FIGURE 20.46 The electron gun and electron deflector of a CRT. Electron gun Electron de flector d = 4.0 cm L= 5.0 cm + ++ +
Older televisions and computer monitors, as well as some modern electronic instruments, use a cathode-ray tube, or CRT, to create an image on a screen. In a CRT, electrons are accelerated by an electric field inside an electron “gun,” creating a beam of electrons all moving along in a straight line at the same high speed. A second electric field then steers these electrons to a particular point on a phosphor-coated glass screen, which
causes the phosphor to glow brightly at that point. By rapidly varying the steering electric field and the intensity of the electron beam, the spot of electrons can be swept over the entire screen, resulting in the familiar glowing picture of a television. as shown a simplified model of the internal structure of a CRT. Electrons—emitted from a hot filament—start with zero speed at the negative plate of a parallel-plate capacitor. The electric field inside this capacitor accelerates the electrons toward the positive plate, where they exit the capacitor with speed v1 through a small hole. They then coast along at this speed until they enter the steering electric field of the deflector. This field causes them to follow a curved trajectory, exiting at an angle θ with respect to their original direction.
a. The CRT designer has specified that the electrons must leave the 4.0-cm-wide electron gun with a speed of 6.0 × 107 m/s . What electric field strength is needed inside the electron-gun capacitor?
b. The steering electric field has a constant strength of 1.5 × 105 N/C over the 5.0 cm length of the deflector. By what angle θ are the electrons deflected?
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