LECTURE 1 : CLASS XII: PHYSICS : ELECTRIC CHARGE & FIELD

CLASS XII   |    PHYSICS    |    ELECTRIC CHARGE

      Notes prepared by Subhankar Karmakar

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ELECTRIC CHARGE

Electric charge is an intrinsic property of elementary particles of matter which gives rise to electric force between various objects.

• Electric charge is a scalar quantity.
• SI unit of electric charge is coulomb (C).
• There are two types of charges- a. Positive charge and b. Negative charge.
• A proton has a positive charge (+e) and an electron has a negative charge (-e).
• Magnitude of charge of a proton and an electron are same.
• Charge of a proton = + 1.6 x 10⁻¹⁹ C
• Charge of an electron = - 1.6 x 10⁻¹⁹ C

POLARITY OF CHARGE

• The property which distinguishes the two kinds of charges is called the polarity of charge.

FUNDAMENTAL LAW OF ELECTROSTATICS: 

• Like charges repel and unlike charges attract each other.

• The charge developed on a glass rod when rubbed with silk is called positive charge.
• The charge developed on a plastic rod rubbed with wool is called negative charge. 

CONDUCTORS AND INSULATORS

• Conductors: The substances through which electric charges can flow easily are called conductors. 
• Insulators: the substances through which electric charges cannot flow easily are called insulators.
• When some charge is transferred to a conductor, it really gets distributed over his entire surface. 
• If some charge is put on an insulator, It stays at same place. 
• The process in which a body shares its charges with the earth is called grounding or earthing.

ELECTROSTATIC INDUCTION

• It is the phenomenon of temporary electrification of a conductor in which opposite charges appear at its closure and and similar charges appear at its further and in the presence of a nearby charged body. 
• When two conductors one or both are charged are in contact with each other total charge is divided into two equal parts. 
• Suppose a conductor A has charge Q and another conductor B has charged q and they are connected with each other, then each of them will have a charge equal to (Q + q)/2.
• Suppose a conductor A has charge Q and another uncharged conductor B are connected with each other, then each of them will have a charge equal to (Q/2).

CHARGING OF TWO SPHERES BY INDUCTION

Steps for charging of two spheres by induction.

1. We hold the two metal spheres on insulating stands and place them in contact.
2. We bring a positively charged glass rod near the left sphere. The left sphere then becomes negatively charged and the right sphere becomes positively charged.
3. We separate the spheres and they now have opposite charges. 
4. We remove the glass rod. The charges on the spheres get redistributed. Positive and negative charges now face each other. 
5. When the spheres are separated quite apart, the charges on them get uniformly distributed. 

CHARGING OF A SPHERE BY INDUCTION

Steps for charging of a sphere by induction

1. We take a metal sphere on an insulating stand and keep a negatively charged plastic rod near it. The near end of the sphere becomes positively charged and far end of the sphere becomes negatively charged.
2. Far end of the sphere is connected to the ground by a connecting wire to the ground. 
3. When the sphere is disconnected from the ground, the positive charge remain in the near end. 
4. When the plastic rod is removed, the positive charge is uniformly distributed on the sphere. 

BASIC PROPERTIES OF CHARGE:

• There are three basic properties of charge. They are 1. Additivity, 2. Quantization and 3. Conservation of Charge. 
• Additivity: Additivity of electric charge means that the total charge of a system is the algebraic sum of all the individual charges located at different points inside the system.
• Quantisation: The quantization of electric charge means that the total charge (q) of a body is always an integral multiple of a basic quantum of charge (e).

∴ q = ne, where n = ±1, ±2, ±3, ±4,.........

• Conservation: The laws of conservation of charge states that 
• The total charge of an isolated system remains constant. 
• The electric charges can neither be created nor destroyed, they can only be transferred from one body to another body. 

