721. Is it possible to sense when a person touches a car, even if the car is painted? - AW
Yes. I wouldn't try to detect mechanical contact, because you'd have trouble differentiating between forces exerted on the car by a hand and those exerted on it by sound waves. But you can tell whether a conducting object (such as a person) is near the car by looking at the car's electric properties. If you were to send electric charge on and off the car rapidly with a source of high-frequency alternating current, you would find that the amount of charge that flowed on or off the car during each cycle would change as the person's hand approached the car. That's because the charges on the car would push or pull on charges in the person's hand and the charges in the person's hand would move. In effect, the person's hand would make the car "larger" and it would draw more charge from your current source. Even if the person didn't touch the car, the nearness of the hand and car would change the way current flowed on and off the car. Such a change would be easy to detect with laboratory equipment and could probably be made by cheap consumer equipment, too. The only complications would be in not detecting everything—passing cars for example—and in not damaging the device with static discharges. Still, I think all of that could be done.