Resposta :
VC = V. ( 1 - e^-(t / RC) )
VC = 20. ( 1 - e^-(t / 100.15.10³.10^-6) )
VC = 20. ( 1 - e^-(2/3t ) )
quando Vc for igual a 15 v
15 = 20. ( 1 - e^-(2/3t ) )
15/20 =( 1 - e^-(2/3t ) )
0,75=( 1 - e^-(2/3t ) )
e^-(2/3t )=1-0,75
e^-(2/3t )=0,25 aplicando ln
ln.e^-(2/3t )=ln.0,25
-2/3t.lne=-1,386 lne=1
t=-1,386.-3/2
t=2,079 s
b)
VC = 20. ( 1 - e^-(2/3t ) )
VC = 20. ( 1 - e^-((2/3).3,5 ) )
VC = 20. ( 1 - 0,097 )
VC = 20. 0,903
VC = 18,06 V