Simple ? from a beginner

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As a gift, my grandfather bought me Basic Electrical Theory. I have a question about the very first unit and he suggested this forum for the answer. If I am in the wrong area or this forum is not meant for such basic questions, please just direct me to where I might search for the answer. Thank you in advance for your assistance!

My question: Static Voltage: The voltage developed is related to the amount of charge deposited on a body and to the capacitance of this body with respect to its surroundings. What determines how much charge is deposited? How long the contact is or the capacitance of the body or a combination of the two? I'm assuming a smaller object generally holds less than a larger body, depending on what it is made of though?
 

realolman

Senior Member
WantToUnderstand said:
As a gift, my grandfather bought me Basic Electrical Theory. I have a question about the very first unit and he suggested this forum for the answer. If I am in the wrong area or this forum is not meant for such basic questions, please just direct me to where I might search for the answer. Thank you in advance for your assistance!

My question: Static Voltage: The voltage developed is related to the amount of charge deposited on a body and to the capacitance of this body with respect to its surroundings. What determines how much charge is deposited? How long the contact is or the capacitance of the body or a combination of the two? I'm assuming a smaller object generally holds less than a larger body, depending on what it is made of though?

That's not very basic in my opinion. I saw your post here with no responses. Now you have a response but not an answer. I don't know . sounds like physics to me.
 

wbalsam1

Senior Member
Location
Upper Jay, NY
WantToUnderstand said:
As a gift, my grandfather bought me Basic Electrical Theory. I have a question about the very first unit and he suggested this forum for the answer. If I am in the wrong area or this forum is not meant for such basic questions, please just direct me to where I might search for the answer. Thank you in advance for your assistance!

My question: Static Voltage: The voltage developed is related to the amount of charge deposited on a body and to the capacitance of this body with respect to its surroundings. What determines how much charge is deposited? How long the contact is or the capacitance of the body or a combination of the two? I'm assuming a smaller object generally holds less than a larger body, depending on what it is made of though?

Just trying to hone in on your question a little here. Static electricity is electricity at rest, also known as static charge. Capacitance is the ability to store electricity in an electrostatic field. I'm in the camp that believes it's a function of both dynamics you've mentioned. How long the pressure is applied (IMHO) is related to how the resistance of the body will impede the "flow" of electrons.
 

emahler

Senior Member
if you don't know the answer, you should become a plumber.....:D

actually good question....not sure, but I would side with the size and capacitance (is that a real word?) of the body....assuming there is enough contact with the required surface to fully charge the body...
 

charlie b

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Location
Lockport, IL
Occupation
Semi-Retired Electrical Engineer
WantToUnderstand said:
What determines how much charge is deposited? How long the contact is or the capacitance of the body or a combination of the two?
It is a combination of the two. Charge cannot make its way onto either side of a capacitor, unless there is something pushing it there. The longer the push, the more charge can be pushed. But there is a limit, and it is the capacitance value that establishes the limit.
WantToUnderstand said:
I'm assuming a smaller object generally holds less than a larger body, depending on what it is made of though?
Sort of. Any two metal objects that are not in electrical contact with each other will have a value of capacitance between them. There is capacitance between the pen on your desk and the watch on my wrist. The value of capacitance depends on the area (not volume) of the two objects, the distance between them, and the nature of whatever material (i.e., air, or oil, or mylar, or a vacuum) that is between them. If I two metal plates that are each 1 square foot in area, and if I separate them by one quarter inch of air, I could calculate a value of capacitance. It would not matter if one plate were 1/16 inch thick and the other plate were 5 inches thick; the value of capacitance would be the same.

That should give you a start. But starting here is going to be a bit tricky. Capacitance and inductance are not easy subjects to learn, and it is not often that new students take them on early in their studies. Best of luck to you.
 
First, thank you for not dismissing me out of hand as just a beginner with no business here! Second, thank you for your input and guidance towards understanding.
I would like to share a bit of info I found on this.

TRIBOELECTRIC SERIES
When we rub two different materials together, which becomes positively charged and which becomes negative? Scientists have ranked materials in order of their ability to hold or give up electrons. This ranking is called the triboelectric series. A list of some common materials is shown here. Under ideal conditions, if two materials are rubbed together, the one higher on the list should give up electrons and become positively charged. You can experiment with things on this list for yourself

TRIBOELECTRIC SERIES
your hand
glass
your hair
nylon
wool
fur
silk
paper
cotton
hard rubber
polyester
polyvinylchloride plastic

CONSERVATION OF CHARGE
When we charge something with static electricity, no electrons are made or destroyed. No new protons appear or disappear. Electrons are just moved from one place to another. The net, or total, electric charge stays the same. This is called the principle of conservation of charge.

