(a) Draw a schematic sketch of an ac generator describing its basic elements. State briefly its working principle. Show a plot of variation of

(i) Magnetic flux and

(ii) Alternating emf versus time generated by a loop of wire rotating in a magnetic field.

(b) Why is choke coil needed in the use of fluorescent tubes with ac mains? 

(a) The working of AC generator is based on the principle of electromagnetic induction.

Construction:

AC generator mainly consists of:

Armature − The rectangular coil ABCD

Filed Magnets − Two pole pieces of a strong electromagnet

Slip Rings − The ends of the coil ABCD are connected to two hollow metallic rings R₁ and R₂.

Brushes − B₁ and B₂ are two flexible metal plates or carbon rods. They are fixed and are kept in tight contact with R₁ and R₂, respectively.
Working –
The angle θ between field and the normal to the coil changes continuously as the armature coil is rotated in a magnetic field. Therefore, magnetic flux linked with the coil changes and an emf is induced in the coil. According to Fleming’s right hand rule, current is induced from A to B in AB and from C to D in CD. In the external circuit, current flows from B₂ to B₁.

Magnetic flux linked with the coil is given by, 

                      ... (1)

Graph between magnetic flux and time, according to equation (i), is shown below:

As the coil rotates, angle θ changes. Therefore, magnetic flux Φ linked with the coil changes and an emf is induced in the coil. At this instant t, if e is the emf induced in the coil, then

The graph between alternating emf vs. time is shown below:
 

​(b) A choke coil enables us to control the current in an ac circuit. If resistance R is used, then a lot of energy will be wasted in the form of heat.