## If black is a high current from the television’s radio receiver and white is a l…

#### If black is a high current from the television’s radio receiver and white is a low current, why do you get a bright spot when you increase the flow of electrons at that instant. Isn’t white a bright spot?

Yes, white is created by a strong flow of electrons. There are two separate circuits here. The current from the receiver section of the television isn’t what is sent through the electron gun. Instead, that current controls the electron gun. When a large current arrives at the electron gun (actually the grid) from the receiver, the flow of electrons toward the screen is pinched off and a dark spot is created. When a small current arrives from the receiver, the electron beam remains intense and a bright spot is created.

## If you stand between the two satellites, would you have light on you?

#### If you stand between the two satellites, would you have light on you?

When two satellites beam their radio waves at you, you are exposed to both of those waves. A normal antenna would not be able to distinguish between them and it would be hard to receive the transmissions of one and not the other. But with a satellite dish, you can easily select the transmissions of one and exclude those of the other. The satellite dish is directional, meaning that it focuses and collects radio waves from a particular direction while ignoring those from other directions. With a satellite dish aimed at a particular satellite, you can receive only transmissions from that satellite.

## What is one doing when changing the brightness, contrast, and color adjustments …

#### What is one doing when changing the brightness, contrast, and color adjustments on a television?

The brightness control determines the maximum strength of the electron beam and thus the peak brightness of the phosphors on the screen. The contrast control determines the extent to which the electron beam current changes between bright regions and dim regions on the screen. If the contrast is high, then even a less-than-white spot in the image may produce full beam current and full brightness in the phosphors and a more-than-black spot in the image may be cast as full black (no beam at all). If the contrast is low, then almost the entire screen will be illuminated by a medium electron beam and the image have no full black or full white. The color adjustments control the relative intensities of the red, green, and blue guns. Because of the way color is encoded in the television signal, the traditional controls are hue and tint, which involve mixtures of red, green, and blue. All these controls involve adjustments to the voltages and currents in the electron guns (cathodes), grids, and anodes of the picture tube.

## How can computer monitors and televisions have images burnt into them over time?

#### How can computer monitors and televisions have images burnt into them over time?

As the electron beam collides with the phosphor coating on the inside of the picture tube, it slowly damages that phosphor coating. Eventually the phosphors are burnt away and the inside surface of the picture tube stops being uniform. To avoid burning specific regions more than others, computers use screen savers that darken the images by turning down the electron beam and keep those images moving about randomly.

## How can the magnets be manipulated in such a way that they can do this moving of…

#### How can the magnets be manipulated in such a way that they can do this moving of the electron beam in such an incredibly small amount of time?

The electromagnets that control the beam are able to turn on and off very quickly. The only limit on the rate at which they can change the magnetic field comes from their inductance. They do resist changes in current passing through them. Fortunately, the television doesn’t move the beam about randomly; it sweeps the beam smoothly. Thus the changes in the current through the electromagnetic coils are also smooth. The television has no trouble ramping the field through the horizontal sweep coils back and forth every 1/15,750th of a second.

## How do high definition televisions differ from traditional ones?

#### How do high definition televisions differ from traditional ones?

High definition televisions have more individual spots of color and brightness than the traditional sets. They may also have a somewhat different aspect ratio (horizontal width vs. vertical height). Creating high definition picture tubes is not particularly difficult since they are now rather common on computers. However, transmitting the increased information needed to paint the picture on a high definition television is a serious problem. One approach is data compression, in which redundant information is eliminated from the signal so that only new information is sent to the television. To avoid making all of the present televisions obsolete, the new high definition television standards are supposed to be downward compatible with those televisions. Unfortunately, trying to serve both types of televisions with the same transmitted signal is going to be a difficult task.

## How do projection or rear projection televisions work?

#### How do projection or rear projection televisions work?

Inside the projection TV, there are three separate picture tubes that work very much like normal black and white picture tubes. One of these tubes creates an image of the red light in the television image, one creates an image of the green light, and the third creates an image of the blue light. In front of each tube, there is a color filter: red for the red tube, green for the green tube, and blue for the blue tube. There is also a projector lens that takes the light leaving the tube and filter and projects a clear image of that light on the screen in front of the projector. The light striking the screen looks exactly like the light leaving the surface of the picture tube. The three images (red, green, and blue) are carefully overlapped so that they mix and you perceive all colors.

## How do the electrons know which spots on the screen to color darker and which to…

#### How do the electrons know which spots on the screen to color darker and which to leave lighter?

The electrons simply deliver energy (their own kinetic energy) to the phosphors they hit. When they are hit by electrons, these phosphors emit light. They fluoresce. The picture determines which spots on the screen should be dark and which ones should be light by controlling the number of electrons that hit those spots; by controlling the current in the beam. When the current hitting a spot is low, that spot glows dimly. When the current hitting a spot is high, that spot glows brightly.

## How do the magnets that redirect the electron beam in the picture tube move it t…

#### How do the magnets that redirect the electron beam in the picture tube move it to the exact point that it’s supposed to?

The electromagnets that steer the electron beam are very carefully designed and constructed so that they steer the beam very accurately. They are coils of wire that are built on a form and then glued together so that they cannot move. There are some adjustments made electronically inside the television set to make sure that the beam follows a very start path as it sweeps across the screen. When you adjust the horizontal and vertical sizes of the picture, you are adjusting the currents flowing through these electromagnets.

## How does a magnet change the picture on a television

#### How does a magnet change the picture on a television—does this hurt the TV?

When you hold a magnet up to the front of a television, you are introducing an additional magnetic field in the system. This field exerts forces on the moving electrons inside the tube and they are deflected. The picture is distorted. With a black and white television, no harm is done because there is nothing to magnetize inside the picture tube. But color television picture tubes contain metal shadow masks that can become permanently magnetic. The picture remains distorted, even after you remove the magnet. To clear up the “damage”, you would have to demagnetize the picture tube. Although this is not a particularly difficult task, it requires a demagnetizing coil and is best done by a professional repairperson. The bottom line is, don’t play with magnets near a color television set.