The microwave oven is a
relatively recent invention. It dates back to 1945 when Percy Le Baron Spencer,
an American Physical Engineer working on radar at Raytheon Corporation,
discovered that a chocolate bar in his pocket had melted due to exposure to
microwaves from a radar unit. Without World War II and the pressing need to
develop radar, we might not have microwave ovens around today.
In a typical radar system, an intense pulse of
microwaves is emitted by a transmitter and then an adjacent receiver looks for
reflected microwaves. If a reflection is observed, the radar unit knows that
there is something out there.
1. Suppose that a particular radar unit uses microwaves with a frequency of 1.000 GHz (1,000,000,000 Hz or cycles per second). (a) What is the wavelength of those microwaves in empty space? (b) How long should the antenna that transmits those microwaves be in order to be maximally efficient at radiating the 1.000 GHz microwaves? (These answers require very simple calculations.)
2. An airplane flies past the radar unit at a distance of 1 kilometer from the unit. How long after the radar transmitter emits its pulse of microwaves will the reflected microwaves reach the radar receiver? Explain.
3. Metals reflect microwaves extremely well, but even if an airplane were made entirely of nonconducting materials, it would still reflect some microwaves. Why?
4. A tiny metal airplane flies past the radar unit. This remote controlled airplane is only 1 centimeters long. The 1.000 GHz radar unit has great difficulty detecting any reflection from this airplane. However a much higher frequency radar unit operating at 100.0 GHz observes the plane easily. Why do the frequencies of the microwaves matter when observing small objects?
You're designing a new visual
device for the music industry. It will be installed at concerts to entertain
the fans with beautiful displays of color. It contains a huge vacuum tube that resembles
a television picture tube--it has a source of electrons in the back, a long
empty space, and a phosphor coating on the inside surface of its screen. When
you're finished building it, its screen will have a large blob of light that
changes brightness and color, and moves up, down, left, or right with the
music.
5. The colored light is produced when electrons traveling through empty space inside the vacuum tube collide with phosphors inside the front surface of the screen. To vary the brightness of this light, you vary the amount of positive charge placed on the inside surface of the screen. Why does changing this amount of charge affect the brightness of the light? (Note: Your answer should make use of the concepts of energy and voltage. Remember that electrons are negatively charged.)
6. To allow for color changes, you put a pattern of colored phosphor dots on the inside surface of the screen and install a shadow mask a centimeter or two behind those phosphors. Holes cut in that mask allow electrons flying straight from the electron source in the back of the vacuum tube to hit only the blue phosphor dots. However, you have a way to vary the color of the light. There is a coil of wire wrapped around the outside edges of the screen and current from the audio amplifier passes through this coil. As the band plays, the current in the coil fluctuates up and down and beautiful colors appear on the screen. Why does running current through this coil allow electrons to turn away from the blue phosphors and hit the other colored phosphors?
7. To steer the electrons to different places on the screen (e.g., up, down, left, or right), you install a number of coils of wire beside the neck of the vacuum tube--near the electron source itself. As currents from microphones on the different musical instruments pass through these coils, the blob of light shifts around the screen. The coils are pushing the electrons around! At first, you use very fine, fragile coils but discover that the electrons inside the tube actually push the coils around, too. Use Newton's laws to explain why the electrons should push on the coils and then identify the type of force that is acting on the coils.
8. After installing your device in a local dance hall, you discover that their "black" or ultraviolet light makes the screen glow. Why does that occur?