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etotheix
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Homework Statement
Homework Equations
For a capacitor: i=C(de/dt)
The Attempt at a Solution
Using Kirchhoff's current law at the note above the resistor R2 I get the following equation, which I believe is right:
C(e0' - ei') + (e0 - ei)/R1 + e0/R2 = 0
Then, since the source has a constant voltage of A, I set ei' = 0 and ei = A and use Laplace to find the eo(t)
e0' + e0( 1/(C*R1) + 1/(C*R2) ) = A/(C*R1)
to facilitate the algebra:
1/(C*R1) + 1/(C*R2) = ( R1 + R2 )/( C*R1*R2 ) = ε1
A/( C*R1 ) = ε2
Then,
sE0 - E0(0) + (ε1)E0 = (ε2)/s
=> E0(0) = 0
E0( s + (ε1) ) = (ε2)/s
using partial fractions:
E0 = (ε2)/( s( s + (ε1) ) ) = A/s + B/( s+(ε1) )
A = ε2/ε1 and B = -ε2/ε1
ε2/ε1 = ( A*R2 )/( R1 + R2 )
E0 = (ε2/ε1)/s - (ε2/ε1)/( s + ε1 )
e0 = (ε2/ε1) - (ε2/ε1)e^(-ε1*t)
e0 = ( A )/( R1 + R2 )*( R2 - R2*ε1*t )
But I know that the answer to this problem is:
e0 = ( A )/( R1 + R2 )*( R2 - R1*ε1*t )
What am I doing wrong? Thanks in advance for any help!