Class 12 Physics Notes Chapter 1 Electric Charges and Fields Important

Have you ever heard of a crunch or seen a spark while dressing up or a synthetic jacket, especially in dry weather? How about some explanation about this phenomenon? Another example of electric shock is the lightning that we see in the sky during thunderstorms. All these experiments are caused by the discharge of electrical charges across the body that have accumulated due to rubbing the insulating surfaces. It also results from the generation of static electricity. Class 12 Physics Notes Chapter 1 Electric Charges and Fields.

Electric Charge-

The term “electricity” is derived from Elektron, a Greek word meaning amber. The properties of matter, atoms and molecules are determined by the magnetic and electric forces present in them. There are also only 2 kinds of an entity called the electric charge.

An experiment conducted also suggested that there are two kinds of electrification wherein (i) like charges repel and (ii) unlike charges attract each other. The property that differentiates these 2 kinds of charges is called the polarity of charge.

Conductors and Insulators-

When an experiment was conducted on electric charges due to frictional electricity, it was found that conductors assist in the movement of electric charge, but insulators do not behave in the same manner. Metal, Earth, Human Bodies are all examples of conductors, while porcelain, nylon, wood all offer high resistance to the passage of electricity through them, as they are insulators.

What are the properties of Electric charge?

An electric charge has three fundamental properties:

• Quantization- This property states that the total charge of a body represents the integral multiple of a basic quantum of charge.
• Additive- This property of electric charges represents the total charge of a body as the algebraic sum all the singular charges acting on the system.
• Conservation- This property states that the total charge of a system remains unaffected with time. In other words, when objects get charged due to friction, a transfer of charge from the one object to another occurs. Charges can neither be created nor destroyed.

Coulomb’s Law-

The coulomb’s law states that the mutual electrostatic force existing between two point charges A and B is proportional to their product which is AB and inversely proportional to the square of the distance between them (rAB). The equation is

FBA=forceonBduetoA=k(AB)÷r2AB

Mathematically,

This law consists of constant terms which are also called a constant of proportionality and is represented by ‘k’ and its values are k=9∗109Nm2C−2 Nm2C−2

Forces Between Multiple Charges-

Even if the mutual electric force between two charges is given by Columb’s law, it does not help to calculate the force on a charge where there are not one but several charges around.  It is have been proved via an experiment that force on any charge due to a number of other charges is the vector sum of all the forces on that charge due to the other charges, taken one at a time.

Superposition Principles

According to the superposition principle, the property of two charges to repel and attract each other remains unaffected even though there is a presence of third additional charge.

Consider q1, q2 and q3 as three charges of a system. Here, if the force on q1 due to q2 is denoted by F12. 

Then, 

Likewise, the force on q1 due to q3,  denoted by F13 is 

Thus, the total force F1 on q1 due to 2 charges q2 and q3 is 

Properties of Electric Field Lines-

Some of the general properties of field lines are:

• Field lines show a continuous curve without having any breakage in a charge-free region
• Two-line never cross each other
• These electric field lines start on the positive charge and end in the negative charge
• Electrostatic field lines do not form any closed loops

Electric Flux-

The total number of electric field lines passing a given area in a unit time is defined as the electric flux. However, we note that there is no flow of a physically observable quantity like in the case of liquid flow. Coming to the definition,  Electric flux  Δθ through an area element ΔS is defined by

Δθ= E.ΔS= E ΔS  cosθ

This is proportional to the number of field lines cutting the area element. The angle θ here is the angle between E and ∆S. In a closed surface, where the convention is already stated, θ is the angle between E and the outward normal to the area element. To calculate the total flux through any given surface divide the surface into small area elements, calculate the flux at each element and add them up. Thus, the total flux θ through a surface S is θ ~ Σ E. ∆S. The approximation symbol is used because the electric field E is taken to be constant over the small area element.

Electric Dipole-

It is a pair of equal or opposite charges A and -B which are separated by distance 2x. The dipole moment vector (let’s assume it as p) has a magnitude 2Ax and is in the direction of the dipole axis from -B to A

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Important Question-

1. (a) The Statement needs explanation ‘electric charge of a body is quantized’.

(b) Why can one ignore the quantization of electric charge when dealing with macroscopic i.e., large-scale charges?

2. Explain how the law of conservation of charge works on this phenomenon when charges appear on both the silk cloth and the glass rod when they are rubbed together.?

(a)Why electrostatic field line is not a continuous curve and cannot have sudden breaks?
(b) Why two field lines never cross each other at any point?

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