# physics class 12 chapter 1- Electric charges and fields

## physics class 12th chapter 1 Electric charges and fields –

Electric charges and fields are two of the most fundamental concepts in physics. They underlie the behavior of all matter and energy, and play a crucial role in many of the world’s most important technologies.

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An electric charge is a physical entity that possesses a negative electric charge. It is made up of elementary particles called electrons. Electric fields are created whenever charges are moved around. They can cause physical objects to move or interact with other objects.

## 1.1 INTRODUCTION

All of us have the experience of seeing a spark or hearing a crackle when we take off our synthetle clothes or sweater, particularly in dry weather. This is almost mevitable with ladies garments like a polyester saree. Have you ever tried to find any explanation for this phenomenon? Another common example of electric discharge is the lightning that we see in the sky during thunderstorms. We also experience a sensation of an electric shock either while opening the door of a car or holding the tron bar of a bus after sliding from our seat. The reason for these experiences is discharge of electric charges through our body, which were accumulated due to rubbing of insulating surfaces. You might have also heard that this is due to generation of static electricity. This is precisely the topic we are going to discuss in this and the next chapter. Static means anything that does not move or change with time. Electrostatics deals with the study of forces, fields and potentials arising from static charges.

## 1.2 ELECTRIC CHARGE

Historically the credit of discovery of the fact that amber rubbed with wool or silk cloth attracts light objects goes to Thales of Miletus. Greece. around 600 BC. The name electricity is coined from the Greek word elektron meaning amber. Many such pairs of materials were known which

## Question 1.1:

Define the term ‘amorphous’. Give a few examples of amorphous solids.

Amorphous solids are the solids whose constituent particles are of irregular shapes and have short range order. These solids are isotropic in nature and melt over a range of temperature. Therefore, amorphous solids are sometimes called pseudo solids or super cooled liquids. They do not have definite heat of fusion. When cut with a sharp-edged tool, they cut into two pieces with irregular surfaces. Examples of amorphous solids include glass, rubber, and plastic.

## Question 1.2:

What makes a glass different from a solid such as quartz? Under what conditions could quartz be converted into glass?

The arrangement of the constituent particles makes glass different from quartz. In glass, the constituent particles have short range order, but in quartz, the constituent particles have both long range and short range orders.
Quartz can be converted into glass by heating and then cooling it rapidly.

## Question 1.3:

Classify each of the following solids as ionic, metallic, molecular, network (covalent) or amorphous.
(i) Tetra phosphorus decoxide (P40:0) (vii) Graphite
(ii) Ammonium phosphate (NH4)3PO4 (viii) Brass
(iii) SiC (ix) Rb (iv) 12 (x) LiBr (v) P4 (xi) Si

Ionic – (ii) Ammonium phosphate (NH4) 3PO4, (x) LiBr
Metallic – (viii) Brass, (ix) Rb
Molecular – (1) Tetra phosphorus decoxide (P4010), (iv) 12, (v) P4.
Covalent (network) – (iii) SiC, (vii) Graphite, (xi) Si
Amorphous – (vi) Plastic

## Question 1.4:

(1) What is meant by the term ‘coordination number’?
(ii) What is the coordination number of atoms: (a) in a cubic close-packed structure?
(b) in a body-centred cubic structure?

(i) The number of nearest neighbours of any constituent particle present in the crystal lattice is called its coordination number.
(ii) The coordination number of atoms
(a) in a cubic close-packed structure is 12, and
(b) in a body-centred cubic structure is 8

## Question 1.5:

How can you determine the atomic mass of an unknown metal if you know its density and the dimension of its unit cell? Explain.