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Physiology & Medicine / Re: Could electricity be used as an alternative treatment for destroying cancer?
« on: 27/11/2019 23:43:07 »
RF ablation is already used to treat cancer (among other things).
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Physiology & Medicine / Re: Is the continuous application of anti-perspirant to your armpits dangerous?
« on: 08/11/2019 13:52:10 »
Dunno about dangerous, but I made myself allergic to the stuff by putting it on broken skin.
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Physiology & Medicine / What's the risk of over inflation during a colonoscopy?
« on: 04/11/2019 13:01:39 »
Does anyone know the incidence of bowel damage from over inflation during a colonoscopy. The information sheet sent with appointments doesn't list over inflation among the potential complications, and I can't find any reference to it anywhere from Dr Google.
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General Science / Re: Why do we sneeze when we look at the sun?
« on: 23/10/2019 22:24:27 »
But what makes an orgasm trigger a sneezing fit?
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General Science / Re: Can you use unlimited mini hydro generators on the end of a faucet?
« on: 23/10/2019 22:15:14 »Hydroelectricity is indeed free of charge as long as you collect the water in your own reservoir at the top of your own mountain and move it through your own pipes. It may be worthwhile comparing your domestic piped water charges with your electricity bill.
UK domestic water costs about £4 per tonne (you pay more for the sewerage to take it away than you do for delivering the clean stuff!). Assume you have a 5 m pressure head from a 1000 liter brake tank in the loft. 1 tonne falling through 5 m will deliver
m g h = 1000 x 9.81 x 5 = 49,050 joules = 13.625 Wh for £4, about 30,000 times the cost of the same amount of domestic electricity.
If you drive your turbine direct from the water main, at say 5 bar pressure, you will reduce the cost to about 3,000 times the cost of mains electricity.
Some people in the UK can use as much water as they like for a flat annual charge. Many years ago I pondered the idea of using it to generate electricity as a provocative way of demonstrating to the water board what a daft tariff system it is. Unfortunately, I had already switched to a water meter by then.
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General Science / Re: Why did grounded Faraday cloth behind my headboard increase EMF signals?
« on: 19/04/2019 18:25:59 »
You have a space in which there are multiple sources all coming from different directions, they then reflect off anything and everything creating multipath interference. Hanging up your conductive curtain serves to do little other than stir the pot, and reflect the signals in a different pattern. A bit like holding up a mirror in a room full of lamps: as fast as you cast a shadow from one lamp you reflect another.
Your detector won’t show this because it’s not selective, it’s measuring the total field and unable to see what’s going on with each source separately. Imagine a classroom full of schoolkids: it’s bedlam because they’re all sat rabbiting. You wonder where the din is coming from, so you take out a gag and place it on one of the kids. You hear no difference, so you take it off and try the next kid, then the next and the next. By the time you’ve been round the whole class you’ve come to the erroneous conclusion that the noise isn’t coming from any of them. What you need is the ability to listen to each one independently so that you can see what effect you’re having on each source one at a time, and in your room, that’s what a spectrum analyser does: you would see a forest of signals, and when you move the detector, or the large objects in the room, the amplitude of the signals would bounce up and down like yoyos.
Failing that, a gag that fits all the kids at once will do, that’s where the suggestion of aluminium foil on the walls comes in. Your problem though is creating a seal: no window, a continuous connection at the seams (including around the door), a sealed air vent, and last but not least, if you want power in the room you need filtering to stop signals being carried in on the power leads.
Or alternatively, you could just sit and wonder why you’re so bothered about it in the first place.
