raid517

OK I'm going to try to put this question the best way I can. But as far as I understand it, an atom will emit a photon when a photon of just the right energy is absorbed, thereby raising an electron from a lower energy state to a higher energy state. More specifically, the photon is emitted when the electron drops down an energy level from it's new excited state, to a lower energy level. From say n=4 to n=2 for example. But my question is what causes the electron to drop an energy level and thereby emit the photon? An example of this might be hydrogen whereby the electron dropping from the n=4 to the n=2 level results is a blue spectral line. However if just the right frequency of blue light is being shone on a sample to allow for an emission of exactly that frequency of light from the hydrogen sample, then why doesn't the electron stay in a continually 'excited' state? What I mean is if work is continually being done on the electron as photons strike it, then why doesn't it just stay continuously in the n=4 energy state? Why would it drop down to a lower energy state an emit a photon of the same frequency as the source of light being directed at it? The only (probably crude) explanations I can think of is that clearly no system is capable of continuously absorbing energy, without at some point releasing some of that energy. Therefore work done on a system, must equate to work produced by that system at some point or other. The only analogy I can think of is perhaps a sealed copper boiler containing water. If this boiler is continuously heated, this will equate to work done, but as the boiler is sealed there is no way for the energy supplied to escape. Therefore the energy of the boiler increases, until inevitably eventually there is an explosion and the energy of the system is transferred by means of work done in a variety of ways. Therefore the electron cannot continue indefinitely to absorb photons, because to do so would result in it becoming increasingly unstable and would presumably cause it to break free of the atom at some point or other. I'm not sure this explains everything though. Just because it would become unstable and would eventually break free of the atom, still doesn't explain (to me) why it may drop from one energy level to another. Therefore the second (again probably erroneous) assumption I can make from this is that in some sense light isn't continuous, but instead comes in packets, rather like a series of tennis balls being thrown at a target, thereby giving the electron sufficient 'time' to absorb the photon, drop from one energy level to another and thereby emit a photon, before being struck by another photon (which is then absorbed), thereby raising it to a higher energy level again - and so on. If this was true (which it probably isn't), it would seem like a very simple mechanical process. So how close, or far off am I? No doubt someone will 'hand me my ass' for trying to work out what's going on here though lol. I just don't feel I have a clear enough grasp of why electrons change energy levels and emit photons to be confident about this. All input would be welcomed.

Thanks guys. I was along the right lines it seems, but you made it much clearer. Thanks! I don't get the part where you say: There clearly isn't sodium in every sample of an unknown mixture you might be given. So are you simply saying that if sodium is present in a flame test, the strongest constituent of the colour produced in the test would be yellow? Wouldn't this depend how much sodium was present? Clearly if there was very little, then the colouration of the flame test by the presence of sodium would be minimal also?

Can someone please explain why you would use a spectrometer rather than a simple flame test to identify the atoms in an unknown mixture? I'm guessing that the spectrometre would divide the spectra into a much more specific pattern, so that you could identify the individual elements in the unknown mixure, wheras the flame would only give you a mix of colours, thus giving the flame a single coulour consisting of all of these different colours. So the flame test would make guessing the elements of the unknown mixture impossible. But I'm not sure if this is the best answer I can give to this question? Maybe they are looking for something more specific, or am I just overcomplicating things?

Please can someone show me the steps needed to go about rearranging the equation Eph = hc/lambda to make lambda (the wavelength of the light) the subject? I'm totally stuck and I have an exam in the morning, I know it should be simple but I'm just not getting it. Any and all help is appreciated.

If an arrow on Earth is fired into the air and reaches a hight of 125 m and penetrates the ground where it lands to a depth of 25 cm, what is it's deceleration? Of course the acceleration due to gravity is 9.8 m s, but I'm stumped on how to calculate deceleration? I successfully calculated the final velocity to 49.5 m s^-1.

