polarity of the emf changes with time. Note from Fig. . that the emf has its extremum value when θ = 90º or θ = 270º, as the change of flux is greatest at these points.
The direction of the current changes periodically and therefore the current is called alternating current (ac). Since ω = πν , Eq ( . ) can be written as ε = ε sin π ν t ( . ) where ν is the frequency of revolution of the generator’s coil.
Note that Eq. ( . ) and ( . ) give the instantaneous value of the emf and ε varies between + ε and – ε periodically.
We shall learn how to determine the time-averaged value for the alternating voltage and current in the next chapter. FIGURE . AC Generator E XAMPLE . In commercial generators, the mechanical energy required for rotation of the armature is provided by water falling from a height, for example, from dams.
These are called hydro-electric generators . Alternatively, water is heated to produce steam using coal or other sources. The steam at high pressure produces the rotation of the armature. These are called thermal generators .
Instead of coal, if a nuclear fuel is used, we get nuclear power generators . Modern day generators produce electric power as high as MW, i.e., one can light up million W bulbs! In most generators, the coils are held stationary and it is the electromagnets which are rotated. The frequency of rotation is Hz in India.
In certain countries such as USA, it is Hz. Example . Kamla peddles a stationary bicycle the pedals of the bicycle are attached to a turn coil of area . m .
The coil rotates at half a revolution per second and it is placed in a uniform magnetic field of . T perpendicular to the axis of rotation of the coil. What is the maximum voltage generated in the coil? Solution Here f = .
Hz; N = , A = . m and B = . T. Employing