Your detector won’t show this because it’s not selective, it’s measuring the total field and unable to see what’s going on with each source separately. Imagine a classroom full of schoolkids: it’s bedlam because they’re all sat rabbiting. You wonder where the din is coming from, so you take out a gag and place it on one of the kids. You hear no difference, so you take it off and try the next kid, then the next and the next. By the time you’ve been round the whole class you’ve come to the erroneous conclusion that the noise isn’t coming from any of them. What you need is the ability to listen to each one independently so that you can see what effect you’re having on each source one at a time, and in your room, that’s what a spectrum analyser does: you would see a forest of signals, and when you move the detector, or the large objects in the room, the amplitude of the signals would bounce up and down like yoyos.
Failing that, a gag that fits all the kids at once will do, that’s where the suggestion of aluminium foil on the walls comes in. Your problem though is creating a seal: no window, a continuous connection at the seams (including around the door), a sealed air vent, and last but not least, if you want power in the room you need filtering to stop signals being carried in on the power leads.
Or alternatively, you could just sit and wonder why you’re so bothered about it in the first place.
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Technology / Re: Why are Some Fridges Hard To Open Again Once Just Closed?
« on: 19/04/2019 18:20:28 »I drove my parent's Ford Torino off the road once due to a stall turning at an intersection. Impossible to steer or stop.I've always been sceptical of accounts like this.
I had a Capri which developed an intermittent fault on the brake servo so that you didn't know whether you were going to get assist until you put your foot on the pedal. It made me jump sometimes, but it was no problem. Just adjust foot pressure accordingly. I continued driving it for a while, until it was convenient to take it to the garage.
When I bought a Cortina with PAS I tried turning off the engine just to see what it was like unassisted. No problem.
The power steering on my Honda Accord once failed suddenly when the crank pulley broke in the middle of a busy gyratory system during the rush hour. It wasn't any problem, I just shoved the wheel harder. When I bought the Accord (with cruise control) I kept getting told stories of how people had been swept along the motorway unable to stop after their cruise controls failed. There was never any explanation of why they would allow that to happen rather than put their foot on the clutch and take it out of gear.
Systems are called fail-safe because they're safe when they fail.
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Geek Speak / Re: Why is my Buffalo external HDD creating files that don't exist?
« on: 19/04/2019 16:50:33 »
I've been kicking this can down a longer road than Theresa May's road to Brexit. I bought a new disk, but before it arrived the backup one started doing the same thing. It had been trouble free for four years, but I'd always sat watching whilst it backed up. On one occasion I left the backup to run by itself, and when I came back it had finished, and the computer had gone into sleep mode. That was when that disk started misbehaving like the other one.
I've just been carrying on and hoping that I'll be able to get a working copy off somewhere if something fails. Much of it is backed up on a third disk, and the originals of the really irreplaceable stuff are on DVD too. I could spend hours assembling a new set of files on the new disk, but I'm not sure I can be bothered if that one's going to do the same as soon as I forget the computer until it sleeps.
Looking online, it seems a common problem, and most people seem to be doing as I have: shut the computer down in order to unplug. As I said above, it's a bit disconcerting that I can hear the head click at the last second when the computer power goes off though. Not much hint of an orderly shutdown there. Sometimes I've been able to reset it by switching the computer into sleep mode and back again, bizarrely.
I keep telling myself I'm going to pick through, and put some of the non confidential material back on the computer, but it's a bit like unbaking a cake, and it gets put off. Meanwhile, I have a merry-go-round: files get temporarily stored on computer in a 'Pending Transfer' folder, then onto HDD, then backup, then I make an SDHC read-only copy to leave in the computer for day to day reference (most of the usage is read only). I actually prefer the convenience of SDHC, if I thought the life was adequate, I'd use it as the main working copy.
I've just been carrying on and hoping that I'll be able to get a working copy off somewhere if something fails. Much of it is backed up on a third disk, and the originals of the really irreplaceable stuff are on DVD too. I could spend hours assembling a new set of files on the new disk, but I'm not sure I can be bothered if that one's going to do the same as soon as I forget the computer until it sleeps.
