OHM’S LAW

DEFINATION: -

“The current which is flowing through a conductor is directly proportional to the applied potential difference at constant temperature is called ohm’s law”.

Mathematically this law can be stated as,

                                              I∞ V

                                              I = V / R

                                Or              V = IR

Where R is constant of proportionality and is called resistance of the conductor. In S.I system the unit of resistance is OHM

OHM: - “One volt is potential difference is applied across any plates in one ampere current then the total resistance is one Ohm”.

              The conductors, which obey ohm’s law, is called OHMIC conductor and that conductor which does not obey ohm’s law is called NON-OHMIC conductor.

EXAMPLES: - Filament bulb, thermister and semiconductor diodes are the examples of NON-OHMIC conductor. Where metal wire is an example of OHMIC conductor.

ELECTRICAL RESISTANCE: -

                    “The measurement opposition offered to the flow of electric current is called resistance”. It arises as a result of continuous bombing of electrons with lattice atoms.

              Mathematically it can be expressed as,

                                R = V / I

Thus resistance is the ratio of potential difference to current. Its units is OHM and OHM is defined as, “one ohm is the resistance of a conductor through which a current of one ampere is flowing when the potential difference across it is one volt”.

                                    1Ω = 1 V A ^–1 
 

RESISTIVITY & ITS VARIATION WITH TEMPERATURE: - From observation we know that resistance of a conductor is directly proportional to the length “l” and inversely proportional to the area of cross-section “A” of the conductor.

              i.e.                           R ∞ L                                        (1)

                                              R ∞ L / A                    (2)

              Combining the above two equations we get

                                              R ∞ L / A

                                Or              R = ρ L / A                                 (3)

                                Or               ρ = R A / L                                (4)

Where ρ is constant of proportionality and is called resistivity of conductor and can be defined as,                    “the resistance of a conductor having unit length and unit area of cross-section”.

It is also called specific resistance and its unit is Ωm.

The reciprocal of resistivity is called conductivity. We represent it by σ and are given by

                                                                σ = 1 / ρ = L / R A

CURRENT ELECTRICITY

ELECTRIC CURRENT: -

            “The rate of flow of charge is called as current”. Mathematically it can be expressed as

                                                I = Q / t

Where I represent the current, Q is the charge, which passes through certain area of cross section in time t.

UNIT: - The unit of current is Ampere and Ampere is defined as,

                        “If one coulomb of charge flows through any cross-section of a

Conductor in one second”

DIRECTION OF ELECTRIC CURRENT: - Let’s consider that a capacitor connected to a voltage source.

It is clear that the charges are static. Now if we connect the two plates by a conducting wire, then charge will flow from one plate to another to neutralize it. But here think that how we can say the charges moved on the wire from one plate to another.In fact from our every day life we know that always flow take place from high to low potential, that is why the old investigator’s decided that in this case the same principle is valid i.e. charge flow from high potential (positive) to low potential (negative). In other words according to their idea the positive charges left the plate, crossed along the wire and neutralized the negative charges on the other plate. Hence this type of current is called CONVENTIONAL CURRENT.

            But now a day it is incorrect, because current through a metallic wire is due to the motion of free electrons. In case of liquids and gases the current is transported both by positive and negative charges, while in case of accelerator the current may be transported due to flow of positive charges.

DRIFT OF ELECTRONS IN METALLIC CONDUCTORS: - A metallic wire consists of large number of free electrons. Therefore electrons move randomly in such a way, that net current in the wire is zero, as shown in the figure. Now if potential difference is applied, so electric field is setup at every point in the wire, due to which all the free electrons experience a force qE in a direction opposite to E. As a result of this force and the continuous collision of electrons with atom, the electron acquires a net drift velocity.

DRIFT VELOCITY: - “The mean velocity gained by electrons in an atom when electric field is setup inside it is called drift velocity”.

SOURCES OF CURRENT: -

                    “A device which supplies current by maintaining a potential difference between two terminal points is called source of current”.

                    Every source of current convert non-electrical energy to electrical energy, some sources of current are given below.

1.                     ELECTRIC CELL: - It convert chemical energy; into electrical energy. Dry cell and automobile battery are the best examples of primary and secondary cells.

2.                     ELECTRIC GENERATORS: - It converts mechanical energy into electrical energy.

3.                     THERMOCOUPLES: -They converts heat energy into electrical energy.

4.                     SOLAR CELLS: - They convert sunlight into electrical energy.

EFFECTS OF CURRENT: -

                    When current is passed through certain medium it produces some effects the obvious effects are:

1.                     Heating effect

2.                     Magnetic effect

3.                     Chemical effect

1)                     HEATING EFFECT: - When current is passes through a metallic wire, the electrons do some work against the resistance of the wire. This work appears in the form of heat. The heating effect of current is utilized in electric heater, hair drier, kettles toaster and electric iron.

2)                     MAGNETIC EFFECT: -       Oersted discovered that a current carrying conductor deflects a compass needle when it is placed near it. Thus a current carrying conductor produce a magnetic field around itself and this is called the magnetic effect of current

3)                     CHEMICAL EFFECT: - When current is passed through electrolyte a chemical reaction take place this is called chemical effect of current.

Energy

DEFINATION: - The ability to do work is called Energy. In S.I system the unit of energy is Joule. Joule is defined as, " One Newton is force is applied on a body and the body moves 1 meter distance then the total energy is one Joule".
There are two kinds of Energy that is P.E (Potential energy )and K.E (Kinetic energy). Infact the P.E and K.E are the kinds of Mechanical energy but commenly used that two kinds of energy.
Potential Energy: - The energy which possess due to its position is called Potential energy. Mathematically it can be represented as,

P.E = mgh

Kinetic energy: - The energy which possess due to its motion is called Kinetic energy. Mathematically it can be expressed as,

K.E = 1/2mv2

Kinematics

Introduction: - It is that branch of physics which deals with that bodies which is moving with out refrence of force. Here in this defination of Kinematics doesn't mean that a body is moving with out force because in whole in this world there is nothing to move with force. If anybody think that earth is moving around the sun where is the force????. Here is the simple answer the earth is moving around the sun but there is also a force involved in this question that force is called CENTRIPETAL FORCE. so here we analysis that body is moving but the word force is not using in any example. Another example is
A CAR IS MOVING ON THE ROAD
In this example we also know that car is moving on the road and it cannot be possible that car is moving by itself but actually there is person is sitting on driving seat and they drive the car but you again read the example and you think that car is moving on the road and we donot use the force in this example so it is called Kinematic
So many examples are there in this branch of Physics.
moon is revolving around the sun
motorcycle is moving in death well game etc.etc.etc

Physics

Defination: - it is branch of science which deals with matter, properties of matter, energy and their mutual relationship is called physics.
In this defination three things are necessary which are discussed below.
MATTER: - If a body has weight and occupied space is called matter. In latest technology matter is defined as,
 If anybody is produced in our sence organ then it is called matter.
PROPERTIES OF MATTER: - There are three states or properties of matter which are Solid, Liquid and Gas
ENERGY: - The ability to do work is called Energy
If these three things that is matter, properties, and energy is mutual relationship with each other then its mean we are talking about only Physics.