Basic questions on Static Charge

[vikramc98406]

Basic questions on Static Charge :

How does static charge accumulation builds huge potential?

What happens to a capicator if more charge is accumulated on it?

 

[ramiabr]

hi,
once you study this you will be able to answer it !! :!:

All the materials are made up of atoms , i.e all the things in earth are made up of atoms , they r the basic building blocks for any element , first accept this fact !

lets c how static electricity is formed , when you rub two metals or like basic physics experiments , like comb in hair , walking in carpet , disturbs lot of electrons in both the materials , so one loses electrons and other gets some electrons !
so both the materials are unbalanced ! so this is how static electricity is produced !! since one material is at different level( lost electrons) , while other got electrons(remains in different level), this is how +ve and -ve polarities are formed !!

in case of capacitors when you charge the capacitors the materials inside the capacitors are such that they ll be excited and go to different potentials ! they remain till you short circut both terminals and bring the potentials at both terminals equally !!

for a given value of capacitor you can store a max specified voltage given by manafacturer anything more than that can damage the capacitor !!
eg a 0.01uf capacitor max of some xvolts you can store !! if you give more than xvolts then you damage the capacitor ..

last one : always remember that static charges are momentary one`s with very high potentials some 50,000 volts ( you can generate with your hands) !! but it ll last for some micro sec 10^-6sec !!!

 

[vikramc98406]

you are right.

When humans touch some sensitive devices (who are have static charge), they discharge giving away huge voltages.

When a capacitor cannot hold voltage more than it can, how come any metallic/human body able to store charge (assuming no discharge path available).

 

[ramiabr]

good quesion ,

capacitor can be thought as a analogous to a water tank , it can store only upto the capacity !! anything more than the capacity can damage the tank !! thats wy storing an ammount of charge depends on the capacitance value !!

if you look deeper into the capacitor it will have two parallel plates of finite length ,
since its of finite area it will have only finite ions in it !! so we can store a finite charge !! typically think the parallel plates are of length less than 0.1cm, thats wy when see the capacitor present in your ceeling fan it will be big !! because needs to store lot of charge !!

but whereas in case of human body , we sense a touch , smell ,etc all these are done by neurons , i.e charge flows throught our body !! so all the parts of the body has got ions like na+ , etc etc , human body can be thought to have lots and lots of ions thats why we are able to produce large potential !!

always remember one thing we can make large potentials but the current strength is very very small !! thats wy we always mention static charge in volts , we never mention it in amps !!

i will give you a better example !!!!
a train operates at 25,000v , an engine driver staying near it and driving it will carry lot of potential , if you keep a multimeter in his hand it will be large !!!! because humans have large no of ions in the body !! the capacitor can be considered as a very very tiny thing before humans !!!!

 

[Kral]

vikramc98406,
A fundamental relationship among charge voltage and capacitance is:
Q = CV, where Q = Charge, C = Capacitance, V = voltage.
Or
V = C/V
So, if the charge is increased, the voltage will increase. An interesting consequence of this is that if you supply a charge to a parallel plate capacitor, and then move the plates farther apart, the voltage will increase, in proportion to the reduced capacitance caused by moving the plates. The energy stored in the capacitor is equal to 1/2CV^2. By decreasing the capacitance, say, by a factor of 2, the voltage will double. This means that the energy stored in the capacitor will double. Where does this energy come from? It takes force to move the capacitor plates apart, so the extra energy comes from ΔE = FD, where f is the force required to move the plates, D is the distance that the plates were moved.
Regards,
Kral

 

[leotim]

if you look deeper into the capacitor it will have two parallel plates of finite length ,
since its of finite area it will have only finite ions in it !! so we can store a finite charge !! typically think the parallel plates are of length less than 0.1cm, thats wy when see the capacitor present in your ceeling fan it will be big !! because needs to store lot of charge !!

The reason of limited voltage in capacitors is not limitation of electrons count in its plate. A 1000µF, 100V capacitors may weight more than 10 grams and each of its plates weight more than 1 grams. Each 1 grams of Aluminium plate, has about 1e21 atoms of Aluminium and if all of its atoms are inoized, it will produce:
1.6e-19*1e21=160 Coulombs of charge. it will produce 160kV on 1000µF capacitors. and it is too higher than 100V.
Realy the limitation on Voltage is strength of isolation between capacitors plates. when the voltage in a capacitor is increased, the electric field between the plates increases too and if this field exceeds the strength of capacitor insolation, it will break down the insolation and the capacitor will be discharged inside itself and the insulation may be damaged.

but whereas in case of human body , we sense a touch , smell ,etc all these are done by neurons , i.e charge flows throught our body !! so all the parts of the body has got ions like na+ , etc etc , human body can be thought to have lots and lots of ions thats why we are able to produce large potential !!

The static charges that is produced in human body, is not due to internal ions such as Na+. It is not a chemical ionization. It is a physical ionazation. Generally, the static charge is not produced in human body it self, but it is produced in clothes or hair and human body just transfers the charges. Static charges could not remain on conductive materials (such as human body).

 

[vikramc98406]

So, When a person(having static charge) touches a metal strip of a pin of an integrated circuit,

Tip of pin and tip of person which is in close contact becomes two plates of capacitor and this capacitance is very low but the voltage difference b/w the two plates is very large which is discharged into the integrated circuit.

Since huge potential drives the Integrated circuit it burnsout.
Am i correct in understandint?

 

[leotim]

Dear vikramc98406
I did not understand exactly what you mean, but damages that is caused by static charges in electronics parts is generally introduced in parts that have a pure capacitance such as a MOSFET gate.

To understand this, suppose a real MOSFET gait pin that can be modeled as a capacitor connected to an ideal MOSFET. In an ideal MOSFET, there is no current on gate pin and it has infinite impedance at this pin. so, if any charge is introduced to the gate capacitor, it could not be discharged.
Now suppose that there is some static charge on your body. When you touch gate pin of that MOSFET this charge is transfered to the gate capacitor. Because the capacitance of gate capacitor is very low (about a little nano Farads) a small charge can cause a large voltage on it: V=Q/C and this large voltage can damage the part.

Hope that I could explain enough well to be understood.

 

[raghuram_msc]

Hi All,

I would like to add that THE CHARGE DOES NOT LIE ON THE PLATES OF A CPACITOR BUT IT IS THE DIELECTRIC WHICH HOLDS THE CHARGE....!!!! Static charges can never get accumulated on the surface of a metal...!!! Please refer any basic solid state physics book....!

Sai

 

[leotim]

Static charges can never get accumulated on the surface of a metal

As I remember, in physics laboratiry, there was a vandergraph generator with a metalic sphere that generates charges on that metalic sphere, am I right?