1A 19 A current is flowing in a 20 H inductor A switch is op

1.A 1.9 A current is flowing in a 20 H inductor. A switch is opened, interrupting the current in 1.5 ms
Part A
Find the induced emf in the inductor.
Express your answer using two significant figures.
E= kV

2.Your little sister is building a radio from scratch. Plans call for a 410
H inductor wound on a cardboard tube. She brings you the tube from a toilet-paper roll (12 cm long, 4.0 cm diameter), and asks how many turns she should wind on the full length of the tube.
Part A
Your answer?
Express your answer using two significant
N= turns

3.
Part A
What\'s the current in a 17 mH inductor when the stored energy is 46 J ?
Express your answer using two significant figures.
I=A

Solution

1. Emf = -L*di/dt

di/dt = (0 - 1.9)/(1.5*10^-3)

emf = -20*(-1.9)/(1.5*10^-3)

Emf = 25.33 kV

2.

inductance for solenoid

L = u0*N^2*A/l

N = no. of turns

l = length in meters

A = area

N = sqrt(l*L/u0*A)

Area= pi*d^2/4 = 3.14*0.04^2/4 = 0.001256

N = sqrt(410*10^-6*0.12/(4*3.14*10^-7*0.001256))

N = 176.6

Number of turns = 177

3.

U = 0.5*Li^2

i = sqrt(2U/L)

i = sqrt(2*46*10^-6/(17*10^-3)) = 0.0735 Amp.