Physics · Electromagnetic Induction · NEET
No. It only converts mechanical energy (the work done to spin the coil) into electrical energy. By Lenz's law, the induced current opposes the rotation, so you must keep pushing the coil against this opposing force. That work you do is what turns into electrical energy — energy is conserved, never created.
As the coil rotates, one face of the coil first moves toward the field lines, then away from them. When it moves the other way, the flux change reverses sign, so the induced EMF reverses direction. Every half rotation the polarity flips, so the current alternates. The EMF follows ε = ε0 sin(ωt), which naturally goes positive then negative.
An AC generator uses two slip rings — each end of the coil stays connected to its own ring, so the output keeps swapping direction and stays AC. A DC generator uses a split-ring commutator that swaps the connections every half turn, flipping the reversed half back so the output stays one direction (DC). Same coil, different connector.
EMF is largest when the coil's plane is parallel to the field (coil moving fastest across the lines, flux changing fastest). EMF is zero when the coil's plane is perpendicular to the field (flux is maximum but momentarily not changing). So maximum flux gives zero EMF, because EMF depends on the rate of change of flux, not on flux itself.
The EMF appears in the rotating coil (called the armature), because that is where the flux is changing. The field magnet just supplies the steady magnetic field. The coil is turned by an external prime mover (turbine, engine, or a hand crank in the lab), and slip rings carry the induced EMF out to the circuit.
The variation of EMF with time for four types of generators are shown as waveforms (a), (b), (c) and (d). Each waveform is periodic and crosses the time axis, reversing sign. Which amongst them can be called AC?
An EMF is generated by an AC generator having a 100-turn coil of loop area 1 m². The coil rotates at one revolution per second in a uniform magnetic field of 0.05 T perpendicular to the axis of rotation. The maximum value of EMF is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It works on electromagnetic induction (Faraday's law): a coil rotating in a magnetic field experiences continuously changing flux, which induces an alternating EMF.
Four main parts: (1) an armature (the rotating coil where EMF is induced), (2) field magnets that provide the steady magnetic field, (3) slip rings that rotate with the coil, and (4) brushes that touch the slip rings and carry the current to the external circuit.
Slip rings keep each end of the coil connected to the same output terminal at all times. This lets the reversing EMF of the coil pass out unchanged as AC. (A DC generator instead uses a split-ring commutator to flip the output into one direction.)
By Lenz's law the induced current opposes the rotation, creating a retarding torque. You must do mechanical work against this torque to keep spinning the coil, and that work becomes the electrical energy delivered. Nothing is created — mechanical energy is simply converted to electrical energy.
An external prime mover — such as a steam or water turbine, a diesel engine, or a hand crank in the lab. In power stations, heat or falling water spins the turbine, which spins the generator coil.
