Properties of Electric Charges Class 12 Physics

    Properties of Electric Charge is a key and high-scoring topic in Class 12 Physics Electrostatics. For CBSE and RBSE board exams, it's important to clearly understand concepts like conservation, quantization, and additivity of charge, as questions are often asked from this section. In this article, you will get simple explanations, important points, and quick revision notes to help you score better.

Introduction to Electric Charge

Electric Charge

    Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. It exists in two types: positive and negative. Like charges repel each other, while unlike charges attract. This interaction is explained by Coulomb's Law, which describes the force between charged objects. Electric charge also follows key principles such as conservation and quantization. These properties are based on electrostatics and play a crucial role in understanding various natural phenomena and modern technologies used in everyday life.

Like and Unlike Charges

What is Electric Charge

    Electric Charge is a fundamental property of matter that enables it to interact through an electromagnetic field. It is responsible for all electric interactions in nature. There are two types of electric charge: positive charge (carried by protons) and negative charge (carried by electrons). A key feature of charge is that it follows the conservation of charge, meaning it can neither by created nor destroyed, and it is also quantized, existing in discrete amounts. The SI unit of charge is the Coulomb (C).

Key details about electric charge:

• Fundamental Property: Electric charge is an inherent property of subatomic particles. Protons carry a positive charge, while electrons carry a negative charge, forming the basis of all electrical phenomena.

• Attraction and Repulsion: In electrostatics, like charges repel each other, whereas opposite charges attract, which explains many everyday electrical interactions.

• Conservation and Transfer: According to the law of conservation of charge, electric charge cannot be created or destroyed; it can only be transferred from one object to another.

• Quantization: Electric charge exists in discrete amounts and follows the principle of quantization of charge, given by 

                    Q = n e

                    where

                    n is the number of electrons

                    e = `1.6  \times  10^{- 19} C`

• Types of Charge: A body becomes positively charged when it loses electrons and negatively charged when it gains electrons.

• Units: The SI unit of electric charge is the Coulomb (C). One Coulomb is equal to the charge of approximately `6.25 \times 10^{18}C` elementary charges.

Types of Electric Charge

• Positive Charge (+): A deficiency of electrons is called a positive charge; it means the object has more protons than electrons.

• Negative Charge (-): An excess of electrons is called a negative charge; it means the object has more electrons than protons.

• Neutral (0): An object is neutral when the number of protons and electrons is equal, so that the total charge is zero.

SI Unit of Electric Charge

    The SI unit of electric charge is the coulomb (C). It is the charge moved when one ampere flows for one second (Q = I t). This derived unit is named after Charles-Augustin de Coulomb.

Properties of Electric Charges

• Definition of Quantization of Charge

 

   Quantization of electric charge means that charge does not exist continuously but in small fixed units (packets). The charge on any object is equal to the charge on a fundamental electron charge (`1.602 \times 10^{-19}` Coulombs) or an integral multiple of it.

Key Aspects of Charge Quantization

• Formula of Quantization of Charge: 

Charge = integer `\times` elementary charge

Q = n e

Where 

Q = Total charge on the body

n = integer (... -3,-2,-1,0,1,2,3,...)

e = elementary charge (charge of an electron = `1.602 \times 10^{-19}C`)

• Origin: This comes from the fact that electric charge is transferred in the form of whole electrons during processes such as charging by friction, so charge always appears in discrete (quantized) amounts.

• Discovery: The idea was first indicated by Michael Faraday through his electrolysis experiments, and later experimentally confirmed by Robert Millikan in 1912.

• Macroscopic Scale: At the macroscopic scale, charge is quantized but appears continuous because the number of elementary charges is extremely large, making individual charge units practically unnoticeable.

• Example: 

Suppose a body has a charge of 

`Q = 3.2 \times ^{-19}C`

Using the formula

Q = ne

So, 

`n = Q/e = {3.2 \times 10^{-19}}/{1.6 \times 10^{-19}} = 2`

This means the body has 2 excess electrons.

Explanation:

• Charge always comes in packets of elementary charge (e).

• Just like you cannot have half a person, you cannot have half an electron's charge.

• That's why n must always be an integer.

Conservation of Charge

    The law of conservation of charge states that in an isolated system, the total electric charge remains constant over time; charge cannot be created or destroyed, only transferred between objects through particles like electrons and protons in physical processes.

Key Principles

• Net Charge Invariance: The algebraic sum of all positive and negative charges in a closed system always remains constant.

• Transfer vs. Creation: Charging occurs due to the movement of electrons. If an object becomes positively charged, it has lost electrons, which are gained by another object.

• Universal Application: This law holds true in all situations, from everyday static electricity to advanced nuclear and particle physics processes.

Examples in Nature

• Static Electricity

• Radioactive Decay

• Pair Production/Annihilation

Key Concepts

• Isolated System: The law of Conservation of Charge only applies if no charge enters or leaves the system.

