CURRENT ELECTRICITY

                           

                                CURRENT ELECTRICITY

Electric Current:
       The controlled movement of electrons through a substance is called current. Electric current is nothing but the rate of change of electric charge through a circuit. i.e I = dQ/dt. Unit of current is   Ampere or coulomb/second.One Ampere represents   6.25 × 10¹⁸  electrons passing second.i.e 

                 Ampere = coulomb/second 

               One coulomb = charge of   6.25 × 10¹⁸   electrons

Electromotive Force:

      Electromotive force (e.m.f) is the force that causes a current of electricity to flow.And the potential difference (p.d) between to points in a circuit is the electric pressure  or voltage required to drive the current between them.The volt is a unit of potential difference and electromotive force. It is defined as the difference of potential across a resistance of 1 ohm carrying a current of 1 ampere.

Resistance:

         The property of the electric circuit which opposes the flow of electric current (due to bond between electrons and protons as well as collisions).practical unit of resistance (R) is ohm (Ω).

Laws of resistance

• Length(l): varies directly proportional to length(l).
• Cross-section(A): varies inversely proportional to cross-section area(A)
• Nature of the material(ρ): ρ is known specific resistance or resistivity in ohms-meter (Ω×m)
• Temperature of the conductor:  varies directly with the temperature.  

                                              R᐀ᑭl/A

Conductance:

         Conductance (G) is reciprocal of resistance.  G 󠀾᐀1/R. S.I unit of conductance is Siemen

And conductivity is the reciprocal of resistivity.    σ ᐀1/ρ unit of conductivity is mho/meter

Temperature effects of resistance:

     The resistance of a resistor increases when temperature of the resistor increases.

                    R₂ = R₁ × ( 1 + α(T₂ - T₁) )

R2 is the resistance at temperature T2 in ohms (Ω).
R1 is the resistance at temperature T1 in ohms (Ω).
α is the temperature coefficient.

Ohm's law:

      "For a fixed metal conductor, the temperature and other condition remain constant the current(I) through it is proportional to potential difference (V) between its ends." 

                                                          V=IR

Limitation of ohm's law:

          Ohm'S law does not apply under the following condition -

1. Electrolytes where enormous gases are produced on either electrode.
2. Non-linear resistors like vacuum radio waves, semi-conductors,gas filled tubes etc.  
3. Arc lamp
4. Metal which get heated up due to flow of current  
5. application like metal rectifiers, crystal detectors, etc   

Resistors in series:

According to the fig . 1.1 equivalent resistanc R = R₁ + R₂ + R₃

Also  the sum of all voltage drops equal to the applied voltage : V = V₁ + V₂ + V3

                                                                                                   : IR = IR₁ + IR₂ + IR₃

fig 1.1

Resistance in parallel:

According to fig. 1.2 equivalent resistance is   1/Req = 1/R1 + 1/R2 + 1/R3

                                                                                 I = I₁ + I₂ + I3      

                                                                            V/Req = V/R1 + V/R2 + V/R3

fig. 1.2

Positive temperature co-efficient:

        With increase in temperature if resistance is increases or current in the circuit is decreases then it is said to be positive temperature co-efficient. e.g-conductor 

Negative temperature co-efficient:

     With increase in temperature if resistance is decreases or current in the circuit is increases then it is said to be negative temperature co-efficient. e.g - semi-conductor.