Police use radar guns to determine how fast you are going as you travel down the highway in your car. The gun sends an electromagnetic wave toward your car. Some of this wave reflects from your car and returns to the gun. The gun then receives this reflected signal and analyzes it to determine how fast you were going. Ever since police started using radar guns, people have been trying to figure out how to elude them.
1. Some motorists use radar detectors to find out if police are checking their speed with a radar gun. What do you need to know about the radar gun to tune your detector so that it sees the gun's signal?
2. An extreme approach to avoiding the police radar gun is to put a radar jammer in your car. This is actually a transmitter that sends a confusing electromagnetic signal back to the police radar gun. If the police radar gun is operating at a frequency of 10 GHz (10,000,000,000 cycles per second), what is the wavelength of the electromagnetic signal?
3. The most passive approach to avoiding radar detection by the police is to put a special covering on your car that absorbs the electromagnetic waves used by the radar gun. Although this covering does not detect or transmit electromagnetic waves it can effectively keep the radar gun from recording your speed. How?
An oscilloscope is a device used to visualize electric signals (currents or voltages). We have used oscilloscopes several times in the demonstrations during class lectures. Old oscilloscopes contain the same kind of picture tube that is in old television sets (an evacuated tube in which an electron beam is scanned across a phosphor-coated screen). The electron beam is typically swept horizontally across the screen in synchronization with the signal you are interested in looking at. The beam is displaced vertically in proportion to the electrical current you send into the oscilloscope. This can be done by sending the current you are interested in visualizing directly to the vertical deflection coils of the picture tube, but often this signal is amplified first. When you are not sending any current into the oscilloscope the electron beam sweeps out a horizontal line across the middle of the screen causing the phosphor along this line to light up. Positive current makes the electron beam move up, above the center of the screen, and negative current makes the beam move down, below center.
4. If you try to use the oscilloscope near a large magnet, what will happen to the trace on the scope? Why?
5. Suppose you send a positive, steady (direct) current into the oscilloscope. Describe the trace that will appear on the oscilloscope.
6. Now suppose you send the signal received on your AM radio antenna into the oscilloscope. Describe the trace on the oscilloscope. Assume that the AM radio station is playing a continuous audible tone.
7. Describe the trace that would be observed if you looked at the signal that is sent to the radio's speakers. Again assume that the radio station is playing a continuous audible tone.
8. In modern oscilloscopes the signal is often sampled at very short time intervals and the voltage or current of the signal is assigned a digital value for each of the sample intervals. Describe in general the difference between representing a current of 4.37 amps using an analog representation versus a digital representation.