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Physics, Astronomy & Cosmology / Re: How do you calaculate the capacitance of two unsually shaped plates?

« on: 21/03/2022 17:46:38 »

The quick way is to calculate the capacitance of the wide end, then either triple it if the spacing is 2mm as in the diagram, or quadruple it if the spacing is 3mm as in the text.

2

The Environment / Re: Why is the air inside a car worse than outside?

« on: 07/11/2021 00:03:30 »

Numerous experiments have shown that cyclists riding in rush hour traffic are breathing cleaner air that the occupants of cars. The explanation offered is usually that the air intakes on the cars are closer to exhaust level than the cyclists face.
https://www.sciencedirect.com/science/article/abs/pii/S0048969701007586
https://can.org.nz/system/files/Research-0402-Emissions.pdf

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Technology / Re: What are some low-tech ways to address climate change?

« on: 06/11/2021 18:56:30 »

like population control, limiting meat-based agriculture/diet, not clear-cutting our forests, different approach to transportation, less materialism etc.

The problem with any of these actions is that it makes you less fit. A country that limits births and keeps its forests will be taken over by the countries that burn the candle at both ends and consume the resources declined by the country doing the better thing. So my contribution is a world government with real authority, that has yet-to-be-identified goals in mind and not necessarily the immediate comfort of voters. Thus it will probably not consist of elected leaders. Such a thing would form if there were a common enemy such as aliens, but we apparently don't consider climate change to be such a common enemy.

I've been arguing this for years, the only way out of a Prisoners' Dilemma is an authority that has the power to enforce Cooperation rather than Defection. This is why I'm convinced that there will be no meaningful action taken on climate change until it's already too late, people won't accept any policies that compromise their own lifestyle until it's their own lifestyle being compromised by the climate.

Recently I have been wondering if it might produce a bit more action to have conferences where the speaker is obliged to say what they've already done rather than what they're promising to do, along with a big display on the wall indicating how far ahead or behind their quota they are.

4

Geology, Palaeontology & Archaeology / Re: Is this possible?

« on: 21/10/2021 18:39:23 »

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

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General Science / Re: How did a dead Duracell AA battery come back to life in my clock?

« on: 14/10/2021 13:01:18 »

Deja vu

https://www.thenakedscientists.com/forum/index.php?topic=81103.msg620803#msg620803

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General Science / Re: How can a plane fly upside down

« on: 28/07/2021 15:04:42 »

https://en.wikipedia.org/wiki/Lift_(force)#/media/File:AirfoilDeflectionLift_W3C.svg

That is very true for initial tack off and for rapid clime but how can this work when inverted. And it can.

Hold your hand flat, like a wing, and stick it out of the window of a moving car (without letting people think you're signalling a turn). You can feel the air pushing your hand up or down as you adjust the angle of attack.

Look at the strange attitude of the fuselage here because the pilot has had to keep the wing angle of attack the same in space, but reversed relative to the plane, the green line shows the horizontal, but the black line is where the horizontal normally lies when the plane is flying the right way up. If the pilot had kept the fuselage nearer horizontal, as it is when not inverted, the wings would have been pushing the plane toward the ground.

Inverted.png (104.38 kB . 369x218 - viewed 7903 times)

7

General Science / Re: How would our weight differ on a revolving to that of a non-revolving earth.?

« on: 26/07/2021 12:37:38 »

I was quoting directly from

https://www.sfu.ca/~boal/211lecs/211lec12.pdf

Hmm, OK.

I follow the maths which concludes with
"ε ~ 0.0017 x 57.3 = 0.1 of a degree"
which is the figure I got. But then having arrived at that, they throw in
"because the rotational motion affects both the surface of the Earth as well as the bob, then S remains perpendicular to the Earth’s surface"
without further explanation, which is the bit I don't follow.

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Geology, Palaeontology & Archaeology / Re: What is the difference between a mountain and a hill?

« on: 26/07/2021 11:32:35 »

There is no universally accepted quantitative definition of mountain and hill.
The OED definitions are as follows:

Mountain: "A large natural elevation of the earth's surface, esp. one high and steep in form (larger and higher than a hill) and with a summit of relatively small area."

