# Significance of RC time constant

1. Oct 8, 2015

### jaydnul

In a simple series RC circuit, when t=RC, the voltage across the resistor and capacitor is the same. Why is this physically significant and why do we choose this as the time constant that dictates the cutoff frequency?

2. Oct 8, 2015

### nsaspook

Review this first. We need some way to calculate the energy (as physical charge) in the capacitor over time into voltage or current values used in circuits.

3. Oct 8, 2015

### jaydnul

Yes, but why is RC used when calculating cutoff voltage? If RC is arbitrary, then cutoff voltage is arbitrary which doesn't make sense to me.

4. Oct 8, 2015

### Staff: Mentor

I think you mean "cutoff frequency" like you posted in your Original Post, not some cutoff voltage...

And even "cutoff frequency" is a bit harsh to use for a simple single stage RC filter. A better term would be the -3dB frequency. Can you say why that would be a better term to use?

5. Oct 8, 2015

### jaydnul

Ya sorry, cutoff frequency.

Is the -3dB arbitrary?

6. Oct 8, 2015

### Staff: Mentor

Well, you could pick -10dB or -20dB instead, so sure, it's semi-arbitrary.

But "RC" is simpler than "some number * RC", and RC gives you -3dB.

Your question is reasonable though. There are some things in physics and EE that are fundamental, and others that we use for convenience. It's fine to ask about the distinctions as you learn more and more in your education. This is a good place to ask such questions.

7. Oct 8, 2015

### nsaspook

The time constant in the RC filter is useful for looking at ...
Think about the electrical properties of an RC network and what -3dB means.

8. Oct 8, 2015

Ok, thanks!