RCL Circuit - Problem due at 11 PM EST tonight, have been working on it for days

[phys62]

1. Homework Statement
A series RCL circuit contains only a capacitor (C = 8.46 μF), an inductor (L = 6.01 mH), and a generator (peak voltage = 77.4 V, frequency = 2.45 x 103 Hz). When t = 0 s, the instantaneous value of the voltage is zero, and it rises to a maximum one-quarter of a period later. (a) Find the instantaneous value of the voltage across the capacitor/inductor combination when t = 4.32 x 10^-4 s. (b) What is the instantaneous value of the current when t = 4.32 x 10^-4 s?


2. Homework Equations
v(t) = V*sin(2pi*ft)
z= square root(R^2 + (Xl-Xc)^2)
I=V/Z



3. The Attempt at a Solution
the instantaneous voltage in part a = 27.77 V. (this is correct)

my answer to part b is incorrect but I don't know why:

X of L=2pi(2450)(.00601)=92.5 Ohms
X of X=1/(2pi(2450)c) = 7.678 Ohms
Z=square root(0^2 - (92.5 - 7.678)^2) = 84.822
I0=V0/Z = 77.4/84.822 = 0.91249 A
I=I0sin(2pi*ft + (pi/2))
I=(0.91249)sin(2pi(2450)(4.34x10^-4) + (pi/2)) = 0.841 A

 

[Kruum]

Going through your answer very quickly, all I can spot is your phase angle. Since it's almost purely inductive you should have a minus where the plus now is. Hopefully I'm not too late.

 

[phys62]

Oh wow... I can't believe that was it. It was too late for full credit, but now I know that for my test tomorrow. Thanks! :D

 

[Kruum]

To further help you determine the sign of the phase angle. If X_L is greater than X_C then you need to subtract the angle, if X_L is smaller than X_C you need to add the angle.