 2
2p
≅ , for r>>a
4πε 0 r 3
• A Dipole Placed in Uniform Electric Field E experiences:
Torque τ ,
τ = pxE
• The Electric Flux:
φ = ∫ dφ = ∫ E.d s is a ‘dot’ product, hence it is scalar.
π
∆φ is positive for all values of θ <
2
π
∆φ is negative for all values of θ >
2
• Gauss’s Law:
The flux of electric field through any closed surface S is 1/ε0 times the total charge
enclosed by S.
q
φ = ∫ E.d s =
ε0
• Electric field outside the charged shell is as though the total charge is concentrated at the
centre. The same result is true for a solid sphere of uniform volume charge density.
• The electric field is zero at all points inside a charged shell.
Portal for CBSE Notes, Test Papers, Sample Papers, Tips and Tricks
