CBSE CLASS XII PHYSICS NOTES PART I

 CBSE CLASS 12 PHYSICS NOTES

 CHAPTER 1

ELECTRIC CHARGES AND FIELDS

 

Here are the notes for CBSE Class 12 Physics Chapter 1 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.

INTRODUCTION

Electric charges and fields are fundamental concepts in the study of electrostatics, a branch of physics that deals with the forces exerted by charges at rest. This study note will break down these concepts into digestible sections, providing clear explanations, examples, and important notes to help you understand the topic thoroughly.

 ELECTRIC CHARGE

The term 'electricity' comes from the Greek word "Elektron," which means amber. This is because the ancient Greeks discovered that rubbing amber would generate static electricity. In physics, electric charge is a fundamental property of matter, influencing how atoms and molecules interact. There are two types of electric charge: positive and negative. The fundamental rule of electric charges is that like charges repel each other, while unlike charges attract. This property is known as the polarity of charge and is key to understanding many electrical phenomena.                                                                     Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric or magnetic field. Charge is a scalar quantity and its S.I unit Coulomb (C).

SPECIFIC PROPERTIES OF CHARGES

1. According to Benjamin franklin, charges are of two types, positive and negative.

2. Like charges repel and unlike charges attract.( Fundamental law of electrostatic)

3. Charges are always associated with mass.

i.e. charge cannot exist without mass whereas mass can exist without charge.

4. When a body is positively charged= Loss of electrons = Mass decreases.

When a body is negatively charged= Gains of electrons = Mass increases.

5. Charge is conserved – The charge of an isolated system remains constant. That means charge can neither be created nor be destroyed.

6. Charge is quantized = Total charge of a body is equal to the integral multiple of fundamental charge ‘e’.

i.e. Q = ± ne , n = an integer (1,2,3,….)

Minimum possible charge =  ± e = ± 1.6 × 10^-19 C.

7. Charge is invariant = The charge is independent of frame of reference. That is charge on a body does not change whatever may be its speed.

8. Charge is additive = Total charge of an isolated system is equal to the algebraic sum of charge on individual bodies of the system.

i.e. if a system contain three charge q₁ ,q₂ , and q₃ then total charge on the system.

Q = q₁ + q₂ + q₃.

CONDUCTOR AND INSULATOR

In experiments with electric charges, it was discovered that conductors and insulators behave very differently. Conductors, such as metals, the Earth, and the human body, allow electric charges to move freely through them. This means that they can easily carry electricity. On the other hand, insulators, like porcelain, nylon, and wood, resist the flow of electric charges. They do not conduct electricity well, making them useful for preventing unwanted electrical flow and protecting us from electric shocks. 

TYPES OF CHARGES

֍ Positive Charges : Carried by Protons.

֍ Negative Charges : Carried by Electrons.

FUNDAMENTAL THEORY OF ELECTRIC CHARGES

Like charges repel each other, while unlike charges attract each other.

PROPERTIES OF ELECTRIC CHARGES

Electric charge has three key properties:

 ADDITIVITY OF CHARGE

The total charge in a system is the algebraic sum of all individual charges present. This means that if multiple charges act on a system, their effects add together to give the overall charge.

 QUANTIZATION OF CHARGE

The charges are quantized meaning it exists in discrete quantities. This property indicates that the total charge on a body is an integer multiple of a basic unit of charge. This means that electric charge comes in discrete amounts rather than continuous values. The smallest unit of charge is the charges of an electron or proton, denoted by e.

e = 1.6 × 10^-19 C

Q= ne

 CONSERVATION OF CHARGE

The total charge in an isolated system remains constant. This is known as the conservation of charge. This principle states that the total charge in an isolated system remains constant over time. Charges cannot be created or destroyed, only transferred from one object to another. This ensures that the total charge of a system is always conserved.

Q = q₁ + q₂ + q₃ + ……. + q_n

METHODS OF CHARGING

There are three methods of charging.

