studiot

Do you normally feel the ground under your feet as you walk? Of course the fish can sense the water in just the same way since both your walking and the fish swimming rely on the frictional contact and (reaction) pressure.

Here is one way to get the best of both worlds. Use a docking station. The laptop can be seen mounted on the docking station, where it just clicks into place and provides desktop like facilities. The special purpose mouse can be seen in the apple standing on one of the laptop pads.

That was meant to mean that there are no dissipative processes.

Interesting, thank you. I know the prevailing winds are westerlies but I wonder if any of the tectonic activity in Iceland has anything to do with this?

Now that I am back home, let us discuss the following closed, isolated system. Consider an adiabatic cylinder containing a frictionless adiabatic piston separating the cylinder into two chambers, each containing a compressible fluid, so that the piston is in the equilibrium position. Let the piston now be mechanically displaced from its position and then released. Are the subsequent changes reversible or irreversible and what is the entropy change of the overall system?

The 'politics' comment was about the obsolescence discussion, planned, built-in or just plain there, which is what my link is all about. But yeah, whatever, computer help is good too.

Hazel, your thread should be in politics or in the lounge, like this one here. http://forum.allaboutcircuits.com/showthread.php?t=95669&page=18 When I was young, one of the legacies of Birmingham being the (metal)workshop of the world was that if you needed a piece (or a thousand pieces) of any type metal in any shape of form, there would be a dozen back steet companies who could supply off the shelf or make cheaply to order. Small companies that had grown up supplying to the big ones. which supplied to the world. Similarly in London, where I lived, there was a place where you could obtain nuts and bolts for any thread ever made. The owner made his fortune out of this simple supply. Nowadays the the West has handed manufacturing on to others, these paragons have all gone and obtaining even simple things is difficult. Sales clerks these days usually don't even know what you are asking for at component level. go well

Ask for some help, instead of bleating. If you were to tell us the details, I might be able to offer some suggestions, as might others. For instance I often buy broken laptops on Ebay - that is broken except for the part I want - this can often make an economic repair. Since yours has a mobo failure I would suggest looking for one with a smashed screen, but functioning mainboard. These can be had in the region £25 - £75 in the UK, probably the same number of $ in the US.

Windows has lots of options to help make life easier. Look at the accessibility options in the XP control panel Also look at the magnifier http://windows.microsoft.com/en-GB/windows-xp/help/turn-on-magnifier

The screen size you can plug into a laptop is not limited by the laptop. Many laptops have an HDMI port which carries both high definition television / video and sound in one cable. Very few desktops have this. Touch screens have not been a success for desktops, but are readily availabale in laptops and tablets. If you buy a laptop beware of closing it with an object such as a pen inside. Broken screen replacement costs run from about £90 to about £350. Be careful of the power connector, they are overweak and the laptop usually has to be dismantled to replace. A good practice is to place a weight (book etc) onto the cable to prevent the full weight of the power supply/lead/interference suppressor dragging on the conputer jack. Do not obstruct the air vents. Many laptops have air vents on the undersurface. I recall one client who burnt out his laptop by working in a heavily upholstered armchair and resting the latop on the padded arm, thus blocking these vents. On the other hand working on a laptop can be worse for your health, so much so that there is a (much ignored) EU health & safety directive that requires workers to be provided with separate screen and keyboard to avoid RSI and eye problems. You are better off with separates if working for long periods, you can also adjust the chair to suit as well. Oh and on screen print size can be adjusted in most programs by adjusting the zoom on either laptops or desktops.

Nicholas, you are obviously keen but have not yet met many of the deeper points and connections of mathematics. These are not introduced in courses taught at your school level and, I'm sorry to say, that some of the groundwork that used to be taught is missing today. So you will have to seek out some of this for yourself. That can be an exciting journey for those who are interested in such matters! You need to fully understand the difference between an equality and an identity. They are different, although we usually use the equals sign for both. Similarly you need to fully understand what is meant by the statement; "X satisfies this equation or that condition". Also important is what mean when we say 'solve an equation'. In elementary mathematics, particularly applied maths or physics etc, this often means "plug in the known quantities and find the unknown quantity" Otherwise called 'plug and chug' Schrodinger's equation is an example of a type of equation that has an infinite number of solutions. But we do not use Schrodinger's equation in isolation, a proper description also comes with some (a set of) 'boundary conditions'. These boundary conditions are additional mathematical statements, which may be equations to solve in their own right, which allow us to pick out a solution from all the infinite possibilities, appropriate to the system in hand. This combination of a key equation, statement, principle, law, or theorem with other information is very common to the point of being the norm. Having worked through all this we extract from the Schrodinger equation, along with the problem boundary conditions, a function or functions functions. Somtimes this function is such that it obeys an uncertainty principle. Note this idea of uncertainty applies to many equations,even simple algebraic ones. For example the equation 3x2+8x+6 = (x2+4x+1) +(2x2+4x+5) is infinitely uncertain since it is true (satisfied) for any or all values of x. (Note this is actually an example of an identity that I mentioned earlier) Whereas the equation 3x2+8x+6 = 2x2+5x+4 is precise (certain) since it is only true for two specific values of x, namely x= -1 and x=-2. (note this is an example of an equality).

In line four if you multiply deltaH /R into both terms in the bracket, you recover line 3 and remove the bracket. All they have done is the equivalent of {-3/2a - 3/2b} take out the common terms 3/2 leaves 3/2{-1/a - 1/b} can you see it now it is simple algebra. Sorry I can't do the latex at the moment.

