Amagnetic field of 0.55 g is directed straight down, perpendicular to the plane of a circular coil of wire that is made up of 550 turns and has a radius of 20 cm. 1) if the coil is stretched, in a time of 35 ms, to a radius of 50 cm, calculate the emf induced in the coil during the process.

The emf induced in the coil is given by Faraday-Neumann-Lenz law:

where 
is the variation of magnetic flux through the coil
is the time interval

The magnetic field intensity is


Since the magnetic field strength is constant, the variation of flux through the coil is given by
(1)
where N=550 is the number of turns, while  is the variation of area of the coil. We can re-write (1) as 



The time interval is , therefore the induced emf is

What do you need on