Does this even make sense? Measuring voltage of a capacitor

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Homework Help Overview

This discussion revolves around a lab problem exploring the properties of a capacitor, specifically measuring the voltage across a charged capacitor using a voltmeter. The original poster expresses confusion about the relationship between capacitance and voltage measurement, questioning the steadiness of the voltage reading and the implications of connecting a voltmeter to a capacitor.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the nature of voltage readings from a capacitor and whether it is appropriate to measure voltage with a voltmeter. Questions arise about the behavior of the voltage over time and the differences between ideal and real voltmeters. The original poster also questions the theoretical background necessary for understanding the measurements being taken.

Discussion Status

Participants are actively engaging with the original poster's questions, providing insights into the behavior of voltmeters and capacitors. Some suggest that the internal resistance of the voltmeter affects the readings, while others emphasize the importance of understanding the underlying principles of measurement. There is a mix of interpretations regarding the expected behavior of the voltage over time.

Contextual Notes

Participants note that the lab exercises may be ahead of the theoretical concepts covered in class, which could contribute to the confusion experienced by the original poster. There is also mention of the lab manager's leniency in grading, which may influence the participants' approach to the assignment.

  • #31
flyingpig said:
But does that voltmeter have this thing called "MOSFET"? It doesn't feel natural to include the word "MOSFET" in my answer.

I doubt it. But that only shows the panel meter. What is the make and model number of the voltmeter? What are its specifications? Especially its input resistance in voltage measurement mode...

We had a discussion about input impedance of voltmeters recently somewhere here (in EE?), and this wikipedia page ended up being helpful. See if it helps you (sorry if you've already seen it or been pointed to it):

http://en.wikipedia.org/wiki/Multimeter

.
 
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  • #32
Yeah I know what that is, but we didn't use it.

But how should I answer the original question?

Is the voltage reading steady? Why might you expect this voltage to vary?

It isn't steady because the resistance on the bench voltmeter is low
 
  • #33
flyingpig said:
Yeah I know what that is, but we didn't use it.

But how should I answer the original question?



It isn't steady because the resistance on the bench voltmeter is low

Correct. Although I think I would call the resistance "non infinite" or "finite" or something, rather than "low". You could use some of the description from the wikipedia page about input resistance (with attribution to wikipedia.org, of course) in your report.
 
  • #34
But I still don't understand what is this MOSET thing. How does this let the current from the capacitor to be "read"?
 
  • #36
But I thought voltage doesn't "move"
 
  • #37
flyingpig said:
But I thought voltage doesn't "move"

Voltage does not move per se. I'm not sure I'm understanding your perspective here.

The external large capacitor has a charge on it, related to the capacitance and the voltage. When you connect the voltmeter's initially uncharged leads to the capacitor, a small current flows in the leads to charge up the input capacitance of the MOSFET preamp in the FET VOM. That puts (almost) the same voltage on the VOM input as was on the capacitor to start with. Very little current flows for the rest of the measurement (only MOSFET gate leakage current). When you disconnect the VOM leads from the external capacitor, they are initially charged up still to that same voltage, but the voltage droops to zero fairly quickly (based on the leakage current and input capacitance).
 
  • #38
Can you not use acronyms...? I hardly know what those technical words mean.
 
  • #39
flyingpig said:
Can you not use acronyms...? I hardly know what those technical words mean.

So you didn't actually read the wikipedia page that I linked for you a few pages back then...
 
  • #40
No I know what a multimeter is, we have one of those.
 
  • #41
flyingpig said:
No I know what a multimeter is, we have one of those.

So what acronym is not defined for you?
 
  • #42
Mosfet, vom, fet
 
  • #43
flyingpig said:
Mosfet, vom, fet

Re-read the wikipedia article please. Thank you.
 

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