COULOMB'S LAW
Charged objects create an invisible electric force field around themselves. The strength of this field depends on many things, including the amount of charge, distance involved, and shape of the objects. This can become very complicated. We can simplify things by working with "point sources" of charge. Point sources are charged objects which are much, much smaller than the distance between them.

Charles Coulomb first described electric field strengths in the 1780's. He found that for point charges, the electrical force varies directly with the product of the charges. In other words, the greater the charges, the stronger the field. And the field varies inversely with the square of the distance between the charges. This means that the greater the distance, the weaker the force becomes. This can be written as the formula:

F = k (q1 X q2) / d2

where F is the force, q1 and q2 are the charges, and d is the distance between the charges. k is the proportionality constant, and depends on the material separating the charges.

http://www.sciencemadesimple.com/static.html
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
080511-1204 EST

WantToUnderstand:

Here are some useful references:
http://www.eskimo.com/~billb/emotor/chargdet.html
http://www.fairchildsemi.com/ds/MP/MPF102.pdf
When I built this circuit I used an MPF103 because I some, added a 1000 ohm 1/4 W resistor between pin1 (drain) and +9 V, used 100,000 ohm 1.4 W between "antenna" and the gate, and a reversed biased 1N4148 from gate to - end of 9 V battery.

Positive charges attract negative charges, and opposite charges repel.

In the test circuit above when the gate goes more positive the LED turns on.

If you rub cotton against glass you make the glass positive. Move the glass toward the antenna and this attracts a negative charge to the antenna leaving the gate more positive and the LED brightens. Use pure cotton, not a blend.

Rub cotton and a Dow Chemical Ziploc bag and the Ziploc will be negatively charged. When you approach the antenna the LED goes dark, and when you move away the LED brightens. Same result with a non-conductive integrated circuit shipping tube.

Cotten with what I thought was Mylar (polyester) film (photographic and means it is coated) was the same as glass.

Some non-coated Mylar film behaves like the Ziploc bag.


If you use Google with the string --- static electricity --- the first few responses are good.

http://www.sciencemadesimple.com/static.html --- you referenced this
http://en.wikipedia.org/wiki/Static_electricity
http://www.school-for-champions.com/science/static.htm

The following was written by one of my old professors, A. D. Moore. The course I had from him was mostly graphical field mapping. In a different area of the course a typically question he asked was --- what is the maximum phase shift from an RC circuit? What is your answer?

I have never seen or read the following book so I have no comment on its value.
http://www.amazon.com/Electrostatics-Exploring-Controlling-Static-Electricity/dp/1885540043

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
080511-1532 EST

johnny:

Not a typo, just plain wrong. Brain brought up wrong word.

"Positive charges attract negative charges, and opposite charges repel."
It should read:
Positive charges attract negative charges, and like charges repel.

.
 

wbalsam1

Senior Member
Location
Upper Jay, NY
gar said:
080511-1532 EST

johnny:

Not a typo, just plain wrong. Brain brought up wrong word.

"Positive charges attract negative charges, and opposite charges repel."
It should read:
Positive charges attract negative charges, and like charges repel.

.

Oh, yeah. Now I see the word "opposite" is improperly used .
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
080512-2017 EST

In your post #1 in the second paragraph you brought up the question of time to charge.

This would generally fall under the area of circuit analysis. So consider that you have a fixed capacitor, resistor, switch, and a battery all in series.

Remember that any instant of time that the sum of all voltages around a closed loop is zero. Thus, the equation for this loop can be written as:
Vb = R*dq/dt + q/C
where Vb is the battery voltage, R is a fixed resistance (a constant), and C is a fixed capacitance. q is the instantaneous value of charge on the capacitor, and i is the instantaneous current and equals dq/dt the rate of change of charge.

After solving this differential equation we get:
vc = Vb*(1-e^(-t/RC) ) where vc is the instantaneous capacitor voltage vs time assuming zero charge on the capacitor at the time of the switch closure. e is the base of the natural logarithm.
See http://en.wikipedia.org/wiki/Natural_logarithm

RC is called the time constant of the RC circuit. The voltage rises to approximately Vb*(1-0.368) = 0.632*Vb in one time constant period. To 0.865*Vb in 2 time constants, etc.

Here is a reference that might help. Note 1/e^(t/RC) is the same as e^(-t/RC). http://www.allaboutcircuits.com/vol_1/chpt_16/4.html
Using % in the equations I think adds clutter. This site was the first to pop up under Google with the search string
series resistance capacitance voltage transient curves

This RC charge curve is the basis of a lot of timing circuits. And it is the problem in building fast response circuits.

.
 
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