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From: CrazyCam on 8 Feb 2007 20:43 jlittler(a)my-deja.com wrote: > On Feb 8, 3:35 pm, CrazyCam <crazy...(a)ar.com.au> wrote: >> GB wrote: >> >> <snip> >> >>> Answer me this one: >>> You take a fridge and put it in a temperature sealed room. You >>> lock the door to the room so that Johno can't upset the test by >>> coming in and getting beers for people every two minutes. You >>> leave the fridge door open, and the fridge running. >>> Does the ambient temperature inside the temperature sealed >>> room rise or fall? >> This is, of course, a trick question. >> >> Everyone knows that a fridge is simply a heat pump, gathering heat from >> inside, and moving it outside, but... with the fridge door open, the wee >> light will be on, thus heating up the insides of the room. > > While all true, you missed that the electric motor of the fridge is > less than 100% efficient and hence some of it's power is also becoming > heat. Hence the room will be heated up by motor heat generated plus > light heat John ....oh, never mind. regards, CrazyCam
From: paulh on 8 Feb 2007 22:03 On Fri, 9 Feb 2007 09:33:57 +1100, sharkey <sharkey(a)zoic.org> wrote: >> I *know* I should know... but just how much is a poofteenth? Is is >> metric / Imperial? How is it measured? > >An Imperial Fluid Poofteenth is a cubic Imperial beesdick, and an >Imperial Poofteenth (weight) is the weight of an Imperial Fluid >Poofteenth of HP Sauce at ISTP (Imperial Standard Temperature and >Pressure: a bright but chilly morning in London now that Spring is here.) > >The Metric Poofteenth is a very small block of Platinum in a jar of pure >Argon in a cafe in Paris, and you can't have it. If this was a webforum and I was admin, I'd 'sticky' this post.... alas... this knowledge'll soon be lost. paulh
From: jlittler on 8 Feb 2007 22:18 On Feb 9, 12:43 pm, CrazyCam <crazy...(a)ar.com.au> wrote: > jlitt...(a)my-deja.com wrote: > > On Feb 8, 3:35 pm, CrazyCam <crazy...(a)ar.com.au> wrote: > >> GB wrote: > > >> <snip> > > >>> Answer me this one: > >>> You take a fridge and put it in a temperature sealed room. You > >>> lock the door to the room so that Johno can't upset the test by > >>> coming in and getting beers for people every two minutes. You > >>> leave the fridge door open, and the fridge running. > >>> Does the ambient temperature inside the temperature sealed > >>> room rise or fall? > >> This is, of course, a trick question. > > >> Everyone knows that a fridge is simply a heat pump, gathering heat from > >> inside, and moving it outside, but... with the fridge door open, the wee > >> light will be on, thus heating up the insides of the room. > > > While all true, you missed that the electric motor of the fridge is > > less than 100% efficient and hence some of it's power is also becoming > > heat. Hence the room will be heated up by motor heat generated plus > > light heat > > John ....oh, never mind. <gets that stricken look as he realises he's missed the subtlety again> <sigh> JL (if I buy the beer will you give lessons ?)
From: Iain Chalmers on 8 Feb 2007 22:26 In article <bigiain-4E0BD6.18012908022007(a)nasal.pacific.net.au>, Iain Chalmers <bigiain(a)mightymedia.com.au> wrote: > Without arguing the authority of wikipedia... > > http://en.wikipedia.org/wiki/Alternator#Automotive_alternators says > automotive alternators achieve between 50 and 60% efficiency. > > http://en.wikipedia.org/wiki/Megajoule says 3.6MJ=1kWh, so 1kWh~=0.28MJ > or 1Wh=0.00028MJ > > http://en.wikipedia.org/wiki/Internal_combustion#Engine_Efficiency says > piston engines get ~ 20% efficiency > > http://en.wikipedia.org/wiki/Gasoline#Energy_content says you get > ~32MJ/l from petrol. > > So, 120W headlights will need 120/0.50 =~ 240W of input power to the > alternator. > > To run them for an hour will require 240*0.00028 =~ 0.067 MJ. > > At 20% efficiency the engine will consume 0.067/0.20 =~ 0.34MJ > > Which is (near enough) 100mL of petrol. > > If you're driving along at 100kmh, you should expect to consume an extra > 0.1L of fuel an hour, which is an extra 0.1L/100km (or around and extra > 1.5mpg). If you're only averaging 50kmh, you'll still consume 0.1L/h to > drive the headlights, but you'll be driving them for twice as long which > will bump your fuel economy up by 0.2L/100km. > > (I think I got all my orders of magnitude right there) I thought up a quick sanity check on these numbers on the way home last night - I'm claiming 250W of engine output power for an hour consumes ~100mL of fuel. If I multiply that up to the ~4L per hour my Spada uses on the highway at around 100kmh, it says I'm using 10kW (4L/hour is 40 times 100mL/hour, and 250W times 40 is 10kW) which is just under half Honda's claimed max power for the bike. Seems to be reasonable to me, I'm clearly in the right order of magnitude - I'd clearly be using more than 1kW and less than 100kW. A rough guess at the errors I've got seems to be maybe +- 20% for alternator efficiency, and probably the same for the estimate of engine efficiency - together they'll swamp any other errors, so lets call it 0.1L/hour +- 40%... Seems to me that is easily enough to be measurable if you're careful... big -- "Everything you love, everything meaningful with depth and history, all passionate authentic experiences will be appropriated, mishandled, watered down, cheapened, repackaged, marketed and sold to the people you hate." Mr Jalopy quoting Hooptyrides (on jalopyjunktown.com)
From: Iain Chalmers on 8 Feb 2007 22:30
"Boxer" <someone(a)nowhere.com> wrote: > My 5.7 litre HSV gets an average of about 16.5L/100km in the city and > 10.0L/100km on the highway. How much does it vary around that average? Do you think you'd notice a systematic ~0.1L increase on the highway? big -- "Everything you love, everything meaningful with depth and history, all passionate authentic experiences will be appropriated, mishandled, watered down, cheapened, repackaged, marketed and sold to the people you hate." Mr Jalopy quoting Hooptyrides (on jalopyjunktown.com) |