Looking online, it seems a common problem, and most people seem to be doing as I have: shut the computer down in order to unplug. As I said above, it's a bit disconcerting that I can hear the head click at the last second when the computer power goes off though. Not much hint of an orderly shutdown there. Sometimes I've been able to reset it by switching the computer into sleep mode and back again, bizarrely.
I keep telling myself I'm going to pick through, and put some of the non confidential material back on the computer, but it's a bit like unbaking a cake, and it gets put off. Meanwhile, I have a merry-go-round: files get temporarily stored on computer in a 'Pending Transfer' folder, then onto HDD, then backup, then I make an SDHC read-only copy to leave in the computer for day to day reference (most of the usage is read only). I actually prefer the convenience of SDHC, if I thought the life was adequate, I'd use it as the main working copy.
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Geek Speak / Re: Why is my Buffalo external HDD creating files that don't exist?
« on: 04/08/2018 17:14:29 »
OK, thanks all.
I have back ups, so I could reformat the dodgy disk , but as I'm not sure how long the problem has been developing or how extensive it is, I might have been backing up duff data. The HDD came with s/w pre-loaded, will I lose that if I clear it? If Recuva needs a different destination anyway, I need a new disk, so I might as well buy one and see what I can assemble from backups onto that. If I wait for a while to see what develops I can then have a go at salvaging the old one and keep it as a spare maybe rather than burn my boat in haste.
My first two disks were for backup, but as I mentioned, this one was to offload confidential working data, so it was spending protracted periods plugged into the computer unlike the others. I wondered if the original problem with ejection might have been precipitated because it stayed plugged in whilst Windows (7) went into sleep mode when the laptop was left unused. Should it matter? Also the light on the HDD sometimes continues flashing for a few seconds after the computer has confirmed it's safe to unplug. I didn't notice at first, so that might be another possibility. Even when I do wait, I sometimes hear a click as if the write head was active at the instant the computer switches the power off, I don't recall either of the previous drives doing that (also Buffalo).
It was becoming a nuisance for portability having the HDD dangling from the computer, so I've been toying with using an SDHC instead, so that it can stay inserted whilst I carry the laptop around. I'm using one as a read-only reference copy of the most useful data at the moment, but I now have the inconvenience of maintain two copies and still fitting the HDD when I need to write. I gather the write life of flash is limited, and assume SDHC ports don't have wear levelling like SSDs, but how limited?
I have back ups, so I could reformat the dodgy disk , but as I'm not sure how long the problem has been developing or how extensive it is, I might have been backing up duff data. The HDD came with s/w pre-loaded, will I lose that if I clear it? If Recuva needs a different destination anyway, I need a new disk, so I might as well buy one and see what I can assemble from backups onto that. If I wait for a while to see what develops I can then have a go at salvaging the old one and keep it as a spare maybe rather than burn my boat in haste.
My first two disks were for backup, but as I mentioned, this one was to offload confidential working data, so it was spending protracted periods plugged into the computer unlike the others. I wondered if the original problem with ejection might have been precipitated because it stayed plugged in whilst Windows (7) went into sleep mode when the laptop was left unused. Should it matter? Also the light on the HDD sometimes continues flashing for a few seconds after the computer has confirmed it's safe to unplug. I didn't notice at first, so that might be another possibility. Even when I do wait, I sometimes hear a click as if the write head was active at the instant the computer switches the power off, I don't recall either of the previous drives doing that (also Buffalo).
It was becoming a nuisance for portability having the HDD dangling from the computer, so I've been toying with using an SDHC instead, so that it can stay inserted whilst I carry the laptop around. I'm using one as a read-only reference copy of the most useful data at the moment, but I now have the inconvenience of maintain two copies and still fitting the HDD when I need to write. I gather the write life of flash is limited, and assume SDHC ports don't have wear levelling like SSDs, but how limited?
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Geek Speak / Why is my Buffalo external HDD creating files that don't exist?
« on: 03/08/2018 11:39:10 »
I have an external Buffalo PNFU-3 HDD which has started going doolally, and everything I do is compounding the mess.
