istok

it is impossible to do such an experiment on Earth, because there is no way to isolate only one small object and record its affection on light beams. You would have much more influence on the light bending from the moon, for example.

Actually, electric chair uses AC, as far as I know. It is less harmful to humans, though, but there is an explanation for that. First there was nothing. And then Edison created DC. It was very little effective. The power plant had to be very near, etc... and then Tesla invented AC (for Westinghouse). It could be transmitted thousands of miles away. Edison was about to fail with DC projects. So he invented electric chair which uses AC. And the public was shocked. Nobody wanted to use AC for a long, long time. But anyway, AC defeated DC after some time, because it is much more effective. And Edison lost the battle. That's why AC is used in electric chairs, although it is less harmful.

Exactly. According to skin effect theory, this is happening because at higher frequencies all the current is "concentrated" in the skin surface. Example: If 1 mA is running mostly through upper 1mm of your skin - at, let's say, 10 MHz, and no current runs beneath (this approximation is OK because of exponential nature of decreasing), than at 100 Mhz, the same 1 mA will be running through upper 0,01 mm. And that tiny surface of the skin is receiving a great amount of current.

Hello, Nikola Tesla proved that there is one thing called the "skin effect". You see, if you have an AC current running through a conductor, such as a wire, then the current is not uniformly distributed in it (if you look at the cross-sectional area). On the surface of the wire, current density is highest. The more you go into the center of the wire, current density is weaker. For example, current density near surface is 1 A/m^2, and near the center of the wire it is 0,01 A/m^2. The intensity of current decrement depends on one thing: the frequency of applied voltage. If the frequency of voltage is very, very high, then almost all of the currrent runs on very surface of the wire. As you decrease the frequency, more and more current is going to the center of the wire. At zero frequency, you have DC current and it is all uniformly distributed across the wire. Actually, the current density falls exponentially with the frequency of applied voltage - in the direction of the radius of the cross-sectional circle of the wire. That explains all. Well, since human body is also a conductor, then the same thing applies to it. If you have a current of very high frequency running through your body (e.g. 100 MHz), then nothing can happen to you no matter what the voltage is. The current is "only skin deep", it goes a milimeter of your skin in depth, and therefore it is not harmful to any vital functions, heart, muscles, and everythig else. Tesla had several performances on various fairs proving this effect. He used to run million volts AC through his body, with very high frequency. And he always remained unharmed. Cute, isn't it? Well, that should be the answer for you. Less frequency - more dangerous. But don't try it at home Greetings from Belgrade, Tesla's capital

I see now. Thanks for deleting my previous account

Hello Dave and others! First of all, I would like to ask the moderator to delete my account with username as an email address (istok@bitsyu.net, a post before), i made it by mistake. That is why I made testing. Well, I am a graduate in engineering, and recently discovered this forum, so I told to myself "Why not joining" I am mainly looking for some hot topics in physics, but maths are of some interest, too. Anyway, I don't know what is generally level of conversation here. Basic, intermediate, or advanced...? I am asking this because I realise many people did not understand your question, which is very basic. Here is my answer: 1) Gradient of x^3 is 3x^2, which, for x =5, makes 3*25 = 75. 2) Gradient of 1/x^2 is -2*(1/x^3), and, just like you said, it is - 2/125 for x=5. Or maybe I didn't understand? Was I supposed to solve it some other way? Istok