COULOMB'S LAW OF ELECTRIC FORCE:

Coulomb's law states that the force of attraction or repulsion between two stationary point charges is

1. Directly proportional to the product of the magnitude of the two charges.
2. Inversely proportional to the square of the distance between them and the force acts along the line joining the two charges. 

q₁ ⁕--------------- r ------------------⁕ q₂

If two point charges q₁ and q₂ are separated by a distance r, then the force F of attraction or repulsion between them is such that

 F ∝ q₁q₂  and  F ∝ 1 /r²

F = k q₁q₂/r²

[ Electrostatic force constant (k) ]

Where k is a constant of proportionality and it is called electrostatic force constant. 

The value of k depends on the nature of the medium between the two charges and the system of units used to represent the physical quantities. 

For the two charges located in free space and in SI units, 

 k = 1/(4πεₒ) = 9 x 10⁹ Nm²/C²

Where εₒ is called permittivity of free space. 

εₒ =  8.85 x 10⁻² C² N⁻¹m⁻²

Dimension of εₒ = [M⁻¹L⁻³T⁻⁴A²]

UNITS OF CHARGE

1. The SI unit of charge is Coulomb and denoted by C. 

2. CGS unit of Charge is two types

(i) in electrostatic CGS unit: statcoulomb or e.s.u. 

1 C = 3 x 10⁹ e.s.u. of charge. 

(ii) in electromagnetic CGS unit: abcoulomb or e.m.u. of charge.

1 C = 0.1 abcoulomb or e.m.u. of charge.

COULOMB'S LAW IN VECTOR FORM

RELATIVE PERMITTIVITY AND DIELECTRIC CONSTANT 

Permittivity

Permittivity is a property of a medium which determines the electric force between two charges situated in that medium. It is denoted by ε. Permittivity of vacuum or free space is minimum and it is denoted by εₒ. Permittivity of other medium are greater than εₒ. 

Relative Permittivity

The ratio (ε/εₒ) of the permittivity (ε) of a medium to the permittivity (εₒ) of free space is called relative permittivity (εᵣ ) of the given medium. 

Dielectric Constant (κ) 

Dielectric constant is defined as the ratio of the force between two charges placed some distance apart in free space to the force between the same two charges placed at same distance apart but in another medium. It is equal to relative permittivity. κ = εₒ. 

Dielctric constant for air = 1.00054

Dielctric constant for water = 80.

SUPERPOSITION OF ELECTROSTATIC FORCES

The principle of superposition states that when a number of charges are interacting, the total force on a given charge is the vector sum of the forces exerted on it due to all other charges. The force between two charges is not affected by the presence othe other charges. 

LECTURE 1: CLASS 10: LIFE PROCESSES

CLASS X   |    SCIENCE    |    LIFE PROCESSES

      Notes prepared by Subhankar Karmakar

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SCIENCE: CLASS X: CBSE (CLASS NOTES)

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A. CHARACTERISTICS OF LIVING THINGS

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The characteristics of living things:

• 1. Living things can move by themselves
• 2. Living things need food air and water
• 3. Living things can grow
• 4. Living things can respond to changes around them. They are sensitive.
• 5. Living things respire
• 6. Living things excrete
• 7. Living things can reproduce.  

B. LIFE PROCESSES

The basic functions performed by living organisms to maintain their life on this earth are called life processes. The basic life processes common to all the living organisms are:

• a. Nutrition and Respiration
• b. Transport and Excretion
• c. Control and Coordination
• d. Growth
• e. Movement
• f. Reproduction

C. NUTRITION

All the living organisms need energy to perform various life processes and they get it from the food they take. 

• Food: 

Food is an organic substance. The simplest food is glucose. 

Carbohydrates and fats are the nutrients which are used by an organism mainly as a source of energy whereas proteins and mineral salts are the nutrients used by an organism for the biosynthesis of its body constituents like skin blood etc. 