It started with a message HDD 'still in use' when I try to eject it. I circumvented that by shutting computer down, then unplugging USB.
Now I have it creating phantom files that aren't there and not listing files that are there.
Eg:
Folder FFF contains a file XXXA. If I search for XXX it lists two files XXXA & XXXB. If I open location for XXXB it opens FFF which only lists XXXA. If I try to open or delete XXXB it says "error: not contained in FFF".
FFF contents lists XXXA with correct date & time, but search lists both XXXA & XXXB with date & time from XXXB
So I tried emptying all other files from FFF into a temp folder TTT, then deleted FFF, & renamed TTT>>FFF. Now XXXA & XXXB are both appearing on search, but neither are in FFF.
I'm rapidly getting to a situation where I'm losing track of what's rubbish and what isn't. I daren't back it up because I don't know whether I'm copying drivel, I can't fix it myself, but I can't take it to a repairer because it contains confidential data.
Ironically, I only bought the HDD so that I could get confidential data off the computer in case it ever needed to go for repair.
Any suggestions welcome before I scrap it altogether and go with the backups. Thanks.
It started with a message HDD 'still in use' when I try to eject it. I circumvented that by shutting computer down, then unplugging USB.
Now I have it creating phantom files that aren't there and not listing files that are there.
Eg:
Folder FFF contains a file XXXA. If I search for XXX it lists two files XXXA & XXXB. If I open location for XXXB it opens FFF which only lists XXXA. If I try to open or delete XXXB it says "error: not contained in FFF".
FFF contents lists XXXA with correct date & time, but search lists both XXXA & XXXB with date & time from XXXB
So I tried emptying all other files from FFF into a temp folder TTT, then deleted FFF, & renamed TTT>>FFF. Now XXXA & XXXB are both appearing on search, but neither are in FFF.
I'm rapidly getting to a situation where I'm losing track of what's rubbish and what isn't. I daren't back it up because I don't know whether I'm copying drivel, I can't fix it myself, but I can't take it to a repairer because it contains confidential data.
Ironically, I only bought the HDD so that I could get confidential data off the computer in case it ever needed to go for repair.
Any suggestions welcome before I scrap it altogether and go with the backups. Thanks.
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General Science / Re: Is a mass bouncing on a spring affected by gravity like a pendulum?
« on: 17/09/2017 17:25:28 »I think you are slightly misquoting me, what I said was "For a pendulum the restoring force is mg (vector)"I thought you were saying "For a pendulum the restoring force is mg (mg is a vector)....."
If it's left open for the user to decide how approximately equal, and how small, it can't not be true.Since sinϴ ≈ ϴ for small ϴ, then F ≈ mgϴ,But that isn't always true.
You are making an approximation. As @chiralSPO said, we often make approximations in order to simplify calculations, and that is valid in both practical and teaching situations.The question was why doesn't gravity affect the spring-mass system, and the answer is because gravity exerts a constant force and not one dependent on displacement. That's true regardless of any approximation in the linearity of the pendulum restoring force, and/or Hooke's law.
For a pendulum Sinϴ ≈ ϴ is only useful for ϴ<10°, although at 20° the error is only around 1%, for ϴ>22° the difference is noticeable.
Similarly for the spring/weight I did say "for a perfect spring/small oscillation" and so using a similar small deflection the variation of g is considered to be zero.
If you use approximations for one you ought to allow them for the other.
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General Science / Re: Is a mass bouncing on a spring affected by gravity like a pendulum?
« on: 16/09/2017 16:53:20 »For a pendulum the restoring force is mgThe restoring force on a pendulum is mgsinϴ.
Since sinϴ ≈ ϴ for small ϴ, then F ≈ mgϴ, that is, the restoring force is proportional to the displacement, which is the condition for simple harmonic motion.
