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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.

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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.

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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

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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.

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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.

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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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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.

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Physiology & Medicine / Re: How do you think thoughts happen?

« on: 01/06/2020 21:51:25 »

AFAIK nobody has ever produced a useful definition of consciousness, so anything approaching an "explanation" is way over the horizon.

Functionally, the brain has a remarkable associative memory compared with any artificial device, but the difference seems to be quantitiative rather than qualitative. 

How do you define "useful"? Is dictionary's definition not useful enough?

At the time John Maddox's wrote What Remains to Be Discovered, there wasn't even a definition of life, and I don't think there is now either. There are a few words that everyone has an intuitive understanding of, but that are all but impossible to define formally. The dictionary is usually fatuous and circular:
Life: the state of being living rather than dead.
Energy: capacity to do work (Work: using energy)
Time: passing from the past into the future (Future: time yet to come)
Consciousness: awareness (Aware: cognizant, Cognizant: aware)

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Physiology & Medicine / Re: Should we take a small aspirin everyday?

« on: 27/04/2017 17:48:44 »

However, some heart conditions can lead to excess formation of blood clots, which could pass to the brain and also cause a stroke or heart attack.

Which ones?

Atrial fibrillation

I wondered if evan_au meant AF, but aspirin is no longer recommended for AF as the benefits are minimal, and the bleeding risk is just as high as with Warfarin and other anticoagulants.

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General Science / Re: Does scientific radicalisation exist?

« on: 19/12/2016 11:26:56 »

Bad capitalism: creating an artificial market for unnecessary crap. Can be harmless (perfume and jewelry  advertisements), self-limiting ("art"), mildly dangerous (illegal addictive drugs) or very dangerous (legal addictive drugs).

Good capitalism: bringing useful products within reach of the majority in a competitive market. Generally better than a controlled economy because you can innovate on a whim and the market will eliminate weak products (in principle).

These morph from one to the other over time. For example, good capitalism brings a product such as mobile phones to the market, but sooner or later the market becomes saturated, and then manufacturers have to devise ever more ways of inducing people to throw away serviceable products and buy new ones. When that extends to a large part of the economy that's bad for both the environment and society.

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Physiology & Medicine / Re: Are nationalism and xenophobia instincts that have the same root ?

« on: 03/12/2016 15:07:10 »

what is best for me conflicts with what is best for everyone?

That's the Prisoners' Dilemma:

https://www.amazon.co.uk/Prisoners-Dilemma-Neumann-Theory-Puzzle/dp/038541580X/ref=sr_1_1?ie=UTF8&qid=1480777598&sr=8-1&keywords=poundstone+Prisoner%27s+dilemma
https://en.wikipedia.org/wiki/Prisoner's_dilemma

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General Science / Re: Why Does Hair Get Darker When Wet ?

« on: 15/09/2016 14:23:42 »

....or any other fibres for that matter. It's interesting that a grey jumper will get darker when it's wet, but a white T-shirt doesn't.

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General Science / Re: How radioactive are people?

« on: 14/12/2015 22:51:53 »

Three years ago I was injected with technetium for a perfusion scan, they tell you to keep away from pregnant women and children for a day or two after that.  []  []