If we use a monochromatic light in Young's double slit experiment we have a bright and dark interference pattern. What is nature of interference fringes if the monochromatic light is replaced by the white light?

In case of white light, we get a central bright fringe surrounded by a set of colored fringes. Central spot is zero path difference point, for all wavelengths and hence a bright spot. Any other point in its neighborhood has some geometrical path difference. Value of this path difference is different for different wavelengths. Therefore no point can satisfy the condition of brightness or darkness, simultaneously, for all wavelengths. thus if one color disappears, we get a coloured fringe.

A body revolves in a circular path, it is continually acted upon by the centripetal force. In spite of this, there is no change in kinetic energy of the body. Why is it so?

Centripetal force is always directed along the radius and therefore always perpendicular to displacement 9 along the tangent).Hence no work is done by the centripetal force. Thus kinetic energy done by the body will remain constant.

Why is electric flux so named?Is it possible that a surface placed in an electric field may have zero electric flux? If so under what circumstances?

Flux is a word which used in connection with something flowing across any cross section such as flux of air or flux of water. In case of an electric field, electric lines of force can be imagined to flow across the given surface. Hence the name electric flux. Yes, it is possible that a surface placed in an electric field may have zero value. This is possible only if the electric lines of force are parallel to the surface or in the other words the angle between the electric lines of force normal to the surface is 90 degree.