What is choke coil? Write its importance. 

Choke coil is a device used in ac to control; the current without significant loss of electrical energy in the form of heat.

It consists of large number of turns of thick insulated copper coil having small resistance to d.c. circuit. The coil has high inductance so it offers a large reactance X_L, in a.c. circuit. This control the alternating current so,Irms = Erms/XL

But the average power consumed in a pure inductance is zero. Thus choke coil is preferred to a resistance in an a.c. circuit to change the magnitude of current. 

D.C. Motor 

A motor is a machine for converting electrical energy into mechanical energy. It is based on the fact that a current carrying conductor experience a torque in a magnetic field. 

Construction 

A.C motor consists of an armature coil DCBA wound on a laminated iron core. It is placed in the magnetic field produced by current flowing through the field winding. The ends of the coil are connected to a split ring commulator which is in contact with brushes B1 and B2 to which a d.c. voltage is supplied.

working

Suppose a current from a battery flows round the armature coil in the direction DCBA. Assuming that the plane of coil is initially horizontal then, by fleming's left hand rule an upward force acts on DC and an equal downward force will act upon D.C. These force form a couple which rotates the coil in clockwise direction. but the couple become 0 when the coil is vertical, but owing to its monentum , the coil can pass the vertical position. the two half of split ring now make contact with the other two brushes as a result of which, the direction of current in the coil is reversed. hence the part of the coil moving upward in earlier condition will move down in this latter condition and viceversa and consequently the coil continues to move in same direction. each time the current pass the vertical position, the current in the coil reverses and thus the coil rotates continuously. the mechanical energy for driving the coil is obtained from the electrical energy supplied by battery. 

the motion of single coil is jerky so in order to obtain uniform motion, the single coil should be replaced by a number of evenly spaced coils, which are connected to a commulator having corresponding number of segments.

Hazards due to radio isotopes
Use of radioisotopes require careful consideration of their human toxicity. An individual handling radioisotope must not only think about its overdose but also should be conscious about its potential threats in the day to come because its effect may be prevalent over a decade. The hazard due to radioisotope can be summarized into following points. 
1. Genetic distortion: Write about mutation and take the example of Hirosima and Nagasaki
2. Cancer: Write about carcinoma formation
3. Blindness:
Properties of Ultraviolet rays and its application:
Properties
1. They don't carry any charged particle
2. It is electromagnetic wave so that it can never be deflected by electric or magnetic field. 
3. Due to high frequency compared to visible light, it has high energy. 
4. It shows florescence effect. 
5. It can effect the photographic plate
6. Its velocity is 3 x 10⁸m/s
Application 
1. Visible light is produced in CFL lamp due to striking of UV in the fluorescent powder coated inside CFL lamp. 
2. This light is used used in UV-Visible spectroscopy. 
3. In lab for analytical purpose
4. As disinfectant: to destroy microorganism 
5. Vitamin D is synthesized in human body in the presence of U.V light.