The Physics of Computers

    One of the newest methods of human-computer interaction and input is via touch screens. There are many such examples of these on the market right now, likely being bought up for the holiday season. Touch screen displays are in consumer electronics such as the HP TouchSmart line of computers, tablet notebooks, as well as the popular MP3 players from Apple and Microsoft, and a multitude of smart-phones available from various mobile carriers such as AT&T and Verizon.
    Many of these touch sensitive interfaces operate of the basis of detecting changes in capacitance in the screen area. This change in capacitance happens when the user touches the screen, altering the charge. While the voltage through the screen remains the same, but the charge changes. These screens also have sensors (usually in the four corners) that can measure the change in capacitance. By determining the change in capacitance from multiple distant points (i.e. corners).

Image F: A diagram of a capacitive touchscreens.

    Another new technology that many companies are using for user input these days is accelerometers. Accelerometers are becoming common place in smart-phones such as the iPhone or Droid, as well as in some video game controllers, such as those for the Nintendo Wii, and PlayStation 3. In devices such as the Nintendo Wii Controllers, a piece of silicon is placed in an electric field, where one end is fixed, and the other end can move freely with movement of the controller. This free end is in an electric field, between a pair of capacitors. As the free-moving end moves about in this electric feild, it alters the electric feild, as the electric feild changes, the capacitors "detect" the change and change accordingly. Through some mathematical magic, these changes in the capacitor are translated into movement on the screen.

Image G: A simple diagram of how an acceleromter (or acceleration sensor) works.