The displacement of a spring is also proportional to the displacement (Hooke's Law), so that also produces SHM. The effect of gravity on the spring/mass is not proportional to the displacement, so it doesn't produce SHM.
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Physics, Astronomy & Cosmology / Re: Could you create, generate, and control a static electromagnetic field?
« on: 07/07/2017 13:46:12 »In fact a dynamo works by moving a conductor through a magnetic field which does work on the charged particles in the conductor which can then be used to generate energy in a circuit. But there's no need for an electric field in this case and therefore no electric potential.If I move a wire through a magnetic field there will be a voltage difference generated between the ends. If I were to place a speck of charged dust nearby, would it not be attracted to one end of the wire because there is now an electric field surrounding the wire?
Quote
I don't see what being a radio engineer has to do with anything.The relevance is that radio engineers all understand the terms EMF and PD in the context of a Thevenin source, as defined above.
Quote
An electromagnetic force (EMF) is defined as that which can produce energy to generate an electric currentOk, so how is that different from PD? Using my definitions of EMF and PD, I could construct a Thevenin source with an infinite resistance, such that the EMF can deliver energy but the PD cannot, would that not then satisfy your distinction between the two terms?
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Technology / Re: Why Can I NOT use an energy saving bulb in a dimmer switch light fitting ?
« on: 07/07/2017 00:32:08 »It's quite complicated isn't it? I could really do with a walk-through diagram to show what is happening at each stage of the waveform to understand how it works.If you install a (free) copy of LTspice, there's a dimmer in the examples folder which you can play with to your heart's content: http://www.linear.com/designtools/software/
This is a plot of relative power as a function of triac trigger angle:
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Physiology & Medicine / Re: Is the apparent sudden rise in gluten intolerance real?
« on: 18/06/2017 18:19:46 »
There was a program about this on TV not that long ago.
Apparently the rise in gluten-free consumption is just a food fad with those who have a self-diagnosed gluten 'intolerance'. The interesting bit was the blind trial: people fed a gluten-free diet, with one group taking gluten as a supplement, and the controls taking a placebo. Sure enough, nobody could tell if they were on gluten or the placebo.
Apparently the rise in gluten-free consumption is just a food fad with those who have a self-diagnosed gluten 'intolerance'. The interesting bit was the blind trial: people fed a gluten-free diet, with one group taking gluten as a supplement, and the controls taking a placebo. Sure enough, nobody could tell if they were on gluten or the placebo.
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Physics, Astronomy & Cosmology / Re: Could you create, generate, and control a static electromagnetic field?
« on: 18/06/2017 18:09:13 »
I'm still quite lost, I'm not sure which bits PmbPhy is retracting and which not (if any).
FWIW, and digressing a bit from electrostatic fields, my usage of the two terms as a radio engineer is that EMF is conventionally the source voltage of a Thevenin source, and the PD is the terminal voltage. My point is that these are both voltages, with no difference in kind, and the terms merely distinguish between two different locations in the equivalent circuit. (Both would be the same magnitude of course when no current flows, as with an electrostatic field.)
Re: a charged particle, if I charged a speck of dust using a van der graaf, and then placed it between two parallel plates connected to a battery, would it not be attracted to one of the plates?
There's a difference in potential between two points in a static electric field but such a field cannot produce a steady current in a resistor whereas an EMF can.This seems to me a bit like arguing that there is a difference, in kind, between the metre used to measure the position of a point in empty space, and the metre used to measure the height of a reservoir that can generate power in a hydro plant. Distance is distance.
FWIW, and digressing a bit from electrostatic fields, my usage of the two terms as a radio engineer is that EMF is conventionally the source voltage of a Thevenin source, and the PD is the terminal voltage. My point is that these are both voltages, with no difference in kind, and the terms merely distinguish between two different locations in the equivalent circuit. (Both would be the same magnitude of course when no current flows, as with an electrostatic field.)
