1

General Science / Re: Why is mains electricity 240v AC?

« on: 28/06/2022 17:23:30 »

The easy way to get an average is still to use a capacitor

If you want an average reading instrument you need to leave the capacitor off. Put a capacitor on the output of a rectifier, and you'll have a (close to) peak reading instrument, because the charge and discharge currents are dissimilar, as shown in the plot above. If you need to smooth the ripple with a capacitor and still retain the mean sensing, you need a buffer between the rectifier and the smoothing.

2

General Science / Re: Why is mains electricity 240v AC?

« on: 19/06/2022 17:37:09 »

No, non True-RMS meters are mean sensing, not peak-sensing, and this is usually stated explicitly on good quality meters.

The average value of a full-wave rectified sine is 0.637 V_peak

Yes, I know:

Like moving coil analogue meters, DMMs (that aren't true RMS) are sensing the DC average of the input, which for a rectified sine wave is 0.637, and the form factor is therefore .7071/0.637 = 1.11. To measure RMS values of waveforms other than a sinusoid, you need to divide by 1.11, and then multiply by the form factor for the waveform in question.

which is not easy to sense or sensible to display.

It's trivially easy to sense, you just leave the smoothing capacitor off the rectifier, and meters don't display mean level, they are calibrated in terms of RMS.

"Mean Sensing, Calibrated RMS":

TM3B.png (680.75 kB . 883x402 - viewed 500 times)

No, non True-RMS meters are mean sensing, not peak-sensing, and this is usually stated explicitly on good quality meters.

The easy way to get an average is still to use a capacitor. (or the mass of the needle in an analogue meter)

The value you use (and that of the shunt resistor) is dependent on the frequency range- which can make things awkward.

A capacitor directly on the output of a rectifier will give you peak, or something close to it, because the rectifier charges it but doesn't discharge it. That's what rectifiers do, only conduct in one direction.

Smoothing at 0, 40ms, & 120ms:

rect.png (23.57 kB . 1150x514 - viewed 502 times)

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Physics, Astronomy & Cosmology / Re: Is mass a number?

« on: 17/06/2022 17:04:24 »

No. The mass is 75 kg, not 75 yards or bananas. The reference must be stated as part of the quantity.

75kg is indeed (or should be) 75 times the mass of the ISK prototype  but it's also 165.35 times the mass of the imperial standard pound, which is a perfectly legal, scientific, trade, maritime and aeronautical measure in countries that are not fully signed up to the metric system.

Now in my book, 75 ≠ 165.35, but it's the same person.

Who are you replying to, you seem to be trying to contradict me with my own argument.

4

General Science / Re: Why is mains electricity 240v AC?

« on: 17/06/2022 16:55:41 »

Alan nearly got the right answer.
If you put a bridge rectifier and a capacitor in the circuit you can measure (pretty nearly) the peak voltage

No, non True-RMS meters are mean sensing, not peak-sensing, and this is usually stated explicitly on good quality meters.
Putting smoothing capacitors in meters would make them susceptible to large errors caused by noise spikes.
https://en.wikipedia.org/wiki/Form_factor_(electronics)

and that is 1.141 times the RMS value (if you have a sinewave).

1.414

5

General Science / Re: Stats question: who's most likely to be at fault?

« on: 16/06/2022 19:15:30 »

If we assume that there are 8 times as many non-cycling road users as cyclists

There's something fishy about the statistics. Chart 1 says that pedal cyclists travelled 200,000,000,000 miles in the UK in 2020. That equals the distance covered by all the cars in the UK during the same period. Does every cyclist really pedal 8 times further than every car driver?
Really?

No it doesn't, it says there are 5.03 billion vehicle miles cycled in 2020, which is 1.8% of the 280.5 BVM for motor traffic.
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1028165/road-traffic-estimates-in-great-britain-2020.pdf

I see that study is uk based. In Ireland cyclists are immune to traffic lights, stop signs and any traffic control one can think of. As you may gather this is a pet grievance of mine. Apologies for off-topic rant, I just couldn't help it!

