CBSE CLASS XII PHYSICS NOTES PART II

 

CBSE CLASS 12 PHYSICS NOTES

 CHAPTER 1 (PART II)

ELECTRIC CHARGES AND FIELDS

 

Here are the notes for CBSE Class 12 Physics Chapter 1(Part-II) Electric Charges and Fields. This chapter explains the basics of electric charges and the electric fields they create. It includes key concepts like Coulomb's Law, which describes the force between two charged objects, and the superposition principle, which helps calculate the total electric field from multiple charges. You'll also learn about electric field lines, their properties and electric flux. These concepts are essential for understanding how electric forces work. You can find the full notes through the link provided for more details.

ELECTRIC FIELD                                                                                                            

An electric field is a region around a charged object where other charges experience a force.

TEST CHARGE

The charge which produces the electric field is called source charge and the charge which experiences the effect of source charge is called test charge.

- Unit positive charged is taken as test charge.

- Its magnitude is very small in comparison to source charge because its own field should not affect the field of source charge.

ELECTRIC FIELD DUE TO A POINT CHARGE                                                        

The electric field (E) due to a point charge q at a distance r is given by:

The direction of the electric field is radially outward for a positive charge and radially inward for a negative charge. The right-hand rule to determine the direction of the electric field around a positive charge.

ELECTRIC FIELD INTENSITY

It is defined as the force experienced per unit positive test charge placed at the point, without disturbing the source charge. It is expressed as,

ELECTRIC FIELD DUE TO MULTIPLE CHARGES:-

ELECTRIC FIELD LINES

> Electric field lines are a visual representation of the electric field.

> The electric field lines originate from positive charge and terminate at negative charge.

> The density of lines indicates the strength of the electric field.

 Continuous Curves: Electric field lines form continuous curves without any breaks in regions where there are no charges. This continuity shows the direction of the field at every point in space. 

 No Intersection: Two electric field lines never cross each other. If they did, it would imply that the electric field has two different directions at the same point, which is impossible.

 Starting and Ending Points: Electric field lines originate from positive charges and terminate on negative charges. This indicates the direction of the force that a positive test charge would experience in the field. 

 No Closed Loops: Electrostatic field lines never form closed loops. They always start on a positive charge and end on a negative charge, or they extend to infinity if no negative charge is present. This distinguishes electric fields from magnetic fields, where lines can form closed loops.

ELECTRIC DIPOLE:

An electric dipole is a system of two equal and opposite charges, typically denoted as +q and –q separated by a certain distance. The dipole moment is calculated as the product of the magnitude of one of the charges and the distance separating them, mathematically expressed as: 

Here, 2a represents the distance between the charges, and the direction of the dipole moment vector is from the negative charge to the positive charge. The concept of an electric dipole is important in understanding how molecules and other systems interact with electric fields.

It is a vector quantity.

Its dimension is ( ATL).

Its SI unit is Cm.

ELECTRIC DIPOLE MOMENT: 

i. It determines the strength of electric dipole.

ii. It is define as the product of magnitude of either charge and separation of distance between them.

iii. It is a vector quantity.

iv. Direction is always from negative charge to positive charge.

IDEAL DIPOLE / POINT DIPOLE:-

Suppose, q- , 2L→ 0 Such that p is finite. Such a dipole of negligibly small size is called as ideal dipole or point dipole.

PROPERTIES OF ELECTRIC DIPOLE

ELECTRIC DIPOLE AT A AXIAL POSITION:

 

ELECTRIC DIPOLE AT AN EQUITORIAL POSITION:

ELECTRIC DIPOLE AT A NORMAL POSITION:

 

DIPOLE IN UNIFORM ELECTRIC FIELD

ELECTROSTATIC FORCE DUE TO CONTINUOUS CHARGE DISTRIBUTION

The region in which charges are closely spaced is said to have continuous distribution of charge. It is of three types given as below:

 

ELECTRIC FLUX

ELECTRICAL FLUX SPECIAL CASE

 Case 1

When θ is less than 90⁰.

Cos θ = Positive

Flux is positive

When the line of force leaves the surface, the flux is positive.

 

Case 2.

When θ is equal to 90⁰.

Cos θ = 0.

Flux is 0.

 

Case 3

When θ is greater than 90⁰.

Cos θ = Negative.

Flux is Negative.

So, When the line of force enter the surface, the flux is Negative.

 

IMPORTANT POINTS ON GAUSS LAW

1. Gauss’s law is applicable for any closed surface, whatever its shape and size is.

2. The surface in which gauss’s law is applied is called Gaussian surface.

3. Flux linked with closed surface is independent of area of the surface.        

4. If the medium is di-electric then Ø_in = q/ε₀k.