1

Physics, Astronomy & Cosmology / Re: What's the brightness of a star 16 times wider but half as hot as the Sun?

« on: 21/04/2021 17:17:59 »

A sphere with a radius of 1 meter has a surface area of 12.57 square meters.

I think you are not being irrational enough.

2

Physics, Astronomy & Cosmology / Re: What's the brightness of a star 16 times wider but half as hot as the Sun?

« on: 20/04/2021 21:24:43 »

star with 16 times the radius of the Sun would have a surface area 255.926 times as high

Really?
I'd have gone for 16 times.

Though there's another complicating factor.
The spectrum emitted will change.

Imagine that the "sun" was at 700K
It would emit virtually no visible light.
But, if you heat it to 1400K it will glow quite brightly.

"Brightness" isn't well defined here.

However, looking at the numbers, I'm pretty sure that the answer they expect is 16.

3

New Theories / Re: New explanation of the working principle of the homopolar motor made by Faraday!

« on: 20/04/2021 11:33:11 »

He really liked your impoverishment.

4

Physics, Astronomy & Cosmology / Re: Can you measure the one way speed of light without synchronised clocks?

« on: 18/04/2021 15:10:46 »

That's interesting. The Wiki article references the idea that the method can be considered a "doppler shift".
Well, maybe it can or not- I guess it depends on your definitions.

But there's a streetlight at the end of the road and it's not been replaced by LED yet, so it is a sodium lamp.
And I can put a sodium lamp in a car and drive towards the streetlight.
And, in principle, i can allow light from both curses to fall on a photodiode and I will get a voltage from that diode corresponding to the difference in frequencies - one of my local sodium lamp and the other of the doppler shifted streetlight.

Since I know my speed and the beat frequency and the wavelength of the light, I think that's enough to let me calculare teh speed of light.

Equivalently, I can use a radar "speed gun" and a vehicle with a known speed to measure the speed of light.


Does that count as "one way"?

5

That CAN'T be true! / Re: Are electric cars environmental greenwash?

« on: 16/04/2021 11:14:51 »

The gas grid presently holds and distributes 35% of the UK's total energy. Until 1963, 50% of the gas was hydrogen.  It has been economically feasible and indeed profitable since 1790.

Why did they stop distributing hydrogen?

They moved from "town gas " to "North Sea Gas", largely because it was cheaper.

https://en.wikipedia.org/wiki/Coal_gas#Conversion_to_natural_gas

6

Physics, Astronomy & Cosmology / Re: How long could humanity survive if the sun went out?

« on: 15/04/2021 15:43:30 »

I'm guessing, about as long as the nuclear submarines can stay underwater.

7

Chemistry / Re: How do I calculate radioactive dose rate at a distance?

« on: 14/04/2021 22:49:26 »

Looks right to me.
I presume that you could, if required, show your working.

8

General Science / Re: Is Washing Dishes with Cold Water as Effective as Using Hot Water?

« on: 14/04/2021 18:07:14 »

Doesn't this evidence from the past, buttress the concept that hot water is the essential ingredient in dish-washing?

No.
Because it says that hot water isn't essential.
It says you can use newspaper instead and then wash them in cold water..

Did you read what you wrote?

9

General Science / Re: Is Washing Dishes with Cold Water as Effective as Using Hot Water?

« on: 13/04/2021 13:29:21 »

it usually takes really high temperature to intensify the reactions.

No, it doesn't.


However the biggest effect of using hot water is probably that it melts the fats.
It's easier to rinse away a liquid than a solid.

10

Chemistry / Re: what is this molecule please?

« on: 11/04/2021 18:49:56 »

Well, most of it is tetracycline.
https://en.wikipedia.org/wiki/Tetracycline
But there's a strange oxime derivative bit at the bottom left.

11

The Environment / Re: Why is it that when green energy is discussed, geothermal is never mentioned?

« on: 08/04/2021 22:08:45 »

Even when the oil runs out, there's hydrogen.

There are no hydrogen mines.

But how could cars be powered by "geothermal energy"?

Use the heat to generate steam, use that to spin a turbine, use that to drive a generator and use the electricity to electrolyse water to make hydrogen or charge batteries.

Pretty much the same way as we use nuclear power.

