Need a machine to impart measurable charge (Coulombs) at will

[Albertgauss]

Hi there,

Does anyone know of a machine/equipment/technology, etc. that allows a person to give charges (Coulombs) to conductors in a measurable way? I've rubbed a ton of plastic rods with fur but that's not too measurable. The Van De Graaff is overkill or it if I try to connect alligator clips and wires from the Van De Graaff to where I want the charge to go to, the charge bleeds off really quick. I admit I have not tried a Windhurst machine yet. If you know of something that can scientifically dispense charge (Coulombs), I would definitely appreciate it.

I'm not looking for lethal or dangerous amounts, its just for student projects at college. I'm basically charging up pith balls and hollow, metal spheres a couple inches diameter, and would like to be able to charge such items in a measurable way where the effects of repulsion and attraction are easily seen. The most charge I work with is what a single, lab-demo Van De Graaff can produce so I don't need anymore charge than that. I am certainly not trying to do anything dangerous.

 

[anorlunda]

So, give us numbers please. If you have two 2-inch balls, how much charge and how much voltage do you need to make the repulsion measurable? Then calculate the energy stored in those charges, and compare that with the energy of an impulse that could be dangerous.

 

[CWatters]

What sort of thing are you trying to charge up? Small objects have some small capacitance but it doesn't take much for charge to leak away through mounting supports or through the air from sharp corners.

Not sure you can measure charge directly without discharging it. Think you need to measure it's voltage using something like an Electrostatic Voltmeter. If you know the objects capacitance you can then calculate the charge.

See what others say.

 

[mfb]

Calculate the capacitance of the setup, charge them to a specific voltage?

 

[anorlunda]

I also question your assertion that a Van De Graff generator is not enough. Why can't you do a demo like the one in this video?

 

[Albertgauss]

nc = nano-Coulomb.

First, I apologize about my first post: it was too vague. Here are some more details.

First, some basic numbers. It looks the charges I am interested in are up to 2000 nC with voltages no more than 20 volts. indeed, the power supply available to me is 20 volt max. I also have a Unilab Digital Coulomb meter that reads charge up to 2000 nC that I was able to get working today. I include an attached jpeg of this digital Coulomb meter I use.

The purpose of this post is to try and obtain something like the "Charger" in the Pasco Coulomb Apparatus in the following video:



At around 5-8 seconds into this video, the person charges the spheres with the tip from some "Charge Source Machine". I think this is a "Charge Probe" but Pasco no longer makes it. If you look on the webpage of the Pasco Coulomb Apparatus

https://www.pasco.com/prodCatalog/ES/ES-9070_coulombs-law-apparatus/index.cfm

and scroll down to "Charging Probe" I think this is what they are using in the video to charge the spheres. It was unavailable when I tried to buy it. I was hoping there was something commercially that is really used in industry similar to the unavailable Charge Probe of Pasco. (which I am thinking of as a "Charge Source").

I am actually trying to charge a lot of things and seeing what projects are available. Some of the things I am trying to do:

It appears that if I charge up a plastic rod with fur, I get about -30 to -50 nC on the tip of the plastic rod, but I can't get a consistent charge each time I rub the fur on the plastic rod. Obviously this is due to the fact that I can't rub fur on a plastic rod exactly the same way each time.

Also, I connected 1.5 volts of a power supply across a Pasco EX 5533, 8 cm radius Capacitor with plate separation of 1 centimeter. Air in between. This Capacitor is found on this webpage.

https://www.pasco.com/prodCatalog/EX/EX-5533_capacitance-experiment/index.cfm

With about 1.5 volts across both plates, my digital Coulomb meter displayed 1000 nC for the charge on a plate of the above Capacitor; I'm not sure if that is right yet. Theoretically, for the Pasco Capacitor listed above, I should get a charge of 2.67(10^-11) Coulombs (too small for my digital Coulomb meter to measure anyway) using q = CV with C = epsilon * Area/(plate separation) or q = 8.85(10^-11)*Pi*((8 centimeter) ^2)*1.5 Volts/(1.0 centimeter) = 2.67(10^-11) Coulombs. It would be nice to have a machine that, if I put a micro-Coulomb on my Capacitor, I know there is a micro-Coulomb on my capacitor so that I could know better why my calculation and Coulomb meter disagree.

The VanDeGraaf is a high voltage on the surface, but when I try and use it as a charge factory and move that charge from the Van De Graaf to something like a pith ball or the Pasco capacitor via banana-jacked chords (one end of the chord taped to the dome, the other end taped to the pitch ball or capacitor plate), then I have trouble transferring charge. I know that sharp points on alligator clips and banana cables bleed off charge and I have experienced this. I also really can't say how much charge is on my pith ball or capacitor at the point where alligator connects to the pith ball or capacitor plate. I think the Van De Graaff dome will destroy my Coulomb meter so I haven't tried to measure the actual charge on the Van De Graaf dome. Nothing I calculated about how much charge I "should have" on my pith ball made sense when I tried charging pith balls this way. The pith balls I have are about a centimeter in diameter.

The Van De Graaff generator itself actually has too much voltage (you get shocked when you try and do anything too close to the dome, not dangerous, but annoying) so if I use it as a charge factory, I have to use cables to move the charge far enough from the dome so I don't get shocked (~ tens of centimeters).

Anyway, given my lack of charge control, charge reproducability, and the other difficulties I mention here, you see that it would be nice to find a device that outputs a controllable amount of charge I can easily measure with the equipment I mention here. That is what I am looking for.

 

[dlgoff]

The purpose of this post is to try and obtain something like the "Charger" in the Pasco Coulomb Apparatus ...

The charging probe is basically a multimeter test lead.

The charge is transferred from the power supply that the "charging probe" is connected to. From page 7 of the PASCO ES-9070 User manual (red underline by me):

 

[DrZoidberg]

1000 nC can't be correct. At 1.5V the capacity would have to be 666 nF. In reality however it is around 20pF. Are you sure that all parts of the experiment - including the capacitor and the coulombmeter - were completely discharged before you did your measurement? Also, all plastic objects should be kept at a safe distance since they are often charged and can interfere.

There is a simple way to always transfer the same specific amount of charge. All you need is a metal sphere on an insulating handle and a constant high voltage source. Since the sphere has a constant capacity it will always pick up the same amount if charge if you touch it to your voltage source. You can then touch the sphere to whatever you want to transfer the charge to.

 

[Albertgauss]

Oh, I see. I didn't find that about the multimeter test lead. I realize I should have dug more. Of course, I have been confused so I didn't know as well what I was looking for. This is all making more sense now, though.

I agree that the 1000 nC can't be correct. I believe the 20 pF as that is what I calculate also.

I will go back tomorrow and report here trying out some of these ideas. I thought I pushed the reset button the Coulomb meter before I charged it up. I will look at that again tomorrow. I didn't know about the plastic objects. Yes, they were close by (but not touching) the Coulomb meter. I will see if this makes a difference.

I will also try more work with the spheres. I forgot about the fact Dr. Zoidberg mentioned in paying attention to other things. I can try that out tomorrow also.