|MadSci Network: Computer Science|
We need to break your question into two parts:
1. What causes static electricity on a computer screen, and
2. What happens when someone touches a computer screen.
Your question pertains more to physics than computer science, and we have to go to physics to get our answer.
The type of computer screen we need to look at employs Cathode Ray Tube (CRT) technology, and is commonly referred to as the "picture tube" in both monitors and television sets.
For a short history and generalized description on CRT monitors and how they function, please see CRT Monitors.
From that site, you will note that a CRT functions by bombarding the screen of a monitor with a stream of electrons. This is where our answer starts.
The first part of our answer has to do with the glass which makes up the screen in the monitor. Glass does not conduct an electrical charge very well and is, in fact, classified as an electrical insulator. Because glass cannot conduct an electrical charge and is being constantly bombarded by electrons(which are negatively charged particles), an imbalance between electrons and the protons in the atoms of the glass which make up the monitor screen develops. Protons are particles which carry a positive charge and, when in balance with electrons, result in a cancelling effect whereby there is a neutral electrical charge.
Now, as we've stated, glass is an insulator, and as such all of those electrons end up with no place to go, so they accumulate on the surface of the monitor screen, creating the imbalance of charged particles commonly referred to as static electricity.
As to our second question, when you come along and touch the monitor's screen, the excess electrons have found a way to move away from the glass, through you. You have just become an electrical conductor, balancing the charged particles.
As a footnote to all of this, many CRT monitor manufacturers are now coating the screens of their products with a metal film which serves to conduct the electrostatic charge away from the screen. No more ZAPS!
For some basic information on electrostatic charges, please see Electricity & Static Electricity from the Science Made Simple web site.
For a description of common misconceptions about static electricity, see Static Electricity Misconceptions on the SCIENCE HOBBYIST web site.
If you have a follow-up question, or need more information, please feel
free to post on MAD Scientist again or contact me directly.
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