Measuring RC Circuit Voltage: Understanding Why CRO is Used

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Homework Help Overview

The discussion revolves around measuring the voltage across a capacitor in an RC circuit using a cathode ray oscilloscope (CRO). Participants are exploring the implications of connecting the CRO across both the resistor and capacitor, particularly in the context of phase differences and circuit impedance.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning how to accurately measure the voltage across the capacitor and whether connecting the CRO across both components provides the correct measurement. There are discussions about the implications of the CRO's impedance and the relationship between the voltages across the capacitor and resistor.

Discussion Status

Some participants are seeking clarification on the measurement process and the role of the CRO in capturing voltage changes over time. There is an acknowledgment of the need to consider the grounding of instruments and the potential effects on measurements, but no consensus has been reached on the best approach.

Contextual Notes

Participants reference lab manuals and instructions that may not have been fully understood, indicating potential gaps in information or assumptions about the measurement setup.

dizi100
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Why the voltage across the capacitor can be measured by connecting a CRO across both the resistor and capacitor ?

actually,i have think of the phase difference and the overall resistance.Still,i can't find the answer.May I get some hints from you?
 
Last edited:
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How else would one propose to measure the voltage across a RC cirucit as a function of time, especially for short time constants?

Using a CRO (oscilloscope) is pretty standard. Of course, the impedance of the CRO needs to be high enough so as not to effect the measurement.
 
Astronuc said:
How else would one propose to measure the voltage across a RC cirucit as a function of time, especially for short time constants?

Using a CRO (oscilloscope) is pretty standard. Of course, the impedance of the CRO needs to be high enough so as not to effect the measurement.

For ac voltage,voltage of the power supply equals voltage across both capacitor and resistor.
If i connect a CRO across both capacitor and resistor,i in fact measure the voltage of the power supply,right?
According to my lab manual,it states that i can mesure the voltage across the capacitor by connecting a CRO across the capacitor and resistor.
i really don't understand...

http://upload.u-state.com/images/1194185727.bmp
 
Last edited:
dizi100 said:
For ac voltage,voltage of the power supply equals voltage across both capacitor and resistor.
If i connect a CRO across both capacitor and resistor,i in fact measure the voltage of the power supply,right?
According to my lab manual,it states that i can mesure the voltage across the capacitor by connecting a CRO across the capacitor and resistor.
i really don't understand...

http://upload.u-state.com/images/1194185727.bmp
The reason has to do with the grounds on your instruments, and I suspect you haven't read the entire instructions that accompany your lab. The voltage across the cap is found as the difference between that across both R-C, which you mention, and that across R, which is found on the other channel of the CRO. You can't measure Vc directly because your CRO and source both have grounds. Once you connect the source as shown, the CRO ground must go to the same place, that is, you cannot put gnd and probe across the cap anymore.
 

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