Hill: "A natural elevation of the earth's surface rising more or less steeply above the level of the surrounding land. Formerly the general term, including what are now called mountains; after the introduction of the latter word, gradually restricted to heights of less elevation; but the discrimination is largely a matter of local usage, and of the more or less mountainous character of the district, heights which in one locality are called mountains being in another reckoned merely as hills. A more rounded and less rugged outline is also usually connoted by the name. In Great Britain heights under 2,000 feet are generally called hills; ‘mountain’ being confined to the greater elevations of the Lake District, of North Wales, and of the Scottish Highlands; but, in India, ranges of 5,000 and even 10,000 feet are commonly called ‘hills’, in contrast with the Himalaya Mountains, many peaks of which rise beyond 20,000 feet. The plural hills is often applied to a region of hills or highland; esp. to the highlands of northern and interior India."

Anyone who wants to start counting them quickly realises you need to define the minimum depression and horizontal separation between adjacent summits in addition to the height above sea level, otherwise you would end up counting every pebble and stone on the summit as a separate hill. There are numerous lists in the UK, Munros, Nuttalls, HEWETTs, FRCCs, Marilyns, etc, each of which have their own definitions, and then there's the Wainwrights, which are completely subjective. (In the case of the Marilyns, there is no minimum height, just a minimum depression of 150m.)

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Geek Speak / Re: Why the median does not change?

« on: 14/06/2021 11:54:22 »

A common example where the median is preferred over the mean is wealth, because the mean gets distorted by a tiny number of mega-rich people.

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Plant Sciences, Zoology & Evolution / Re: How can I keep my grass short?

« on: 31/05/2021 18:32:22 »

The south side of my lawn started sprouting moss after the neighbour put up a six foot fence blocking most of the sunlight. After a few years it's now a thick carpet of moss with a few sparse wisps of grass poking through it.

11

Technology / Re: Microwave side door gap, is it safe?

« on: 01/04/2021 21:50:43 »

I'd like to come back to this question - why do microwave ovens make a loud noise?

So far, I haven't seen any convincing answers.

You've had the answer, you just don't want it. The fan makes a loud sound of rushing air, then there's a hum from the transformer and the solenoid around the magnetron.

If you switch the oven onto a reduced power you'll hear the timer clicking in and out as it reduces the duty cycle, and you'll hear the hum whilst the magnetron is running, but not when the magnetron is off. Note how the "roaring noise" remains when the magnetron is off: that's because it's coming from the fan and not the magnetron.

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Technology / Re: Microwave side door gap, is it safe?

« on: 01/04/2021 20:19:43 »

why does the magnetron make a noise?

It doesn't. It's a thermionic valve, there are no moving parts.

It has a large, rapidly varying current running through it, and a magnet wrapped round it.
Since it's not infinitely rigid, it will vibrate.

The power transformer feeding it will also make a noise.

Yes, but that's a hum, not a "roaring noise".

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Physics, Astronomy & Cosmology / Re: What is the current and pd?

« on: 12/02/2021 14:34:33 »

Firstly, the usual rule for exam questions is that if you aren’t given information, it’s because you aren’t expected to use it. Secondly, considering the trivial nature of the exercise, and the non-triviality of solving it analytically with a diode characteristic, I’d consider that to be a hint, too. Thirdly, my experience of a career in electronics is that you rarely if ever get all the information you want, but the boss won’t be very impressed if you just throw your hands in the air and tell him it’s all hopeless, and there’s no point in doing anything. Generally, you’re expected to do what’s reasonable and defensible with the information you have, and to state what assumptions you have made, if any.

No manufacturer’s data sheet I’ve ever seen specifies the diode characteristic numerically as a polynomial, they will normally specify voltage at one or two currents and temperatures, and include a graph or two. Since the voltage across a diode is similar between types, and remains relatively independent of current, it is usual to assume the voltage is a fairly constant standard for most purposes. Indeed in many applications it would be considered bad design if the diode characteristic were critical.

Assuming a constant voltage drop across the diode leaves a simple exercise that can be solved by the application of Ohm’s and Kirchoff’s laws. Alternatively, a typical diode characteristic in graphical form can be used to solve the exercise by drawing the load line on the characteristic to find the operating point at the intersection of the two lines.

14

COVID-19 / Re: Will delaying the second vaccine jab affect how well it works?

« on: 26/01/2021 13:59:08 »

Have Pfizer published their argument?