• Nutrition:

Nutrition is a process of intake of nutrients like carbohydrates fats proteins minerals vitamins and water by an organism as well as the utilisation of this nutrients by the organism.

• Nutrient:

A nutrient can be defined as a substance which an organism obtains from the surroundings and uses it as a source of energy or for the biosynthesis of its body constituents like tissues and organs. It may be organic or inorganic substance. 

D. MODES OF NUTRITION

Modes of Nutrition means methods of preparing food or obtaining food by an organism. There are mainly two modes of Nutrition. 

• 1. Autotrophic, and 
• 2. Heterotrophic

E. AUTOTROPHIC NUTRITION:  

• Autotrophic nutrition is that mode of nutrition in which an organism makes or synthesizes its own food from the simple inorganic materials like carbon dioxide and water present in the surroundings. 
• The green plants have an autotrophic mode of nutrition. Therefore, all the green plants are autotrophs. The autotrophic bacteria also obtain their food by the autotrophic mode of nutrition. Most of the bacteria are not autotrophic the organisms having autotrophic mode of nutrition are called autotrophic organisms or just autotrophs. 
• The autotrophic organisms or autotrophs contain the green pigment called chlorophyll which is capable of trapping sunlight energy. 

How does autotrophs prepare their food?

The autotrophic organisms contain the green pigment called chlorophyll which is capable of trapping sunlight energy. This trapped sunlight energy is utilised by the autotrophs to make food by combining organic materials like carbon dioxide and water present in the environment by the process of photosynthesis. Thus the autotrophs make their own food by photosynthesis. 

F. HETEROTROPHIC MODE OF NUTRITION

• Heterotrophic nutrition is that mode of nutrition in which an organism cannot make or synthesize its own food from simple inorganic materials like carbon dioxide and water and depends on other organisms for its food.
• All the animals have a heterotrophic mode of nutrition. Most bacteria and fungi also have heterotrophic mode of nutrition. The organisms having heterotrophic mode of nutrition are called heterotrophs. 
• Those organisms which cannot make their own food from inorganic substances like carbon dioxide and water and depend on other organisms for their food are called heterotrophs. The non green plants like yeast are also heterotrophs.

G. TYPES OF HETEROTROPHIC NUTRITION

A heterotrophic organism can obtain its food from other organisms in three ways. Therefore, there are three types of heterotrophic nutrition.

• 1. Saprotrophic nutrition
• 2. Parasitic nutrition and 
• 3. Holozoic nutrition. 

• SAPROTROPHIC NUTRITION
• Saprotrophic nutrition is that nutrition in which an organism obtains its food from the organic matter of dead plants, dead animals and rotten bread etc. 

• The organisms having saprotrophic mode of nutrition are called saprophytes.
• Saprophytes are the organisms which obtain their food from like rotten leaves, dead and decaying animal bodies and other decaying organic matter like rotten bread.
• Examples. Fungi like bread moulds, mushrooms and yeast and many bacteria are saprophytes. 

• PARASITIC NUTRITION
• The parasitic nutrition is that nutrition in which an organism derives its food from the body of another living organism called its host without killing it. 
• The organism which obtains the food is called a parasite and the organisms from whose body food is obtained is called the host. A parasite is an organism plant or animal which feeds on another living organism known as host. 
• Example. Most of the disease causing organism are parasites. Parasitic mode of nutrition is observed in several fungi bacteria a few plants like Cuscuta and some animals like plasmodium and roundworms. 
• Malaria parasite: Plasmodium is known as  malaria parasite.

• HOLOZOIC NUTRITION
• The holozoic nutrition is that nutrition in which an organism takes the complex organic food materials into its body by the process of ingestion the ingested food is digested and then absorbed into the body cells of the organism. 
• The undigested and un absorbed part of the food is thrown out of the body of the organism by the process of egestion. 
• Examples. Man, cat, dog, cattle, deer, tiger, lion, and amoeba have the holozoic mode of nutrition.