Discussion in 'General Electronics Chat' started by Trang, Jun 21, 2016.

1. ### Trang Thread Starter New Member

Jun 21, 2016
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Last week, I had had the pratical about how to use the Oscilloscope then I had the graph and the formula for Vr and Vc are:

The question of profesor is why the resistor voltage lead the capacitor voltage by 90?
Thanks all guys for help!

2. ### MrChips Moderator

Oct 2, 2009
12,635
3,454
Hmmm, you are missing some crucial information. What resistor? What capacitor?

You need to show a circuit diagram.
What is the value of the resistor and the value of the capacitor?
The phase angle is a function of frequency. What is the frequency?

How much calculus background do you have?
What is the derivative of a cosine function?

3. ### Trang Thread Starter New Member

Jun 21, 2016
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Really sorry for my shortcoming, here is all about my pratical

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
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The simple answer is that capacitor takes time to charge. Resistor is simply passing some current through, as soon as there is current flowing through the resistor, you have voltage across resistor. So voltage across resistor is establishing at the speed of current flowing through it, fast in other words. Capacitor does not work that way. Read the physics of the capacitor, it should be in a textbook that discusses physical composition of capacitor and the mechanics of capacitor operation.

Jun 21, 2016
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6. ### Trang Thread Starter New Member

Jun 21, 2016
5
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And I'm sorry because of posting two same thread.
Because I thought that I shouldn't post this problem in this article which is only for Project so I tried to delete this thread but cannot find this button...

7. ### crutschow Expert

Mar 14, 2008
13,498
3,374
A capacitor requires charge be added to it before its voltage can change, a resistor doesn't.
So when you apply a voltage to the RC series combination, the resistor will have voltage across in instantly, but the capacitor voltage will slowly rise as it becomes charged from the current through the resistor.

The same thinks happens when the voltage drops and the capacitor starts to discharge. You now have to remove charge before the capacitor voltage can drop which means the capacitor voltage again lags the resistor voltage.

8. ### MrChips Moderator

Oct 2, 2009
12,635
3,454
btw, the picture of the scope screen in post #1 does not match your question about 90° phase shift.
What is shown in the post #1 screen shot is 180° phase shift.

What gives?