Electricity Part - 2

 

                                            ELECTRICITY PART-2

• Resistance : When electricity is flowing through a conductor crores of electron are flowing through that conductor. Those electrons collide with each other. As a result the electrons get obstacles to flow. This obstacle is called the resistance of that conductor. 

The more the electrons are on the face of collision, the more the resistance will be created and the more the heat will be produced in the conductor. So, the production of heat in a conductor carrying electricity is directly proportional to the resistance of the conductor.

• Ohm's law : If temperature, material and other physical condition remain unchanged, the electric current will be directly proportional with the potential difference of a conductor. 

If 'V' is the potential difference of a certain conductor and 'I' is the electric current then according to Ohm's law It can be written that 'V' varies as 'I' 

Or, V=RI  ,  where R=Constant and it is called the resistance of the conductor.

• Resistance from Ohm's Law : According to Ohm's law V=RI 

OR, R=V/I.

So, the resistance of a conductor is the ratio between potential difference and electric current of that conductor.

• Specific resistance : Resistance of a conductor is directly proportional to the length of the conductor. So, R varies as l, where l is the length of the conductor.

The resistance of a conductor is inversely proportional to the area of cross section of the conductor. So, R varies as 1/A   ,   Where A is the area of cross section of the conductor.

Resistance is dependent on the material of a substance. The resistance of silver is lower than all other metals. 

So, it can be written that R varies as l x 1/A 

Or, R= Specific resistance x l/A  ,  Where specific resistance is constant for a certain material. 

The resistance between two surfaces of an unit cube made of a certain material, is called the specific resistance of that material. In SI system the unit of resistance and the unit of specific resistance are respectively Ohm and Ohmmeter.

Ohmmeter : The resistance between two surfaces of an unit cube of a certain material with side of one meter, is called one Ohm meter.

What is the meaning of the statement - the specific resistance or resistivity of copper is 1.72 x 10^-8 Ohm meter ? 

The answer is : The resistance between two surfaces of an unit cube made of copper with side one meter, is 1.72 x 10^-8 Ohm.

• Combination of resistances and equivalent resistance of a circuit : When more than one resistance is used in an electric circuit, those resistances in all together is called the combination of resistances.

The combination of resistances is of two kinds. Such as - (i) series combination of resistances , (ii) pararel combination of resistances.

when the last  end of the first resistance of some resistances in a circuit is connected with the first end of the second resistance, the last end of the second resistance with the first end of the third resistance and so on, the combination of the resistances is called the series combination. The picture of series combination is given below.

 

When the first ends of some resistances in a circuit is connected in one point and the other ends in the another second point then the combination of resistances is called parallel combination. The picture of parallel combination is given below.

 

If one resistance is connected instead of a combination of resistances in a circuit and the amount of electric current remains the same as before then that one resistance is called the equivalent resistance of the combination of resistances. The circuit is given below to show the equivalent resistance.

Equivalent resistance in series combination : 

Suppose R1, R2 ohm are three resistances in a series combination in a circuit, 'R' ohm is their equivalent resistance and I Ampere electric current is flowing in the circuit. So, it can be written that R=R1 + R2

Equivalent resistance in parallel combination : 

Suppose R1,R2 ohm are three resistances in a parallel combination in a circuit, 'R' ohm is their equivalent resistance and I Ampere electric current is flowing in the circuit. So, it can be written that 1/R=1/R1 + 1/R1. 

• Heating effect of electric current flowing and Joule's law : When a certain amount of electric current is flowing through a resistance for a particular time, heat is produced in the resistance. This is called the heating effect of the electric current flowing.

The amount of heat can be calculated by Joule's law. 

Joules's Law : Let us suppose the amount of heat is 'H', the value of resistance is 'R', the amount of electric current is 'I' and the time is 't' .

The first law : H varies as I^2, where R and t remain constant. 

The second law : H varies as R, where I and t remain constant.

The third Law : H varies as t, where Ra nd I remain constant.

Combination of Joule's law : H=I^2Rt/J,  where the constant J is called Joule equivalent. 

The value of J in SI system is 1, So, the equation is H=I^2Rt .

The value of J in C.G.S system is 4.2 Joule/cal or 4.2 x 10^7 erg/cal, So, the equation is H=I^2Rt/4.2