
Answer:
A) 11.1 ms
B) 5.62 Ω
Explanation:
L ( inductance ) = 10 mH
Vcc = 14V
A) determine the required on time of the switch such that the peak energy stored in the inductor is 1.2J
first calculate for the current  ( i )  using the equation for energy stored in an inductor hence
i = [tex]\sqrt{\frac{2W}{L} }[/tex] Â ----- ( 1 )
where : W = 1.2j , Â L = 10 mH
Input values into equation 1 Â
i = 15.49 A
Now determine the time required  with expression below
i( t ) = 15.49 A
L = 10 mH, Vcc = 14
hence the time required ( T-on ) = 11.1 ms
attached below is detailed solution
B) select the value of R such that switching cycle can be repeated every 20 ms
using the expression below
Ï„ = [tex]\frac{L}{R}[/tex] Â ---- ( 2 )
but first we will determine the value of Ï„
Ï„ = t-off / 5 time constants
 = (20 - 11.1 ) / 5  = 1.78 ms
Back to equation 2
R = L / Ï„
 = (10 * 10^-3) / (1.78 * 10^-3)
 = 5.62 Ω