• Scalar Quantity: Charge is a scalar quantity; charges are added algebraically, considering their signs (+q or - q).

• Role of Electrons: Charge transfer occurs exclusively through the movement of electrons, not protons.

• Mass association: Charge cannot exist without mass; charging a body changes its mass (negatively charged body gains mass; positively charged body loses mass).

Additivity of Charges

    In physics, the additivity of charges is a fundamental property stating that the total electric charge of a system is simply the algebraic sum of all the individual charges located within it.

        Total charge = algebraic sum of individual charges.

Key Points

• Algebraic Sum: Since electric charges can be either positive or negative, their signs must be taken into account while adding them.

• Scalar Nature: Electric charge is a scalar quantity (it has magnitude but no direction), which is why we can add them like simple numbers.
• Formula: 

        Total charge = algebraic sum of individual charges.

        `Q = + q_1 + q_2 + q_3 + ... ... ... + q_n`

• Example:

    Suppose we have three charged particles `q_1 = 2 C`, `q_2 = - 3 C` and `q_3 = 5 C`

    Total charge 

    Total charge = algebraic sum of individual charges.

       `Q = + q_1 + q_2 + q_3`

       `Q = + 2C - 3C + 5C`

So, the net charge is

       `Q = + 4C`

• Charges simply add like numbers (taking signs into account).

• This follows the principle of Conservation of Charge - charge is neither created nor destroyed, only transferred.

Scalar Nature of Charge

    Electric charge is a scalar quantity with magnitude only and no direction.

Invariance of Electric Charge

    The invariance of electric charge is the principle that the total electric charge of an isolated system remains constant, regardless of the reference frame, motion, or any physical process.

    Hence, the charge of a particle is the same whether it is at rest or moving at very high speeds (even close to the speed of light).

Conclusion

    In Electrostacs, electric charge is a fundamental scalar quantity governed by key properties: conservation, quantization, and additivity. Understanding positive and negative charges, Coulomb interaction, and invariance helps Class 12 students build a strong conceptual base for exams and real-world electrical and technological applications.

Properties of Electric Charges MCQs

1. What is the SI unit of electric charge?

   a) Joule

   b) Ampere

   c) Coulomb

   d) Volt

2. Which subatomic particle carries a negative electric charge?

   a) Proton

   b) Neutron

   c) Electron

   d) Positron

3. Which of the following materials is an insulator?

   a) Copper

   b) Rubber

   c) Silver

   d) Aluminum

4. What happens when two objects with the same charge are brought close together?

   a) They attract each other.

   b) They repel each other.

   c) They remain stationary.

   d) None of the above.

5. What happens when a positively charged object is brought near a neutral object?

   a) The neutral object gains a positive charge.

   b) The neutral object gains a negative charge.

   c) The neutral object becomes positively charged.

   d) There is no effect on the neutral object.

6. Which of the following is a unit of electric potential difference?

   a) Ohm

   b) Volt

   c) Ampere

   d) Watt

7. What is the term for the process of transferring electric charge by direct contact?

   a) Induction

   b) Conduction

   c) Radiation

   d) Insulation

8. Which law describes the force between two charged objects?

   a) Newton's Law

   b) Kepler's Law

   c) Coulomb's Law

   d) Faraday's Law

9. What is the charge of a neutron?

   a) Positive

   b) Negative

   c) Neutral

   d) Variable

10. Which type of current flows in only one direction?

    a) Alternating current

    b) Direct current

    c) Variable current

    d) None of the above

Short Answer Type Questions

1. Define electric charge and explain its two types.

2. How do conductors differ from insulators concerning the movement of electric charge?

3. Describe the process of charging by induction.

4. Explain why a charged object attracts a neutral object.

5. What is electric potential difference, and how does it relate to electric charge?

6. Discuss the significance of Coulomb's Law in understanding electric charge interactions.

7. How does lightning occur, and what role does electric charge play in the process?

8. Differentiate between direct current (DC) and alternating current (AC).

9. How does the concept of electric charge apply to the function of electronic devices?

10. Can electric charge be created or destroyed? Explain your answer.

Answer MCQs

1. What is the SI unit of electric charge?

        c) Coulomb

2. Which subatomic particle carries a negative electric charge?

        c) Electron

3. Which of the following materials is an insulator?

        b) Rubber

4. What happens when two objects with the same charge are brought close together?

        b) They repel each other.

5. What happens when a positively charged object is brought near a neutral object?

        b) The neutral object gains a negative charge. (due to polarization)

6. Which of the following is a unit of electric potential difference?

        b) Volt

7. What is the term for the process of transferring electric charge by direct contact?

        b) Conduction

8. Which law describes the force between two charged objects?

        c) Coulomb's Law

9. What is the charge of a neutron?

        c) Neutral

10. Which type of current flows in only one direction?

        b) Direct current

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