Class 12 Physics Alternating Current Transformers

Transformers

  • A transformer is a device that changes voltage  from one value to another.
  • Power at the input end  is equal to the power at the output end.
  • Only the voltage will increase or decrease.

Alternating Transformer

Principle:-

Transformers work on the principle of Mutual induction.

Mutual Induction :- Suppose there are 2 inductors if some current flows through coil(1) ,there will be change in the current as a result there will be change in the magnetic flux, as a result there will be  change in the magnetic flux in the coil (2) and because of which emf is induced in the coil(2).

Construction:-

A Transformer consists of  :-

  1. Primary coil:-
    1. Primary coil has ‘n’ number of turns of wire over a piece of soft iron core.
    2. It is the input end.
  2. Secondary coil :-
    1. Secondary coil has ‘n’ number of turns of any wire(like copper etc.) .
    2. It is the output end as we receive output from this end.
  3. Soft iron core :-
    1. The hysteresis curve for iron is extremely thin because of which it covers minimum possible area.
    2. As the  area  of  the  hysteresis  loop  of  iron is very less  therefore the energy lost by the transformer will be very less.
    3. Permanent magnet is not suitable to use in transformers because the energy lost will be huge.

Two arrangements for winding of primary and secondary coil in a transformer:

(a) two coils on top of each other, (b) two coils on separate limbs of the core.

Working :-

  • An input voltage(AC source)  is applied across the primary coil. As  a result alternating current is produced in the primary coil.
  • The alternating current will give rise to alternating flux is produced in the coils.
  • Because of change in the magnetic flux emf will be induced .
  • There will be 2 Emfs produced in the circuit. 1. Self –induction 2. Mutual induction.
  • There will be self - induced emf in the primary coil , because of change in the magnetic flux in the primary coil there will be corresponding change in the magnetic flux  associated with the secondary coil which will give rise to induced emf in the secondary coil.
  • Mutual induction takes place in the secondary coil.
  • Induced emf in the primary coil = ep = -Np (df/dt)  
    • Where (df/dt) = rate of change magnetic flux and Np = number of turns in the primary coil.
  • Mutual induction in the secondary coil es = - Ns (df/dt)
    • Where Ns = number of turns in the secondary coil.
  • Assuming  resistance =0 in both primary and secondary coils.
  • Therefore ep = Vp (Voltage across primary coil)  
  • Vp = -Np (df/dt)  (equation(1))  and
  • es = Vs (Voltage across secondary coil) = - Ns (df/dt)  (equation(2))
  • Dividing equation(1) with (2):-
      • (Vp/Vs) = (Np/ Ns)
      • =>  Vs = (Ns/ Np) Vp
  • Power at the input end is same as the power at the output end.
  • Therefore Pintput = Poutput
  • => IpVp =  IsVs

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