It seems to me that it would revolve around whether giving half rations risks breeding a vaccine-resistant virus similar to antibiotic resistant bacteria from under-dosing antibiotics.

15

Physics, Astronomy & Cosmology / Re: Why do Microwave Oven Plates Spin ?.

« on: 03/01/2021 15:13:49 »

No, it's not vital, my first microwave had no turntable, and yes, it is to try to make the food cook more evenly.

The microwaves come from one side (usually the plastic covered aperture on the right), but then get reflected around off the inside of the oven, so at any point there will be multiple waves reflected from different parts of the oven walls. When two waves are incident on the same point in space the effect depends on their relative phase, if they are in phase they add up to one bigger wave, and if they are out of phase they will tend to cancel each other out. There are an infinite number of possibilities in between which will give rise to all sorts of other levels too, two waves equal in amplitude and out of phase will cancel each other out completely. This means that there will be different levels of microwave power all over the oven: hot spots and 'cold' spots. The idea is that if you keep the food moving each part will get a share of the cooking.

It doesn't work very well in my experience, anything that isn't a liquid, and can't be stirred frequently, doesn't cook evenly. I gave up thawing/cooking chicken breasts a long while ago, but it's great for reheating stews/casseroles etc.

https://en.wikipedia.org/wiki/Wave_interference#/media/File:Interference_of_two_waves.svg

16

Technology / Re: The most advanced space technology you ever heard about

« on: 01/01/2021 15:03:31 »

Voyager 1 is the space technology that has advanced furthest from earth so far.

17

That CAN'T be true! / Re: I don’t understand physics: does anyone understand physics these days?

« on: 27/12/2020 18:27:26 »

I love physics at the Newtonian mechanics end of the scale, but the cutting edge stuff leaves me disillusioned and impatient at the slow progress.

18

General Science / Re: Frankenstein's Battery

« on: 06/12/2020 17:49:23 »

The test strips on the battery apply quite a heavy load, so that they won't indicate the battery is good when it hasn't sufficient power to operate the heaviest load the manufacturer thinks you may want to use it in. Clocks, on the other hand, use an absolutely minimal amount of power, which is why batteries in them last for years. I save all my AA and AAA cells when they're too flat to run the appliance they were used in, and then use them in the mantlepiece clock. A 'flat' battery will run the clock for many months, particularly if it had previously been used in a heavy load. The test strip is telling you nothing.

Batteries operate more efficiently when they're warm, so it is possible that a slight warming of the room might tip the voltage over the threshold at which the clock starts working. It's unlikely though, if it's been standing for years the self discharge of the battery would have taken the voltage low enough for it not to re-start with a slight temperature increase.

My money says it's almost certainly an intermittent fault, and not a flat battery. The clock stopped working because of the fault, started working spontaneously because that's what intermittent faults do, and then stopped again when you disturbed it.

Intermittent faults can be the bane of an electronics engineer's life, because if the symptoms won't present themselves when you're trying to find the fault, there's a limit to what you can do to root them out. Many intermittent faults boil down to loose/broken/corroded components or connections, in which case heating, cooling, thumping, prodding and vibrating are usually the first resort, not least because they're quick and simple. As well as thermal expansion disturbing a dodgy connection, temperature variation can also affect component parameters, as can supply voltage, but that's less likely in this case.

On a clock, the only external connections you'll find are the battery terminals, if they look ok and aren't corroded, you won't get any further without pulling it apart. That might be easier said than done though, they're not usually designed to be serviced, and might not come apart without breakage. Inside, you're likely to find components that are too small to repair without considerable skill and experience. A cracked chip component for example is likely to be a lost cause, because even if you had the skill to replace it, you won't have any means of establishing its value once it's broken, and without instruments.

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Question of the Week / Re: QotW - 28.11.19 - Do hairs grow out grey or turn grey?

« on: 12/11/2020 15:36:46 »

My hair grows brown, then turns grey, if I pluck a hair it's easy to see that the root and tip are different colours. It's also conspicuous that I come out of the barbers less grey than I went in.

20

Physics, Astronomy & Cosmology / Re: How does static electricity work?

« on: 11/06/2020 23:59:48 »

The current charging the handle capacitance is tens of amps for a few ns, the current leaking to ground through the door is 100uA for 20ms, my guess is that it's the former that you feel, not the latter.