I did consider generalising the question so that it wouldn't descend into an anti-cycling rant, but I assumed it ought not to be necessary on a science forum.

Red Light Car Bike.JPG (284.43 kB . 1449x533 - viewed 490 times)

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General Science / Stats question: who's most likely to be at fault?

« on: 16/06/2022 17:22:28 »

Referring to this page, there are 3782 cyclists at fault for failing to look properly, and 7565 non-cyclists. If we assume that there are 8 times as many non-cycling road users as cyclists, does it follow that:

1.   Cyclists are four times more likely to be at fault because they’re producing half as many incidents from 8 times fewer vehicles.
2.   Cyclists are 16 times less likely to be at fault because non-cyclists are still producing twice the incidents despite having 8 times fewer potential targets for careless road users to collide with.
3.   From the product of 1&2 above, cyclists are four times less likely to be at fault overall.
4.   Cyclists are just half as likely to be at fault, because those who haven’t crashed are not relevant.
5.   None of the above.

Personally, I’m inclined to think it’s (3), because the probability of an accident must be the probability of finding a careless road user multiplied by the probability of them finding something to hit.

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Technology / Re: What Is The Benefit Of Cryogenically Treating Cables ?

« on: 15/06/2022 18:22:59 »

A fellow philistine and musician pointed out that composers "hear" their music in their heads. Problem was for thousands of years that real instruments take up real space, so an actual performance distributes the sound sources in 3 dimensions. Edison solved the problem by getting all the sound to come out of one hole, and for 100 years you could hear what Beethoven wanted you to hear. Then some idiot invented stereo and splattered the orchestra all over the stage again.

For my money, two speaker mono is best. The sound fills the room without any sensation that it's coming out of a box in the corner like you get with single speaker mono, or alternating between two boxes with stereo.

But generally, I prefer Prof. Richard Wiseman's approach to connoisseurs.

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Physiology & Medicine / Re: Can two blue eyed parents have a brown eyed child?

« on: 09/06/2022 19:28:18 »

I used to have a GF with central heterochromia: green eyes with a brown fringe around the pupil. Mine are blue, I don't know what colour eyes our child would have had if we'd had one.

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General Science / Re: Why is mains electricity 240v AC?

« on: 07/06/2022 18:25:08 »

They are usually accurate to +/- 1 digit, but don't necessarily display what you are expecting for the reason given above. The basic voltmeter chip is a DC instrument so for AC you add a bridge rectifier and a 1.41:1 voltage divider to give an estimate of the rms voltage, assuming the waveform is sinusoidal and the rectifier forward voltage is negligible.

Digital multimeters use dual-slope integration ADCs because the integration smooths noise and ripple, and because the integration period can be made a multiple of 10ms for further rejection of mains related hum. They are also independent of integrator time constant and the clock frequency. Like moving coil analogue meters, DMMs (that aren't true RMS) are sensing the DC average of the input, which for a rectified sine wave is 0.637, and the form factor is therefore .7071/0.637 = 1.11. To measure RMS values of waveforms other than a sinusoid, you need to divide by 1.11, and then multiply by the form factor for the waveform in question.

True RMS meters are only really helpful if you don't know the waveform, and have a resistive load. In the common case when you have a load running from a rectifier and capacitor input filter you have neither, and a more accurate estimate of power can be had by multiplying the peak voltage by the mean current if you don't have a power meter.

±1 digit on a 3½ digit DMM would be an accuracy of ±0.05%, which is tall order even for a good quality instrument like a Fluke. The 1 digit on digital instruments is in addition to the basic accuracy, of 1%, or whatever.

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General Science / Re: What is the strongest plastic?

« on: 01/06/2022 12:06:38 »

Latex rubber.

Latex is porous.

11

Physiology & Medicine / Re: Link between supercentenarians and smoking?

« on: 31/05/2022 10:51:24 »

"a good deal of" is not an objective way of measuring things. Relevant questions are how many smokers vs non-smokers reach the age of 100, and what proportion of centenarians are smokers compared with nonagenarians, octogenarians and septuagenarians.

