The voltage across the the final voltageacross the

SOL'N: a) The following general form of solution applies to any RC circuit with a

single capacitor:

For time t = 0–, the current source will be off and the capacitor will have

flow through the resistor, resulting in a voltage across the resistor (and

capacitor) of igR.

b) The following general form of solution applies to any current in any RC circuit with a single capacitor:
i(t ≥ 0) = i(t → ∞) + [i(t = 0+) − i(t → ∞)]e−t /RThC
In the present case, this applies to the resistor current:

� iR(t ≥ 0) = iR(t → ∞) + [iR(t = 0+) − iR(t → ∞)]e−t /RThC We have the same Thevenin resistance and time constant as before:

iR(t → ∞) = 30 V /2 kΩ =15 mA
Substituting values, we have the following result:
� iR(t ≥ 0) =15 mA + [0 −15 mA]e−t /1µs=15 mA−15 mA⋅ e−t /1µs

