# Experiment to show that current leads voltage by 90° in capacitor (Using RC circuit & Oscilloscope)

#### Edward Yuen

Aug 10, 2020
2
Current leads voltage by 90 degrees in capacitor.
How to show that it is true by simple experiment?

Last edited:

#### Martaine2005

May 12, 2015
4,539
Fascinating.
Now tell us why voltage leads current in an inductor.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,091
How to show that it is true by simple experiment?
Measure both using a 2 channel oscilloscope, see the traces being displaced by 90 °.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,091
Fascinating.
Now tell us why voltage leads current in an inductor.
Not really an answer to the question, is it?

#### Martaine2005

May 12, 2015
4,539
Not really an answer to the question, is it?
The OP didn’t ask a question, despite having a question mark at the end. He had a long YT video linked in his message with oscilloscope examples and lots of math formulas.

Edit: I believe this was the video
Martin

Last edited:

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,091
The OP didn’t ask a question
I'm sorry to disagree but this is imho a valid question:
How to show that it is true by simple experiment?
The video is no longer visible and wasn't when I first read the post. I had to look up the history

@Edward Yuen : See what confusion editing your post and removing content may cause?

#### Martaine2005

May 12, 2015
4,539
I edited my message with a link to the video.

Martin

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,091
I edited my message with a link to the video.
Thanks, that's what led me to look for previous content in the first post.

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,792
The video that was posted on YouTube by @Edward Yuen seemed somewhat self-serving to me, plus I didn't really see how he actually proved the ninety-degree voltage vs current relationship for sinusoidal wave forms applied to a capacitor. This was "experimentally verified" with only three discrete frequencies in a video that lasted ten and a half minutes. More than anything else the video seemed to be a showcase for a digital storage oscilloscope and a signal generator. Maybe if I put on a set of head-phones and played the video again with sound it would make more sense. However, I trust the math and the theory of how capacitors and inductors work and use actual components when I want to build something.

BTW, the classic way of demonstrating the current vs voltage relationship in capacitors, as well as inductors, is with a Lissajous pattern on an X-Y oscilloscope display.

#### Wollowstone

Mar 26, 2018
18
You can try the (excellent, imvho) video from Texas Instruments on their site: https://training.ti.com/ti-precisio...y?context=1139747-1139745-14685-1138800-13124

It is clear that on the time domain (generally what is available on a 2 channels scope), the "time delay" is not ALWAYS at 90 degree for each and every frequencies. At 0 Hz, the cap acts like an open circuit and that very high frequency, the same cap acts like a short, like a piece of wire. The time delay (and so, the "angular shift") seen on the scope is thus dependent of the frequency. And it is only for a very specific frequency that your circuit will show exactly at 90 degree "shift" (or, again, time delay) and that frequency will be dependent of the arrangement, sure, but of the resistance R and the capacitance C (assuming a single occurrence of each). The mentioned video could greatly help you to make the proper setting for the experimental curves showing what you wish to present, on the time domain of the scope.

#### Edward Yuen

Aug 10, 2020
2
(1) I also used 1.2 k ohm, 1 micro farad capacitor and 4Vpp 100 Hz Sine Wave. The result is 89.28 ° So, you can use other resistor and capacitor. (2) My scope will jump from 148 micro second to 152 micro second (instead of 149 micro second) when I measured the phase difference using cursor of scope. So, there is experimental error due to my accuracy of my scope. (3) If the accuracy of your scope is better than the accuracy of my scope, the angle may be 89.99 ° or 90 ° . For example, if each jump of the cursor of your scope is 1µs ( Δ x/ Δ t). (Each jump of my scope is 4µs)
Here is my experiment:

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