Know Current Electricity | Part 1 | Definitions, Formulas, S.I. units...

Current Electricity

Electricity is an important source of energy in our everyday life. Current Electricity is charges in Motion

     The electric current is the rate of flow of charges, flowing across the area of the conductor.

     S.I. unit 'Ampere'.

     When 1 coulomb of charge flow through a conductor in 1 second, the current flowing is said to be 1 ampere.

Ohm's Law 

        As long as physical state (material, dimensions & temperature) of conductor remains the same, the electric current flowing through a given conductor is directly proportional to the potential difference applied across it.

Formula

                            I ∝V

                            I = V 

                                  R

                     or   V=RI

      Where R is constant of proportionality. It represents the opposition to flow of current and is known as 'resistance'.

Formula

                           R=V

                                 I

      The graph of current versus potential difference across conductor is straight line as shown

      Resistance is given by the ratio of applied p.d. across the conductor to the resulting current through it.

Formula

  R = V =                    1                            

          I        slope of I-V characteristics

S.I. unit Ohm (Ω)

1 ohm = 1 volt

              1 ampere

Note:- Reciprocal of resistance is called conductance. S.I. unit (S or Ohm^-1)

Resistance

      Solids conductors contains about 10²² free electrons in 1cc. The atoms are tightly bound are current is carried by negatively charged electrons. 

      When no external potential difference is applied, the electrons move in random direction and collide with atoms. The average number of electrons crossing any section in one direction is equal to average number of electrons crossing that section in opposite direction in given time. Thus there is no net flow of electrons through any section of the conductor. Hence there is no current.

Drift Velocity

     There is continous flow of electrons across the section of the conductor. Atoms of any material are always in state of vibration because of thermal energy. While moving, electrons continuously collide with vibrating atoms and their motion is opposed and electrons move with constant velocity called drift velocity.

Limitations of Ohm's Law 

     Although Ohm's law is obeyed by various materials, there exists some material and devices for which the linearity of relation between V and I is not obeyed.

     The dotted curve represents ideal Ohm's law and the solid line represents actual observations. According to Ohm's law current increases with p.d. But, when current increases temperature of the conductor increases, which inturn increase resistance of conductor and hence slightly decrease in current. Ohm has not considered this temperature dependence in his relation.

This figure shows the nature of I-V curve for semiconductor

      All the ohmic and non-ohmic materials have different applications in electric and electronic circuits. Conductors are used to apply the p.d. to various components and carry the current.