What is the RC time constant for this problem?

[mopar969]

See attachment for problem. I am stumped on how to find the rc time constant in this problem. Please help me get started. Thanks.

 

[gneill]

Write the equivalent impedance of the circuit. There will be real (resistive) and imaginary (capacitive) components.

 

[Andrew Mason]

See attachment for problem. I am stumped on how to find the rc time constant in this problem. Please help me get started. Thanks.

You need to replace all the capacitors and resistors with their single equivalents. You then multiply them together to get the time constant.

The resistors are easy. They are in series so just add them. The capacitors are trickier. Can you figure out the equivalent capacitance of those capacitors?

AM

 

[mopar969]

I thought that the resistors are in parallel if not why not? Also the part I am stuck on is determining what capacitors are in parallel or series please help explain.

 

[gneill]

I thought that the resistors are in parallel if not why not? Also the part I am stuck on is determining what capacitors are in parallel or series please help explain.

Both ends of the resistors are not connected together, so they are not in parallel.

Three capacitors are connected in series (end to end). That three capacitor combination is in parallel with the last capacitor.

Start by finding the equivalent capacitance of the three that are in series and replace those three with that equivalent capacitance. Continue to simplify until you're left with one total resistance and one total capacitance.

 

[mopar969]

So, i got a 4000 ohm resistor in series with a 40 uFarad capacitor but now what do I do. Thanks.

 

[gneill]

So, i got a 4000 ohm resistor in series with a 40 uFarad capacitor but now what do I do. Thanks.

What's the time constant value for a circuit with resistance R and capacitance C?

 

[mopar969]

I think the equation I need to use is t=RC is this correct and f so do I need to convert uFarads to farads? Thanks.

 

[gneill]

I think the equation I need to use is t=RC is this correct and f so do I need to convert uFarads to farads? Thanks.

That's the correct expression. And yes, you need to pay attention to the powers of ten.

 

[mopar969]

I used the formula and got 4/25 ohms per farad is this correct and is this the corect units for the rc time constant?

 

[gneill]

The numerical value is fine. The units are Ohm*Farad = [V]/[A] * [Coul]/[V] = [Coul]/[A] = [A][T]/[A] = [T]

So the result is in units of time (seconds).