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John Cuthber

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Posts posted by John Cuthber

  1. Given a product of two primes n, and n+2 the product is going to be very close to (n+1)^2

    So you can take the nearest integer to the square root of the product, and the factors will be  that +/- 1


     imagine I give you the product 19043

    the square root of that is 137.996...

    Very close to 138

    And the integers 1 away from it are 137 and  139.

    And those are the factors of 19043.



  2. 50 minutes ago, ScienceNostalgia101 said:

    You listen to the people who have a pattern of being right, and dismiss out of hand the people who have a pattern of being wrong.

    So, we dismiss Trump out of hand - because he was so wrong about lots of things.
    And, on that basis- we know that his views are often wrong.

    So, for example, since he didn't like the WHO, we can reasonably deduce the the WHO is probably a good thing.


  3. 10 hours ago, neonwarrior said:

    Hello everyone,

    Could you possibly enlighten me which principle soil ph meters use?

    I understand that they have an aluminum and copper electrodes. But I cannot find any information about this one (only the one with silver wire coated with AgCl)

    Thanks in advance


    I'm not sure, but I think that they rely on the fact that the aluminium corrodes faster in more acidic  soils and thus sets free more electrons which pass through the meter and give a larger reading.

    Or they may work on a  principle is similar to this.


    41 minutes ago, Bufofrog said:

    It depends on your needs, but it is much less expensive to buy kits for testing pH than to buy a pH meter.  

    How would that help find out how the meter works?

  4. 1 hour ago, exchemist said:


    P.S. On the subject of gas fridges, I remember someone's girlfriend being conned into thinking that, for caravans, one could have a gas television.

    You can.

    Most of the UK’s electricity is produced by burning fossil fuels, mainly natural gas (42% in 2016)



    Fridges move moisture about, but they don't "produce" much.
    A lettuce doesn't steam as much as a roast.

  5. 5 hours ago, iNow said:


    I always wonder if that was meant to be ironic.
    It is, at root, a commentary on people...

    Nearer the topic; Someone once described the House of Lords as "A very civilised way to look after the elderly".
    I sometimes think that government research labs are a very civilised way to look after us nerds.

  6. The octaves are important- as Exchemist has explained. If two notes are in harmony then the octaves above them will also be in harmony.
    The division of the octave into 5, 8 or 12  (or more) is arbitrary and largely cultural.

    Having said that, there does seem to be something about the pentatonic scale that people all seem to "get".


  7. 8 hours ago, altaylar2000 said:

    Okay. To begin with, why do you think that the charge can only be in relation to the earth and is necessarily discharged into the earth?

    OK, imagine I get some sort of electrical generator- perhaps a Wilmshurst machine, and I put it on an insulating slab like a thick sheet of polythene or something.

    And I set it running (let's say it has a battery drive motor, and a remote control- so I can do that without anything being connected to Earth). such that it charges the two spheres - one positive and the other negative with respect to eachother.


    And then imagine that I bring an earthed wire near to one of the spheres.


    What do you think will happen?


    8 hours ago, John Cuthber said:

    You don't know much about this, do you?


    5 hours ago, altaylar2000 said:

    The earth has different potentials in different places.

    Not very, and not for long. It is generally quite conductive.

    5 hours ago, altaylar2000 said:

    And ground is not a low resistance path.

    It doesn't need to be.

    Static sparks are typically thousands of volts.

    The capacitances involved are of the order of pico farads.
    So even a bad earth connection with a resistance of 10 ohms would allow a current of hundreds of amps to flow. And that would discharge teh capacitor in a period of the order of a nanosecond or less.


    But for that to happen, the current would need to start and stop within a nanosecond.

    And that, in turn would mean that the rate of change of current was very large-hundreds  of gigamps per second.

    And that could only happen if the inductance of the circuit was small enough.

    But to drive a rate of change of current of 100 GA/S with a few KV you need an inductance of about 10 pico Henries or less.

    And even a few inches of wire has much more inductance than that.

    So the thing that limits the discharge in electrostatics is generally the inductance, rather than the resistance.

    So... why did you point out the the resistance isn't all that low?



  8. 3 minutes ago, altaylar2000 said:

    there is need about 5-10 killovolts to do that

    And that is well within the range of typical static electricity.

    You don't know much about this, do you?

    5 minutes ago, altaylar2000 said:

    Such a small charge will not pass through plastic

    It isn't the charge that makes a difference, it is the voltage (though the two are related).

  9. 19 minutes ago, altaylar2000 said:

    light switch usualy plastic, not conductor

    True but:
    It's next to the two earthed metal screws that hold the cover in place, and you can make sparks by putting your finger near them.

    |It's also relatively thin plastic and may well conduct at the high voltages involved.

    And, of course, it may be a metal switch  or a metal cover.


  10. 1 hour ago, altaylar2000 said:

    I'm talking about most, not all. European genes are present there, I did not deny it

    What you are talking about is a map that shows the presence of a gene in roughly 90% of "some population" in parts of N America.

    Well, if the population of the USA is 14% African Americans, 10% Irish Americans and 5% Italian Americans (virtually none of whom would have the gene) then the population represented on that map is not the population of the USA, is it?


    So, what are you talking about?

    Do you even know?

    33 minutes ago, altaylar2000 said:

    Your claim is the opposite of the map you have linked to.

    You are going to need to work very hard to convince us that you are worth listening to.

  11. 1 hour ago, altaylar2000 said:

    Also it means that almost all americans goes from Siberia

    That's unrealistic, given how many are from, for example, Ireland ot Italy.
    It's even more absurd once you consider African Americans who form  about 14% of the population.

    Are you able to explain this, or is your assertion simply incorrect?

    7 minutes ago, altaylar2000 said:

    But in a slave society, the elites are always in the minority, and if this were the case, the number of African Americans would significantly outnumber the "white people"

    By which argument, most people in France, Germany < England and so on would be black.
    But they are not.

    And so we know that the argument is wrong.

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