How to measure a capacitance of an isolated object?

AI Thread Summary
Measuring the capacitance of an isolated object, such as a conductive sphere or coil, requires a reference point, as capacitance is defined between two surfaces. A typical method involves enclosing the object within a large sphere and measuring the capacitance between the object and the sphere, which approximates the capacitance as the sphere's radius increases. Sporadic readings from an RLC meter when touching one probe to the object indicate that a proper setup is necessary for reliable measurements. Theoretical values suggest that the capacitance of a single object can be approximated, but practical measurement requires careful consideration of the surrounding environment. Accurate measurements converge as the reference sphere's radius approaches infinity.
Artlav
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How can you measure a capacitance of an isolated object, like a conductive sphere or a coil?

Can it be done using a typical RLC meter?
I've tried touching only one end of the probe to the object in question, and that seems to give sporadic readings, but within the order of magnitude of the theoretical expectation.

How to do it reliably, if there is a way?
 
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Artlav said:
How can you measure a capacitance of an isolated object, like a conductive sphere or a coil?

Can it be done using a typical RLC meter?
I've tried touching only one end of the probe to the object in question, and that seems to give sporadic readings, but within the order of magnitude of the theoretical expectation.

How to do it reliably, if there is a way?

By definition, capacitance is between two objects or surfaces. You need to measure the capacitance of your "isolated" object with respect to something.
 
I believe that the capacitance of a single object is measured wrt a sphere of large radius approaching infinite. E.g. the capacitance of the Earth is around 710 uf, IIRC from memory. The large sphere enclosing the object is 1 electrode, with the object being the other. Taking the limit as the radius of the sphere approaches infinite gives the capacitance of the object. I believe Halliday & Resnick physics texts have illustrative examples of this.

Did I help?

Claude
 
hi claude! :smile:

but how do you measure it?
 
tiny-tim said:
hi claude! :smile:

but how do you measure it?

I'm not sure as to how it is measured. Capacitance of a single object is valid theoretically, but I'm not sure about it being a measurable quantity. I would say a good approximation is to enclose said object inside a sphere whose radius is large compared with the object dimensions, then measure capacitance between object & sphere. That should get you in the ball park. Of course as the sphere radius increases, the measurement gets more accurate, eventually converging. Maybe tonight I'll look this subject up. I have several good reference texts. Off the cuff, that would be my approach. Comments welcome.

Claude
 

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