Re: a charged particle, if I charged a speck of dust using a van der graaf, and then placed it between two parallel plates connected to a battery, would it not be attracted to one of the plates?
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General Science / Re: Does A Fully Mirrored Room Require Less Light To Light It?
« on: 18/06/2017 17:21:02 »A couple of years ago m neighbour knocked on the door and pointed out that the front garden wall was a few inches over his side of the boundary. I forget how much, but the angle it made with the front of the house was about one degree off the normal. I told him that that's where it had been since the house was built, but he was still suspicious. He went quiet when I showed him that the rooms of the houses were even more off square though. I suppose if the mirrors are perfect, the light would keep going round until it found its way back to the flame eventually in any shape room though..........Quote from: vhfpmrBut a corner reflector returns a beam of light parallel to the incident beam, not convergent on the source.I agree - but then it will bounce off the diagonally opposite corner; won't this put it back on another parallel path, towards the candle flame?
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Physics, Astronomy & Cosmology / Re: Could you create, generate, and control a static electromagnetic field?
« on: 03/06/2017 12:48:29 »I believe that what Pete is saying is that a battery does not necessarily have an electric field about it. An electrochemical (galvanic) battery is not a capacitor, which has an EMF that is determined purely by a macroscopic electric field. Batteries instead have an EMF that is determined by microscopic electric fields (atomic scale), and so when completely isolated from a circuit, a battery is unlikely to have a significant dipole moment (uneven distribution of charge within the battery)
Imagine a simple electrochemical cell, containing a piece zinc metal at one end of a tube, and a piece of copper at the other end. The copper end of the battery contains an electrolyte solution composed of copper chloride dissolved in water (Cu2+ and Cl– ions), while the zinc side is dissolved sodium chloride (Na+ and Cl– ions), and a thin membrane separates the two electrolytes, allowing chloride ions to cross, but not copper. When the two electrodes are not connected to anything, no electrochemical reaction occurs, and the tube shouldn't have any significant electric field about it. (you could measure the amplitude and direction of the field anywhere around the cell, and would probably not observe any--if you touch a meter to the two ends of the cell it would detect the EMF, but the potential difference between points in space each a micron away from the electrode is probably pretty close to zero).
Once a circuit is completed, electrons flow from the zinc electrode to the copper electrode, which dumps the electrons into the empty orbitals of the Cu2+ ions, reducing them to copper metal, and chloride ions move to the zinc end of the cell. So negative charge (moving either as electron or chloride) has moved all around the circuit, but the cell itself doesn't have any change in charge distribution on a macroscopic scale, only changes in the composition of matter within.
You've lost me.
What distinction are you drawing between EMF and PD?
If there's a voltage between the terminals of a cell, then there's an electric field in the space surrounding the cell, which is what I assume evan_au was getting at. The electric field looks similar to the magnetic field surrounding a bar magnet:
https://qph.ec.quoracdn.net/main-qimg-dd8c242f454fa5fc88a4efb47f5dfb55
Or are you arguing that a voltage only exists when a current is flowing? In which case how would the current ever start flowing in the first place?
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General Science / Re: Does A Fully Mirrored Room Require Less Light To Light It?
« on: 03/06/2017 11:35:42 »
Isn't it a matter of common experience that the paler the walls are decorated the brighter the room? Mirrors are just one step paler than white paint.
most of the visible light would bounce of corner reflectors (which form every corner of the room), and end up getting absorbed by the candle flame and turned into invisible heatBut a corner reflector returns a beam of light parallel to the incident beam, not convergent on the source.
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Physics, Astronomy & Cosmology / Re: Could a contact lens fool a biometric iris scanner?
« on: 27/05/2017 12:37:56 »
Not only can you use a fake iris, but you can engineer a fake iris from the data stored in an iris recognition system:
https://www.schneier.com/blog/archives/2012/07/fake_irises_foo.html
https://www.schneier.com/blog/archives/2012/07/fake_irises_foo.html