Class XII Physics Chapter 1 notes

Physics Class 12th Chapter-1 Electric charges and fields

Charge:

It is an inherent characteristic of matter. It is of two types-

Positive charge: Charge developed on glass rod when rubbed with silk cloth.

Negative Charge: Charge developed on plastic rod when rubbed with fur.

~Unlike Charges attract, like charges repel.

Process of charging:

1.Charging by rubbing-

Easy removal of an electron from an atom by providing some energy to these electrons by various means of rubbing the two objects together.

2.Charging by Electrostatic Induction-

In this process, charged object is brought nearer to another uncharged object in which opposite charge appear at its closer emd and similar charge at uts farther end.

Example-

A glass rod which is charges positively is brought near an uncharged sphere then sphere is charged. Free electrons present in metal are attracted toward the rod while centre of rest positive charge of an atom is repelled. This shows production of opposite charge on front face and similar charge on rear end.

(To produce only one kind of charge, the rear end of metal sphere is earthed.)

Properties of charge-

1.Quantization of charge:

Charge produced on an object is always an constitutive multiple of charge of an electron.

i.e.  Q = ne, Where n= 1, ±2,… and so on.

e: Charge on each electron=

1.6×10−19 C

 

2.Conservation of charge:

It shows that charge can neither be created nor destroyed or the total charge remains conserved.

q1+q2=0 shows conservative nature of charge.

 

3.Additivity of charge:

It states that total charge possessed by a body is the algebraic sum of all charges present on it.

Q=q1+q2+q3…..

Unit of charge in Cgs system is Coulomb.

1C= 6.24 x 10^18 electrons.

 

Coulomb’s Law:

It states that force of attraction or replusion between two point charges is directly proportional to the product of charges and inversely proportional to the square of distance between them.

 

Dielectric Constant:

It is defined as the ratio of relative permittivity of medium to the relative permittivity of free space.

OR

It is defined as the ratio of force experienced by the charged particles when they are placed in air or vacuum to the force experienced by the charged particles when they are placed in medium same distance apart.

If q1, q2 > 0

then, replusion will occur.

If q1, q2 < 0

then, attraction will occur.

 

Limitations of Coulomb’s Law:

1. Only applicable on point charge at rest.
2. Only applicable on those cases where square law is obeyed.
3. It is difficult to apply Coulomb’s law when charges are in arbitrary shape.

Electric series:

Fur < Flammel < Glass < Perspex < Cotton < Silk < Leather < Wood < wax < Amber < polythene < Plastic < PVC.

Force between multiple charges:

If there are more than two charges in a medium then net force on each charge is the vector sum of all the forces on it due to other charges.

1. e.

 

Continuous Charge distribution:

• LINEAR CHARGE DISTRIBUTION-

The linear charge density is defined as charge per unit length.

• Surface Charge density

The surface charge density is defined as charge present per unit area.

• Volume charge density-

The volume charge density is defined as charge per unit volume.

Electric field

Space around a rest charged particle in which its interaction with other charges can be experienced.

Electric field intensity:

Force experienced by a test charge in electric field at any point where we want to calculate it.

Note:We always take test charge positive and minimum.

Note: Source charge q must remain at its original position. If test charge is brought at any point around q, then q will bound itself to experience an electrical force due to it and will tend to move. Thus, we take test charge nearly negligible  so that force is small but electric field intensity is finite.

Electric Field lines: Path followed by a test charge particle in a electric field if we free the charge to move.

Properties of electric field lines:

1. Electric field lines of negative charge is always inwards and of positive charge is outwards.
2. These lines originate from positive charge and end at negative charge.
3. These lines start from surface and end at surface. The do not form close loops.
4. Tangent at any point on curve line gives electric field direction.
5. These lines never intersect.
6. These lines are always perpendicular at starting and ending point.

 

 

Note: Magnitude of field is represented by density of field lines.

i.e. Nearer the charge particles, higher the density. Away the charge particles, lower the density.

Electric Dipole:

It is pair of equal and opposite charges separated by small distance 2a.

Total charge on a dipole is zero.

It is a vector quantity and its direction is  always from negative to positive.

Dipole Moment :

It is the strength of dipole whose magnitude is equal to the product of either charge and distance between them.

Electric Flux:

It is defined as the no. of field lines crossing through area placed normal to the field at a point.

Gauss’s Theorem:

Total electric flux over a closed surface S in vacuum is one by Absillinn not times the total charge  Q contain inside S

 

Applications of Gauss’s theorem:

1. Electric Field due to Infinite Wire:
   In order to calculate electric field due to infinite wire, we Contemplate a long wire whose length is L and Charge density is λ. Also, let us consider a surface on which charge is uniformly distributed which is cylindrical. The flux via terminal of surface is zero because electric field is radial in direction.
   Also, unit vector and electric field are outward and normal to curved surface in same direction thus, angle is zero.

The electric flux is

E × 2πrl

Using Gauss’s Theorem,

 

1. Electric Field due to Infinite Plate Sheet:

Consider a plane sheet, which have uniform charge density say, σ and area A.

We know that,

 

Electric field and curved surface are perpendicular to each other, hence, no electric flux is produced. So, total electric flux produced is,

Φ = 2EA

Thus,

1. Electric Field due to Thin Spherical Shell:

Consider a thin spherical shell of radius r and centre O. Let positive charge q be spread over the surface uniformly.

 

 

Electric Field outside the Spherical Shell:

Consider a thin spherical shell of radius r and centre O. Let positive charge q be spread over the surface uniformly.

At any point P, imagine a sphere S with centre O and radius r. Then,using gauss’s theorem,

Electric field inside the shell:

According to figure, r >> R

As there is no charge inside the shell thus, E=0 for r >> R