12

Physics, Astronomy & Cosmology / Re: Is mass a number?

« on: 30/05/2022 11:56:33 »

Most physical quantities require Units......mass is one quantity that has units

as a ratio of two numbers with the same units....the atomic mass unit is dimensionless

But by that argument a mass in kg or pounds or anything else is dimensionless too, as it's also the ratio of a mass to that of a standard. The point about dimensions is that they specify which standard you're using as the denominator in your 'dimensionless' ratio.

75kg is just shorthand for "if the reference is the kilogram, the mass is 75", or "the mass is 75 times that of the kilogram".

Then just to confuse the issue, there are units like the radian and the bel......
 

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Cells, Microbes & Viruses / Re: Monkeypox: Could it be similar to cowpox, and just a mild variant of smallpox?

« on: 28/05/2022 18:10:26 »

Wonder how effective the smallpox vaccines are after a few decades of being used, as I had the pressure injection at primary school, many years ago.  But will say the Covid vaccines are pretty effective, still here, despite having had it likely at least twice, but both times a negative test result, which is still possible with the quick tests. Last one was just like a moderate case of flu, while those who are antivax seem to be selecting themselves for Darwin awards, with a very high probability of winning one.

Darwin requires natural selection to take place before reproduction. Darwinism has failed to stop hereditary early age breast cancer or heart failure.

https://en.wikipedia.org/wiki/Darwin_Awards

14

Plant Sciences, Zoology & Evolution / Re: Have animal reactions to mirrors and windows been studied?

« on: 28/05/2022 00:50:28 »

https://twitter.com/buitengebieden/status/1523065975654195201
https://twitter.com/buitengebieden/status/1527524346872332289
https://twitter.com/philipnolan1/status/1515260865892732928
https://twitter.com/Yoda4ever/status/1519881660744962049

15

Physiology & Medicine / Re: What is the best blood pressure monitor for home use?

« on: 26/05/2022 15:11:11 »

My average BP:

Measured by the NHS, 1985 to 15.9.20: 127
Measured at home, 15.9.20 - 26.5.22: 122
Measured by the NHS, 15.9.20 - 26.5.22: 151

16

Marine Science / Re: Why is the ocean salty?

« on: 26/05/2022 14:10:16 »

There are salts in the soil which get flushed into the rivers and then the sea by rainfall. When the sun evaporates the water to form more clouds and rain the salt gets left behind in the sea, so if you repeat that cycle for billions of years the salt in the sea becomes more and more concentrated.

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That CAN'T be true! / Re: holcomb generator?

« on: 25/05/2022 22:18:59 »

I can't believe people can buy into this

Many people do not have even a basic understanding of science.

It's not so long ago since I saw a university biology lecturer with a PhD post this video of a perpetual motion machine on Twitter asking if it was genuine.

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Physics, Astronomy & Cosmology / Re: How can I pass a large DC current through mercury?

« on: 24/05/2022 11:47:33 »

If you're trying to drive the mercury along the length of the groove in the plastic, with copper electrodes 1cm apart at either side of the groove as you describe, then:

The resistance of the mercury will be 19μΩ, not 4.8mΩ;
the voltage required to drive 1kA will be 19mV, not 4.8V;
the power will be 19W, not 4.8kW;
and the temperature of the mercury will rise at 0.2°/s, not 51°/s.

I'll leave someone else to calculate how many seconds it takes to eject the mercury.

19

Physiology & Medicine / Re: Why do I find it easier to run up steep hills than walk up them

« on: 21/05/2022 18:42:04 »

A walking stance begins to feel uncomfortable and I have to bend my back forward a lot

You're not making the mistake of trying to keep your heels on the ground, are you?

20

Physics, Astronomy & Cosmology / Re: Why is leg room on transport so limited ?

« on: 09/05/2022 01:14:02 »

To cram more passengers in obviously.

If you increase seat spacing and have fewer seats you lose customers on every full flight, but only benefit <1% of the population who didn't have enough legroom. It's not cost effective.

I'm 6'5", I could write a book about what it's like living in a society of shortarses, but what's the point? Like it or lump it.