Saturday, August 11, 2012

WHAT ELECTRIC CURRENT IS....




CONDUCTOR OF ELECTRICITY


           All matter does not conduct electricity well.It is well known that metals conduct well.The fact can be well explained by the theory of free electrons.Metals have valencies 1,2 or 3; that means atoms of metal have 1,2 or 3 electrons at the outermost shell.So metallic atoms have a tendency to detach the outermost electrons from the atom to acquire the electronic structure like the "Noble Gas". Due to heat energy these electron become free from the atom.As a result electrons and positively charged atoms (Cations) are generated.These electrons can move freely within the metallic surface and are called "the FREE ELECTRON"). These electrons give rise to current electricity.

          If we apply potential difference at the ends of a conductor then these free electrons moves from lower potential to the higher potential region.This gives rise to electric current from higher to lower potential.There are two types of electrical flow- 1.DC
                                                 2.AC

DC CURRENT :


         The electric current in which current flows in only one direction irrespective of time ; then the flow is known as DC( Direct Current). Here the potential difference does not vary with time, also the current does not vary.

AC CURRENT :


         The electric flow where the magnitude of current as well as of the potential difference changes sinusoidally with change in time is called the AC (Alternating Current). Instant magnitude of AC Voltage and current can be represent as
       V(t) = Vo Sin (wt)
And E(t) = Eo Sin (wt)

       Here Eo And Vo is called the peak value; w is called angular frequency.





           Time gap, between two consecutive positive or negative peaks, is called "TIME PERIOD". Reciprocal of this time period is called the frequency. The  frequency of a proper AC signal is 50/sec.

EXPLANATION OF AC WAVEFORM

             Famous scientist Michael Faraday said that If there is relative motion between a magnetic field and closed conducting loop then electric current is generated. This happens as magnetic flux density linked with the loop changes continuously with time.

          The following figure how the current is generated due to movement of a magnet into a loop






           If "B" be the magnetic field flux density, "A" be the area of the loop, θ be the angle between the magnetic lines of force and the perpendicular to the loop and "n" be the no. of turns of the loop then magnetic flux linked with the loop is 
                             Φ = nBA Cos(θ)   ................................(1)

       In a dynamo a magnetic shaft rotates within a rectangular coil.Thus the angle θ changes from 0 to 2л

    Then the induced emf linked with the loop is , 
                                    E = - dΦ/dt
                                       =  nBASin(θ) dθ/dt
                                       =  nBAωSinθ 
                                                           Where ω is the angular frequency of rotation                                                             =  Eo Sinθ .............................(2)

 MORE ABOUT AC

       1.AVERAGE VALUE

                           The average value of DC is constant but average value of AC averaged over the full cycle is zero.The average is done as follows :
       <E> =( ∫E(t) dt )/ ∫dt
               =(∫EoSinθdt)/ ∫dt
               =(∫Eo Sin (ωt) dt)/ ∫dt   , the limit of both the int. being  0 to T (time period)
               =0
        Average value of AC emf is non zero over a half cycle.In that case both of the above integration is done with lower limit 0 and upper limit T/2.

       2. RMS VALUE

         The rms value of AC emf is given as 
         Erms =( Eo2Sin2θ dt/ ∫dt) 
                 =( Eo2Sin2 (ωt)dt/ ∫dt)         limit of integrations 0 to T
                  =(Eo) / 2


        *NB>* Thus we can understand why AC is more dangerous than DC. Here we see the peak value of AC emf Eo= √2 Erms. When we speak about 220 Volt AC then 220 Volt is its rms value. Therefore the peak value is 220√2=311 Volt (aprox.). But in DC 220 Volt means its peak value is 220 Volt.                      
    Though 220 Volt DC is not good for us, 220 Volt AC is more harmful.













  

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