12

Physics, Astronomy & Cosmology / Re: What causes nuclear power plant meltdowns?

« on: 04/04/2021 11:55:04 »

Chernobyl surely should have had an off button

All power reactors have an "off" button, which stops the nuclear chain reaction.

However, the residual heat continues to produce around 6-7% of the reactor's output power, even after the fuel rods are fully inserted. This drops below 1% after a day or so.

Chernobyl off button was what killed it, the quick insertion of  the control rods blew it up, because they had a moderator at the end. Steam pockets ensued and not a good outcome!

If I (being very careful of the language I use, nuclear reactors and explosions I'm sure flag up at gchq) rendered a reactor incapable of being controlled by consoles, could I cut a rope to drop the control rods? If one control rod jammed could I drop the rest? Would the reactor have passive cooling capability? A clever design in  pipework(s) to a radiator(s) radiator on the roof(s) would be good. Nuclear reactors seem not to have the aarospace standard of duplicates and triplicate.

They had triplicate systems.
They failed to take account of "common mode failure".

All three cooling systems were washed out at once by a single common cause.
It would also have washed the radiators off the roof..

So a quadruplicate system may not have worked any better.

13

Physics, Astronomy & Cosmology / Re: What causes nuclear power plant meltdowns?

« on: 04/04/2021 11:52:03 »

"Fission" involves the artificial and unnatural splitting of atoms of heavy elements such as Uranium. 

Why do you think it is unnatural?
https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

14

Physics, Astronomy & Cosmology / Re: What causes nuclear power plant meltdowns?

« on: 02/04/2021 13:00:58 »

The exercise was executed as planned, so this was not a matter of "pilot error" but must be seen as deliberate sabotage.

No.
Because sabotage has some sort of point to it- financial, political whatever.
They just screwed up. But there is a reason for that.

The German military command told their troops that Enigma was uncrackable. The troops believed them and got "sloppy"- which is how Enigma was cracked.

The Designers said the Titanic was unsinkable, so the captain didn't prioritise the lookouts.
So nobody saw the iceberg until it was too late.

The managers of the space shuttle project thought that the risk of failure was "One in a million"- and they still thought that after the first failure...


The guys running Chernobyl had been told "RBMK reactors are relatively simple and generally problem-free ". or as they put it "safe as a samovar".

We know what happened next...

15

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

« on: 01/04/2021 21:55:47 »

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

It is not the answers' fault that you are not convinced.

16

Question of the Week / Re: QotW - 21.03.29 - Why does chilli stay on your hands so long after cooking?

« on: 01/04/2021 20:20:05 »

Beans were farty long before Blazing saddles was filmed.
https://en.wikipedia.org/wiki/Pythagoreanism#Vegetarianism

These days, we even know why they are windy.
https://en.wikipedia.org/wiki/Baked_beans#Flatulence

17

Physics, Astronomy & Cosmology / Re: What causes nuclear power plant meltdowns?

« on: 01/04/2021 10:29:14 »

"What usually causes nucular power plant meltdowns?"
Nothing.
That's why they don't "usually" happen.
They are actually very rare.
Wiki records just 3 major ones and another score of minor ones.
Since 8 of those were in Russian subs, you can draw your own conclusion about "what causes meltdowns".

https://en.wikipedia.org/wiki/Nuclear_and_radiation_accidents_and_incidents#Nuclear_meltdown

18

The Environment / Re: What would happen if all of humanity vanishes in one second?

« on: 29/03/2021 19:50:15 »

With conservation of matter, everything we have ever dug up out of the earth still exists somewhere in one form or another. 

But with the exception of gold- which we dig up in many places, but then bury again at the national banks, we dig up concentrated deposits of minerals, and then distribute them all over the place.

It's going to be a challenge to find shallow coal for a long time.

19

The Environment / Re: What would happen if all of humanity vanishes in one second?

« on: 29/03/2021 19:41:27 »

A lot of cars would crash quite quickly. The planes that were in the air at the time would take rather longer to fall out of the sky.
Some ships might sail on for days or weeks before they hit something, or ran out of fuel.

20

New Theories / Re: Any theories in physics you don't agree with?

« on: 29/03/2021 11:14:23 »

A theory can still be shown to be false

At that point, it stops being a scientific theory.