## Illustration 23.4: Practical Uses of Charges and Electric Fields

charge =  x 10-8 C | initial velocity = cm/s | electric field = x 104 N/C

When you first play this animation, there is no electric field present (position is given in centimeters and time is given in seconds). Input values for the electric field in the y direction and run the animation again. You will notice that the charge is deflected from its original straight-line path. Restart. Consider what happens when you

• increase the charge.
• increase or decrease the initial velocity.
• change the sign of the electric field.

Can you make the charge hit the green target? What happens if an object of zero charge is shot into a region where there is an electric field? The simple ideas demonstrated in the animation have been used in several real world applications.

Ever wonder how your computer and television monitor (those that are not flat-panel LCD or plasma) work? The image you see on the screen is produced using a cathode ray tube (CRT). The CRT uses a heated filament (not unlike that found in a light bulb) to produce electrons traveling at high velocities. The electron is shot into a region of constant electric field. Since the electric field exerts a force on charges, the electron is deflected from its path. The amount of deflection depends on the strength of the electric field and the velocity of the electron before it encountered the field. By controlling either the initial velocity or the field strength, the spot where the charge hits the screen is controlled. In the case of the CRT, the charge is not varied since it is a stream of electrons that is produced. The screen is coated with a substance (phosphor) that will emit light when struck by an electron. The rate at which electrons are striking the screen is very fast, allowing a complete picture to be built up. The applet shows electrons being deflected up and down. Notice that another set of plates could be added to control the right and left movement of the electron. Most new CRTs use magnetic fields to control the electron, but the basic idea is the same. Since the location where the electron hits the screen is directly related to the strength of the electric field, a CRT can be used to measure the electric field. Oscilloscopes use this idea to measure voltages (by measuring the electric fields produced), and display time-dependent voltage information on a screen.

Years ago, most printing was accomplished by impacting something mechanical against the paper. Typewriters and dot-matrix printers both used this concept. Today, high-quality, versatile printing is often done using an ink-jet printer. If you have a printer connected to your home computer, it is probably an ink-jet. The ink-jet printer works by squirting tiny drops of ink on the paper. These drops are very small, with a diameter less than a human hair. The number of dots a printer can place in an inch is specified by the dpi (dots per inch) number and is often around 1200 or more in the horizontal direction. There are two technologies used to drop the ink on the paper: continuous-ink printing and forced-ink printing. Forced-ink printing is the most common today and utilizes some method of forcing the ink to drop when it is directly over the desired location. From a physics point of view, the continuous-ink printing method is more interesting because it controls the location of the ink drop with an electric field.

When an ink drop is ejected from the ink cartridge, it is given a computer-controlled charge by the printer and then passed through an electric field. The location where the drop lands on the paper is determined by the charge on the drop. The same basic idea that is used to light up a particular portion of your television or computer screen can also be used to place ink on a particular location of your paper.

Illustration authored by Melissa Dancy.

Physlets were developed at Davidson College and converted from Java to JavaScript using the SwingJS system developed at St. Olaf College.