As you announced in another post, you keep well clear of the covid and anti-vaccine threads, where much of the trolling takes place.
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As you announced in another post, you keep well clear of the covid and anti-vaccine threads, where much of the trolling takes place.
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The ideas that current only flows in circuits or that voltage is only defined for conductors is a simplification of the reality. A simplification that works well enough most of the time but still a simplification.
Consider two concentric spheres of conductive material forming a capacitor, now consider we make the outer sphere larger without changing the size of the inner spehere, you might think the capacitance would go to zero, but it doesn't, it instead asymptotically approaches a fixed value known as the self capacitance as the size of the outer sphere goes to infinity.
Since the self capacitance is essentially "capacitance relative to infinity" it follows that the capacitance between two objects is always greater than or equal to the series combination of their self capacitances. The self capacitance of the earth is rather large compared to that of any manmade object, so the series combination of the self capacitance of the earth and any manmade object is roughtly equivilent to the self capacitance of the manmade object and the self capacitance of an object can be seen as a lower bound on it's capacitance to earth.
A sphere of radius r has a self capacitance of 4πε₀r, ε₀ is about 9 picofarads per meter, so 4πε₀ is about 110 picofarads per meter or about 1.1 picofarads per centimeter. In other words an object whose size is of the order of centimeters will have a self-capacitance of the order of picofarads.
Now I=2πfVC which is about 75000 C for a UK mains supply. So an object with a size of the order of centimeter connected to a 240V 50Hz supply will have a current of the order of hundreds of nanoamps flowing in and out of it. That is certainly small, but not so small it can't be detected. It seems quite plausible to make a volt stick that can work "in free air" with no other conductive objects (other than the object being measured) nearby.
The wrinkle of course is the Faraday cage effect, if the voltstick is surrounded by material that is electrically coupled to the mains, then rather than the current flowing in and out of the voltstick it will flow in and out of the material surrounding the voltstick.
Consider two concentric spheres of conductive material forming a capacitor, now consider we make the outer sphere larger without changing the size of the inner spehere, you might think the capacitance would go to zero, but it doesn't, it instead asymptotically approaches a fixed value known as the self capacitance as the size of the outer sphere goes to infinity.
Since the self capacitance is essentially "capacitance relative to infinity" it follows that the capacitance between two objects is always greater than or equal to the series combination of their self capacitances. The self capacitance of the earth is rather large compared to that of any manmade object, so the series combination of the self capacitance of the earth and any manmade object is roughtly equivilent to the self capacitance of the manmade object and the self capacitance of an object can be seen as a lower bound on it's capacitance to earth.
A sphere of radius r has a self capacitance of 4πε₀r, ε₀ is about 9 picofarads per meter, so 4πε₀ is about 110 picofarads per meter or about 1.1 picofarads per centimeter. In other words an object whose size is of the order of centimeters will have a self-capacitance of the order of picofarads.
Now I=2πfVC which is about 75000 C for a UK mains supply. So an object with a size of the order of centimeter connected to a 240V 50Hz supply will have a current of the order of hundreds of nanoamps flowing in and out of it. That is certainly small, but not so small it can't be detected. It seems quite plausible to make a volt stick that can work "in free air" with no other conductive objects (other than the object being measured) nearby.
The wrinkle of course is the Faraday cage effect, if the voltstick is surrounded by material that is electrically coupled to the mains, then rather than the current flowing in and out of the voltstick it will flow in and out of the material surrounding the voltstick.
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I presume you are referring to the fact that I 'keep well clear' of Covid (and vaccine) threads in the DIYnot GD forum (and all suchlike places). That is certainly true, although my 'keeping well clear' was recently thwarted by the mods moving one of my threads to GD (where I have essentially subsequently ignored it!) !
However, more generally, a substantial part of the last couple of years of my life has necessarily involved discussions about Covid etc. - but in groups (both 'professional' and 'lay') where sensible discussion is usually (albeit not always!) possible.
Kind Regards, John
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