1
Electric charge and field
Electric charge:
Methods of charging:
1.By friction:
Properties of electrical charge:
1.Quantization of charge:
Q= 2+4+6-5= 7C
Electric charge:
- Intrinsic properties of particles
- It can be Positive or negative in nature.
- Electrically neutral: Amount of positive and negative charge are same on Particles.
- Electrically Charged: Amount of positive and negative charge are not same on Particles.
- Particles with the same nature repel each other.
- Particles with opposite nature attract each other.
Methods of charging:
1.By friction:
- Two bodies will be initially neutral.
- One body rubs with another.
- Finally Opposite charge will appear on both bodies.
- One will be charged and another neutral.
- Physical contact is required.
- Same types of charges will appear.
- Charge will transfer from one body to another, hence the sum of the charge on bodies will be constant.
- One body will be charge and another will be neutral
- Initially a neutral body would be earthed.
- Keep both bodies closer. i.e: Physical contact is not required.
- Opposite charge induced on the neutral body.
- Charged body will not lose its charge.
Properties of electrical charge:
1.Quantization of charge:
- Charge on the body is the integer multiple of charge on an electron.
i.e: where n=1,2,.........
- Total charge on an isolated system is constant.
- Isolated system means: the system through its boundary change is not allowed to escape.
- Total charge on the body is an algebraic sum of total charge located anywhere on it.
- While adding signs taken into consideration.
Q= 2+4+6-5= 7C

4.Charge is Invariant:
Coulomb's law:
Force between point charge is proportional to the product of magnitude of charge and inversely proportional to square of distance between them.
ie,
Where, in air
.Note:
- Charge does not depend on the speed of the body.
Coulomb's law:
Force between point charge is proportional to the product of magnitude of charge and inversely proportional to square of distance between them.
ie,
Where, in air
.Note:
- Applicable for point charge only.
- Not applicable for distance less then m.
- Electrostatic force is consevative force.
- Comparatively stronger than gravitational force.
- Coulomb's law obey newton's third law.

Electric field:
Space around a charge in which its influence can be felt by any other charged particle.
Electric field intensity: Force experienced by test charge in presence of other charge particle.
Note:
Space around a charge in which its influence can be felt by any other charged particle.
Electric field intensity: Force experienced by test charge in presence of other charge particle.
- Test charge is always considered positive.
- Let a charged particle 'q' is experiencing a force 'F'
Note:
- Test charge is always considered small because large magnitudes may disturb the original charge distribution. And then we get electric field disturbed configuration.
- For our convenience we took a magnitude of test charge of 1 C.
Direction of electric field due to point charge:
- Positive point charge:

- Negative point charge

Note:
Lines of force:
- Electric field due to point charge is spherically symmetrical.
- Force experienced by point charge 'q' in electric field 'E',
- Electric field due to system of charge,
Lines of force:
- Imaginary lines
- Tangent at any point on this line gives direction to the electric field.

Properties of Electrical lines of forces:
- Originate from positive charge.
- Converge at negative charge.
- Numbers of lines originating or terminating from charge 'q' C is
- Two lines never cross each other.
- These lines can never be closed loops. ie: originating and converging at same point.
- From surface conductor electrical lines of force start or end normally.
- Tangent on it gives direction of electric field vector.
- Closer lines show more intensity points.



Electric field from charged body
Electric dipole:
Electric field intensity due to electrical dipole:
Direction: Same as direction of dipole vector
Direction: Opposite of direction of dipole vector
Direction:
Net Force on dipole
Force on dipole due to charge = Force on charge due to dipole.
Let, the electric field due to the dipole at a given point of charge is E.Force on charge 'q' due to dipole,
Hence, Force on dipole,
- Charged ring
- Infinite line charge
- Uniformly charged disc
- Oppositely charged sheet
Electric dipole:
- Two equal and opposite charges at small distance.
- Dipole moment
- Direction of dipole moment is from negative charge to positive charge.
- Unit of dipole moment 'Coulomb-metre'
Electric field intensity due to electrical dipole:
- Along axial line:
Direction: Same as direction of dipole vector
- Along equatorial line:
Direction: Opposite of direction of dipole vector
- At any general point:
Direction:
Net Force on dipole
- Due to point charge:
Force on dipole due to charge = Force on charge due to dipole.
Let, the electric field due to the dipole at a given point of charge is E.Force on charge 'q' due to dipole,
Hence, Force on dipole,
- Due to uniform electric field:
- Due to non-uniform electric field:

- Due to another dipole:
Where, = Force on dipole 1 due to 2.
= Electric field due to 2 nd dipole.
Torques on dipole:

Electric flux
Gauss's law:
Flux through any closed surface
Also,
Where, Elementary area of gaussian surface.
Selection of Gaussian surface
- Measure the amount of field line passing through the cross section.
- Electric flux
- Direction:
- Consider Negative: when flux inter to the surface.
- Consider Positive: when flux leaves the surface.
Gauss's law:
- Gives relation between electric field and charge.
- Useful to calculate electric fields due to symmetric charge distribution.
Flux through any closed surface
Also,
Where, Elementary area of gaussian surface.
Selection of Gaussian surface
Charge Distribution | Gaussian Surface | Electric field |
Point charge | Spherical | Radial |
Spherical charge | Spherical | Radial |
Linear charge | Cylindrical | Radial |
Planer charge | Planer, Parallel to charged distribution | Normal to the surface |