Question

a long solenoid of radius 2 cm has 100 turns/cm and carries a current 5 ampere

Answer 1

Its actually easy let PHi be the magentic flux

PHI=B1A2N2

here B1 = MUoN1i

here put N1 as 100 X 100 (because it is 100 turns per /cm )

and i = 5A

now N2=100 and A2 is area of the inside coil

THen PHIvalue in 19.76 X 10-4

E = dPHI/dT here dPHI = 2N2B1A2

therefore i = E/R = 2 X 19.76 X 10^-4 / /20 = 1.976 10^ -4