Class 12 Physics Electromagnetic Induction AC Generator

AC Generator

  • An electrical generator is a device that converts mechanical energy to electrical energy.
  • The generation of alternating currents (ac) is based on the phenomenon of electromagnetic induction.
  • Whenever the magnetic flux changes an emf will be induced in the coil.
  • AC generator is a device that converts mechanical energy into electrical energy(alternating currents).
  • AC generator is an example of inducing an emf or current in a loop by varying the magnetic field and area vector.


  • Current is induced in a loop through a change in its orientation or a change in its effective area.
  • Induced emf is produced either by changing θ or by changing the area vector.
  • Direction of current is given by Fleming’s right hand rule.
  • Direction of the current in the circuit changes by the up and down movement of the loops.


  • It consists of 2 poles(north and south) of a magnet in order to have uniform magnetic field.
  • There is a coil of rectangular shape also known as armature.
  • The armature is connected to 2 slip rings.
  • Slip rings helps in electrical contact with the brushes.It does not change current direction.
  • The slip rings provide a continuous connectionwith the wire around the armature.
  • These slip rings are attached to carbon brushes.
  • The rectangular coil is capable of rotating about an axis which is perpendicular to the magnetic field.
  • The axis of rotation is known as axle.



  • At initial time t=0;θ =0 where θ = angle between magnetic field (B) and area vector.
  • Magnetic flux Φ =BA as cos 0 =1.
  • When the coil rotates with the angular velocity ω;
  • After time t, θ = ωt;
  • Φ(t) =BA cosθ = BA cosωt (because θ is changing as a result magnetic flux is also changing)
  • Therefore change in flux occurs; By Faraday’s Law there will be induced emf.
  • Induced emf e=- N (dΦ/dt); where N = number of turns.
  • = –Nd(BA cosωt)/dt
  • e=NBAωsinωt
  • As sinωt is a periodic function,therefore it varies from -1 to +1.
  • Minimum value of e= - NBAω and maximum value of e=+ NBAω.
  • Therefore e =e0sinωt where e0=NBAω
  • Therefore Induced current will change periodically.
  • As the current is changing periodically that is why it is called as alternating current.
  • Rod is getting rotated by some external means; therefore rotation of rod is mechanical energy.
  • This mechanical energy is getting converted toinduce emf because of which induced current is produced.
  • Therefore because of mechanical energy alternating current is generated.As a result electrical energy is generated.


Types of Generator:-

  1. Hydro-electric generators:Power of flowing water is used to rotate the rectangular coil.
  2. Thermal generators: - When thermal power is used to rotate the rectangular coil.
  3. Power generators

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