# Basic(?) Electroscope/Electrometer Question

• jds10011
In summary: I suppose it's not too bad to try the simple experiment first.In summary, electroscopes can be used to measure the deflection angle of a charged rod, but it is difficult to calibrate them for charge measurements. However, by connecting the electroscope to a HV source, the calibration process can be done.
jds10011
Hi folks; apologies in advance if this is covered elsewhere or posted in the wrong place.

I am a high school physics teacher, and I've been tasked with designing some new labs. I'm apparently confused by something rather simple here, so try not to be too hard on me. We have electroscopes that have angle markings for the deflection, 0 through 90 degrees (yes, I'm aware, this technically makes them electrometers, but no one seems to call them that). Of course, we can use this to make basic comparisons about the amount of charge various objects have, and also naturally use them to give students instructions (e.g. transfer enough charge that the deflection is about 45 degrees). However, every manufacturer of these electroscopes claims they can be used for quantitative measurements of charge amounts and relate to calculations of the coulomb force. I tried a rather naive experiment on my own time of using a single charged rod and seeing what distance from the electroscope produced what deflection angle. While of course I saw that further away was less deflection, I couldn't readily discern any sort of inverse-square-like pattern; in fact, it rather clearly was not.

Is there a standard experiment that anyone is aware of that utilizes gathering the deflection angle data, even if for fairly basic or "hand-wavy" use? I'd like ideally to have them learn something quantitative that isn't too far over their heads. This is a standard introductory physics course (not honors or AP, which also exist in the building), so the math can't be too tedious (though I'd consider giving them a derivation and a formula to use if it were something plausible for students to follow at this level). If there is a way to salvage some sort of sense out of the type of procedure I outlined above (hold a charged rod at various distances, look at angle measurements), that would be the ideal sort of experiment I'd like to see them get some sort of intuition out of. Any thoughts anyone has would be much appreciated.

Below are images of the sorts of electroscopes we have, though the foil type we have are particularly terrible, and the vane/bar/needle/whatever-you-call-it type are quite good, so I'd like to use those for this. I am willing to sacrifice one to mass the particular components, if that data would be needed, though I'd prefer to save an experiment of that type for the AP courses (I presently do one with holding up charged pieces of tape, measuring the angle between them, massing the tape, and eventually finding an estimation of the number of extra/missing electrons on each piece of tape).

P.S. I'd dearly love to avoid any pith-ball experiments, as the kids are amazingly capable of breaking them instantly in my experience.

I've never tried to use electroscopes like these for actual charge measurements. Instead, as part of our college-level intro physics electrostatics lab we hang two equal-mass balloons on long strings from a single point on the ceiling, then charge them up (approximately) equally by rubbing them with fur (or a sweater), which makes them repel each other. Measure the angle of deflection, the balloon mass, and the string length, and you can calculate the product of the two charges with the help of a free-body diagram. Then if you assume the two charges are actually equal, just take the square root.

To calibrate such electroscope for charge measurements is rather difficult.
But connecting it to a HV source you can calibrate for voltage measurement.

zoki85 said:
To calibrate such electroscope for charge measurements is rather difficult.
But connecting it to a HV source you can calibrate for voltage measurement.

Hmm... I'm curious if you can clarify what the difficulties are? Thanks for your reply.

jtbell said:
I've never tried to use electroscopes like these for actual charge measurements. Instead, as part of our college-level intro physics electrostatics lab we hang two equal-mass balloons on long strings from a single point on the ceiling, then charge them up (approximately) equally by rubbing them with fur (or a sweater), which makes them repel each other. Measure the angle of deflection, the balloon mass, and the string length, and you can calculate the product of the two charges with the help of a free-body diagram. Then if you assume the two charges are actually equal, just take the square root.

This is similar to the scotch tape procedure I was mentioning with the AP courses. I'm not necessarily interested in making actual measurements of the charges, I'm more interested in getting them to see a general pattern, such as an inverse-square relationship between force and distance, and that this is true regardless of the sign of the charges. Something more quantitative would be great, but not necessary for a basic activity for students at this level. Reinforcing some general concepts and trying to build some basic intuition is a big step for them.

jds10011 said:
Hmm... I'm curious if you can clarify what the difficulties are? Thanks for your reply.
Problems with small capacity and capacitive couplings are sufficient to cause troubles.

Here is an experiment that could work. It's 38 minutes into the video.

## 1. What is an electroscope/electrometer?

An electroscope/electrometer is a scientific instrument used to detect and measure electric charge. It consists of a metal rod or needle suspended by a conducting wire, which can be easily affected by the presence of electric charge.

## 2. How does an electroscope/electrometer work?

An electroscope/electrometer works by using the principle of charge induction. When a charged object is brought near the metal rod or needle, the electrons in the conductor are repelled, causing the rod/needle to move. The amount of movement can be measured to determine the strength and polarity of the electric charge.

## 3. What is the difference between an electroscope and an electrometer?

The main difference between an electroscope and an electrometer is their sensitivity. An electroscope can only detect the presence of electric charge, while an electrometer can measure the exact amount of charge present.

## 4. What are some common uses of an electroscope/electrometer?

An electroscope/electrometer is commonly used in experiments and demonstrations to detect and measure electric charge. It is also used in various industries, such as in the testing of electronic devices and in atmospheric research to measure the levels of ions in the air.

## 5. Can an electroscope/electrometer be used to measure other forms of energy?

No, an electroscope/electrometer can only detect and measure electric charge. Other forms of energy, such as heat, light, and sound, cannot be measured with an electroscope/electrometer.

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