1. FRICTION =If we rub one body with another body then transfer of electrons takes place from one body to another body. The transfer of electrons take place from lower work function body to  the higher work function body.

POSITIVE	NEGATIVE
1. Glass rod.	Silk cloth.
2. Woolen cloth.	Plastic object, Rubber shoes, Amber.
3. Cat skin.	Ebonite rod.
4. Dry hair	Comb.

Clouds become charged by friction. 

2. ELECTROSTATIC INDUCTION (without direct contact between two bodies)

The phenomenon of temporary electrification of a conductor in which opposite charge appear at its closer end and similar charges appear at its farthest end in the presence of a nearby charged body is called electrostatic induction.

3. CONDUCTION = The process of transfer of charge by direct contact between 2 bodies is called conduction.

COULOMB’S LAW

Coulomb’s law quantifies the force between two point charges. Coulomb's law describes the electrostatic force between two point charges. If two stationary point charges q₁ and q₂ are placed a distance r apart, the magnitude of the electrostatic force F between them is given by:

Let the force between two charges are directly proportional,

F α q₁ q₂   ………………… (1)

The force between them is equal to the square of the distance between them,

PERMITTIVITY= Permittivity is the quantity that determines how far the medium permits the electrical interaction between two charged bodies.

 = 8.85 × 10¹² C²  N^-1 m^-2

Where, ε = Permittivity of the medium/ electrical permittivity

ε_o = Permittivity of free space.

ε_r = Dielectric constant (k) / relative permittivity.

Units of charge

Where,  is called absolute electrical permittivity of the free space.

The S.I. Unit of charge is coulomb. In the above equation if q₁ = q₂ = 1C and r = 1 meter. Then,

So, one coulomb is that amount of charge that repels an equal and similar charge with a force of 9 × 10⁹ N. When placed in vacuum at a distance of one meter from it

RELATIVE PERMITTIVITY (ε_r) /  DIELECTRIC CONSTANT (k)

The ratio of the permittivity of the medium to the permittivity of the free space is called relative permittivity (ε_r) or dielectric constant (k).

Relative permittivity or dielectric constant has no unit and dimensionless.

Symbol = ε_r or k

For vacuum, k = 1

For metal, k =  (infinity)

For water, k = 80

COULOMB’S LAW IN VECTOR FORM

This means that the two charges exert equal and opposite forces on each other so they obey Newton third law of motion.

CHARACTERSTIC OF COULOMB’S FORCES:-

1. Applicable or valid for point charges which are at rest.

2. Obeys inverse square law,

3. It is a long rang force.

4. Coulomb’s force is inactive when the separation between two charges is less than one Fermi                  (10^-15m).

5. It is a central force i.e., it act along the line joining the centres of two bodies.

6. Coulomb force depends on the medium within which charges are placed.

7. Coulomb force is not affected by the presence of o0ther charged bodies near it.

8. it obeys Newton’s third law of motion.

KEY POINT

Direction of force: The force acts along the line joining the two charges.

 NATURE OF FORCE

REPULSIVE: If the charges are of the same sign (both positive and both negative).

ATTRACTIVE: If the charges are of the opposite sign (one positive or one negative).

 CONSERVATIVE NATURE

The electrostatic force is conservative, meaning that the work done in moving a charge between two points is independent of the path taken.

 INVERSE SQUARE LAW:

The force follows the inverse square law, meaning the force decrease with the square of the distance between the charges.

 FORCE BETWEEN MULTIPLE CHARGES ( SUPERPOSITION)

When dealing with multiple charges, Coulomb's law applies to the interaction between any two charges. However, calculating the net force on a specific charge in the presence of several other charges requires a different approach. The net force on a charge due to multiple other charges is determined by taking the vector sum of all the individual forces exerted on that charge by each of the other charges, considered one at a time. This principle is known as superposition position.

 SUPERPOSITION POSITION

The superposition principle states that the interaction between any two charges is not influenced by the presence of other charges. In simpler terms if you have multiple charges in a system the force between any two specific charges remains the same, regardless of other charges in the vicinity.