A substitution test with a friend's monitor is best if at all possible, many also laptops have a monitor output socket of some sort. If it is a separate monitor you may be able to test it with a friend's computer. Yes there are , or mostly were, computers, PC and Apple that ran the mouse from the monitor, but they are dying out so unless yours is one of these, most computers have no direct electronic intraction between mouse and monitor. This is a good time to ask you to tell us more about the computer in question and the symptoms.

An interesting application of art in science by Leonardo Da Vinci from the BBC http://www.bbc.co.uk/news/health-28054468

There is more to it than trilateration v triangulation. In order to obtain a 'position fix' you require the intersection of a minimum of two intersecting position lines, in 2D, and 3 lines in 3D. A position line is provided either as a distance (circle) or direction from a known point, thus 2 or 3 known points are required. The known points may be either actual points in 2D or 3D space or they may be projections of actual points onto a standard surface, eg a sphere. Calculation is then performed on the triangles, rather than quadrilaterals or other figures to establish coordinate differences. These triangles may be spherical triangles as in the case of astro work or they may be euclidian, corrected for relativity. The use of additional known reference points and position lines provides multiple estimates of the fix, which may be weighted and adjusted statistically to improve accuracy.

There is one vital fact missing from your list. At what temperature is the bar 500mm? (you will also need to look up the temperature coefficient of expansion of steel.) Then calculate the maximum and minimum possible measurements, given the other data.

No, offence Hazel, but check the spelling in your post against swansont's steganography Stenography! As to GPS, I'm surprised no one has mentioned that it is not possible to perfectly match 3D space coordinates, as used by satellites to the surface of the Earth, on a flat 2D map. This has to to with ordinary geometry and nothing to do with relativity. Also transmission to the receiver is not perfectly known, and affected by conditions and the terrain. We used to call this 'ground swing' in the old days. You can find lots about it on google.

Even if you can't do the problem, you could tell us what you understand by adding more lamps in series. You might also like to think about what happens to the power source voltage when you add lamps. Remember voltage and current are connected by someone 's law.

I did note in post#24 that most of the definitions and much of the maths applies only to finite systems. The question arises is the universe finite or infinite, and, if infinite, how do you treat it? Let us say you calculate a finite entropy 'increase' of S. Can you prove that infinity plus S is greater than infinity? (Sorry I can't present maths symbols from Dundee.) Now let us say you have a system, finite or infinite, and you use S = k ln(w) to calculate the total system entropy. ln(w) is a function that has no upper bound so as w tends to infinity ln(w) does not tend to a finite limit but increases without bound. So does an infinite system have infinite entropy? Can a finite system have infinite entropy? Consider in say the Morse curve where w is a discontinuous independent variable, below a certain level, but continuous above, where there must therefore be an infinite number of energy levels. Consider also Caratheodory's (hope I spelled it correctly) version of the second law: For any equilibrium state there are nearby equilibrium states that are inaccessible to the system. I said I was happy to discuss entropy, but I don't see how the above is relevant to reversibility.

The book I referred to in post#8 should provide the non expert with a real feel for the subject, without needing to know the intricate details of the physics, via the history of who measured what and why, the proposed explanations, and what Katy did next.

+1 for an honest question, honestly put. The first measurements contrary to the accepted science of the day led Einstein to his theories. He subsequently made several changes (refinements ?) to particularly the General Theory, in the light of later measurements. Both his theories also yielded predictions that were tested and found accurate. In fact, no one has ever found measurements contrary to the final versions. However it is also true that there are other more complicated potential explanations that also fit the known facts. During the last century there were even more competing theories but several failed as a result of some new measurement. There is a fascinating book just out by the Professor of Astrophysics at Oxford University that charts the history of relativity. The Perfect Theory Pedro F Ferreira

Thanks for the reply, presume your essay is now long sent in. I wouldn't recommend playing politics with what is really your future knowledge base. OK so your prof has poor English, but there are a great many very, very clever foreigners so better politics would be to find a way to get the best out of him. You would gain more this way in the long run. Go well in your studies.

Once again you are either misunderstanding standard thermodynamic terminology or you are redefining it to terms of your own. This is logical nonsense since including the heat source as part of the system leads to the inescapable fact that the heat transferred from the surrounding to the system equals precisely zero (First Law). This, in turn, leads to the inescapable conclusion that whatever the process, whatever the final system and surroundings the entropy change is always precisely zero (Second Law).

In response to post#25 Since both your answers were totally flippant, I see no point discussing this topic further with you.

@JC You made two substantive points, so dealing with them in order; This thread is clearly labelled with the term thermodynamic, so surely it is reasonable to employ thermodynamic terminology. Introducing terms from mechanics is particularly difficult and confusing since within that system perpetual motion is admissible. Note I did not say perpetual motion machines. Force is an important quantity in mechanics, but very minor to totally unimportant in thermodynamics. So can we please stick to conventional thermodynamic terminology? As to you second point, I noted that you replaced the variable, entropy, with another, enumeration of energy levels along with a constant. Since you did not define your new variable or display a chain of mathematics leading to its relevance I regard the variable entropy as still undefined in this thread. Now I will happily widen the discussion to entropy and the connection between the Boltzmann equation and classical thermodynamics. The first thing to note is that the enumeration variable is not a continuous variable, so there is a mathematical issue in employing it in connection with classical continuous variables, calculus and so forth. Yes it can be overcome but care is needed. The second thing to note is that classical thermodynamics does not attempt to offer calculation of absolute entropy of a system in the same way as statistical mechanics. The Boltzmann formula refers to system entropy, whereas the Clausius one refers to entropy difference. The distinction is rather like the distinction between voltage and voltage difference. There is a further difficulty employing an extensive state function like entropy in that the definitions, open closed and isolated only make sense for finite systems. Both S and delta S have singularities at zero and infinity. I think that is enough to be going on with if you are serious about a sensible technical discussion.