# How static electricity gets build up and how it gets discharged?

I get electrocuted all the time by contacting metal object, by running a cart in the grocery store, opening the car door, touching the door knob. I have reseached various article of how static electricity is created and how it is discharged but none has satisfied me. Can someone explain it to me, how it is created?

• Some thousands of voltage is generated when you walk on carpet or even on dry concrete, floor. How this charge gets accumulated continuously and never gets discharged?
• Why it discharges through your fingers, when it is really built up on your shoes or clothes. If you touch you clothes with the door knob nothing happens but if you touch your finger, boom, you get shock? How does the whole process work?

• Question: Some thousands of voltage is generated when you walk on carpet or even on dry concrete, floor. How this charge gets accumulated continuously and never gets discharged?

Answer: See at end under "research" for charge generation methods.
In your case rubbing or "triboelectric effect" is almost certainly the main mechanism.

Importantly - the generation methods do not contain reciprocal discharge mechanisms.
For discharge to occur the conditions for discharge need to be met separately. If they are not met - eg warm dry conditions with good insulation to other differentially charged objects then discharge will not occur. Charge will accumulate on the capacitor that the body forms relative to ground. For a given amount of charge, the lower the capacitance the higher the voltage.

$$C={Q \over V}$$

$$V={Q \over C}$$

$$Q = C V$$

                              Energy:


$$E={1 \over 2}C V^2$$

So for a fixed charge, halving capacitance means double voltage, and voltage is a squared factor in energy.

• Question: Why it discharges through your fingers, when it is really built up on your shoes or clothes. If you touch you clothes with the door knob nothing happens but if you touch your finger, boom, you get shock? How does the whole process work?

Answer: Discharge will occur when there is a low enough resistance path to some other body for the charge to drain through it - ie for a current to be created. There will ALWAYS be a discharge path BUT it may be of such large resistance that the discharge time is long. If the time to discharge is more than say ten seconds to half discharge then you may notice the effect and if the half-charge discharge time is more than one minute you are almost sure to discover it.

Charge is stored in capacitance. That's a whole area in its own right - but your body has a capacitance if typically 100 pF to ground - see Wikipedia body capacitance article. The objects attached to your body that generate most of the charge have low capacitance compared to that of your body. Charge is stored in proportion to capacitance so your body acts as a reservoir for the charge "generated" by your clothing.

You will feel pain from "shock" when discharge energy rate and magnitude fulfill some complex relationship. If the discharge current is too low you will not feel pain or shock sensation.

Touching clothes etc to a grounded object provides a very high resistance path to ground so charge transfer is slow. Charge does flow from your via clothes to ground but the all imporant enegy transfer rate is low.

If you touch a grounded object wit part of your body then charge stored on your body has a low resistance path to ground and you will be much more likely to fel "pain" than if the discharge path was low resistance. If you touch hair or dry clothing to ground, the discharge path resistance may be 10's to 100's of megohms and you will feel no pain 0 even though these objects generated the charge that your body stored.

1000 V on a 100 pF capacitor contains about 0.1 millijoule of energy !!! - and you may feel the discharge. A 10 kV voltage on 100 pF stores about 10 mJ or energy and you WILL feel the discharge if you touch ground.

If a group of 6 or so people all hold hands and shuffle across a static inducing carpet and one person extends a finger and touches an unattached person then all 7 people get a substantial shock but especially the two at the main contact point. Ask me how I know :-).

Warning: When fully charged it is a bad idea [tm] for the finger pointer to touch a filing cabinet with their outstretched finger !!!. Ask me how I know :-). In the latter case the filing cabinet will be a large capacitance in its own right and at ground potential due to leakage via its path through its base and carpet etc.

Research:

Most of your questions seem to be answered quite well by Wikipedia's static electricity page.

They include links to charge generation by:

You will usually be experiencing mainly a combination of Triboelectic and induction effects.

• @clabacchio - Almost. But Q = CV. I am not competent with the markup language. I would not use 6 lines like that if I knew how to fit it onto 1. Must learn :-). Ref Wikipedia Commented Feb 21, 2012 at 10:59
• OOOh sorry stupid mistake...and I'm still reasoning about the statement in brackets, even if now I'm closer to understand Commented Feb 21, 2012 at 12:46
• check the edit to see if it matches with your previous statement Commented Feb 21, 2012 at 12:52
• +1 I will do some more research, answer is reasonable though :) A quick question, body acts as one 100pf capacitor or it literally has millions of 100pf Capacitors? or the 100pf capacitor is made of millions of small may be even smaller capacitance? Just a thought Commented Feb 26, 2012 at 19:57
• 100 pF is whole body capacitance. | Mount a 1 megohm + resistor on your keyring and try discharging via that.ven quite a lot lower may be enough. This limits peak current and increases discharge time. You can also get "heel grounders" for shoes - but those are intended forassembly or lab environments Commented Feb 28, 2012 at 1:37

This is a current problem in industrial environments (think of conveyor belts or escalators). It is important to safely get rid of the accumulated charge. In electronic laboratories workers usually wear grounded bracelets, which are connected to the ground through a large resistance (e.g. 1 MOhm), never directly! (Serious electric shock hazard!)

On the other hand the tiboelectric effect is used to charge high voltage generators like the Van de Graaff generator. One of the earliest 'toys' of this type was the Leyden jar.

If you experience the 'pinching' frequently, it might be due to the fact that the material of your clothes is especially suitable for triboelectric generation and the soles of your shoes are made of a very effective insulator. So it's probably time for a wardrobe change... ;)

• I really have different type of shoes. I wear normal clothes and I have had experience with one that generates statics even rubbing with itself. But why the charge had to discharge through my finger. I happened with me that I use key to discharge it first, that does not happen and I still get shock. Commented Feb 26, 2012 at 19:47

Static electricity gets build up by two processes-friction and induction.This static electricity gets stored in both the objects involved in the process of induction and friction. But after sometime there is a imbalance of charges on the objects (positively and negatively charged).So in order to balance the number of electrons and protons in both the objects one object gives some of the electrons to the other object.This is known as electric discharge.