Can someone please explain in simple language and in step by step terms how to tackle a problem like this? I have to answer a question that is very similar to this. (Only the numbers have been changed). But I haven't got a clue where to start. At a guess my instinct is to find a way to convert these numbers into the same or similar units? I have of course used different numbers here and I am not trying to cheat or anything (it's just a practice question anyway), as I really would like to understand how to do this. But my maths isn't strong (I'm an adult learner BTW), so I would appreciate as simple and clear a breakdown of this problem as possible. With thanks!

Hi, how do I calculate the tidal volume and ventilation rates for two test subjects from the number of observed breaths per minute taken during an experiment? I also need to calculate the stroke volume of the heart and the cardiac output, but I have no clue how to do this? The relevant details are as follows: Subject 1 Breaths per minute at rest: 24 Breath per Minute during exercise: 29 Heart Rate during rest: 97 Heart Rate during exercise 165 Subject 2 Breaths per minute at rest: 16 Breath per Minute during exercise: 22 Heart Rate during rest: 74 Heart Rate during exercise 23 Any help anyone can offer would be greatly appreciated. Merged post follows: Consecutive posts mergedEdited with weight info in case needed. Subject 1 Breaths per minute at rest: 24 Breath per Minute during exercise: 29 Heart Rate during rest: 97 Heart Rate during exercise 165 Age 27 Weight 84KG Subject 2 Breaths per minute at rest: 16 Breath per Minute during exercise: 22 Heart Rate during rest: 74 Heart Rate during exercise 23 Age 17 Weight 72KG BTW can this even be done? I can't find any info on the web anywhere about it.

Hi can someone please help me to understand these questions? I'm completely stuck. Any help anyone can offer would be very much appreciated! I'm a bit lost, but would probably be OK if someone explained it. Merged post follows: Consecutive posts mergedTo be clear I get (I think) that in the first question that the disease has to be recessive since neither of the parents are affected. In the second question, if the disease is recessive this should give the numbers 1 and 2, 3 and 4, 10 and 11, since these all had offspring who were affected, but they weren't affected themselves. in the 3rd question if I do a punnet square diagram, the genotypes produced from a cross between 10 and 11 could be: Homozygous dominant: 1 out of 4, 25% chance or a probability of 0.25 Heterozygous: 2 out of 4, 50% chance or a probability of 0.5 Homozygous recessive: 1 out of 4, 25% chance or a probability of 0.25 Is this correct? I'm not sure about the last question, but this isn't relevant to what I'm doing anyway.

Hi, hopefully this isn't too dumb a question. However if the first law of thermodynamics states that energy cannot be created or destroyed, would this not imply that whatever existed immediately prior (given that there is no time period we can refer to as 'prior') to the big bang (and therefore before the planck epoch) must also have been another form, or state of energy? If this is the case, then couldn't it be used to imply that something did indeed exist during this period and that consequently the universe can't be said to have emerged from nothing? Or alternatively, is it believed that the laws of physics only came into being after the big bang? Is there no case in which the laws of physics (and specifically in this case the laws of thermodynamics) could be said to have been continuously present, even from the very earliest beginnings (and prior to this) of the Universe itself?

Hi I have to do a biology write up and it seems to be going OK so far. The title of the essay is "The Effect of Changing the Concentration of an Enzyme on the Rate of an Enzyme Catalysed Reaction". Obviously in this case, the increased concentration of enzyme results in an increased rate of reaction, up to a given point, when finally all of the active sites of the enzyme are used up. However the question that occurred to me (and one not asked in the essay) is what would happen if you just kept adding more substrate and simply replenished the substrate as it was used up? In other words, would the enzymes continue to work indefinitely? Would they never stop working? And if they did stop working, at what point would this be and why?

Hi, can anyone please give me some clues on what types of cell transport mechanisms I should be looking out for when thinking about Neurons and white blood cells? Any explanation of these would be much appreciated.

Hi help I'm stuck! I need to write an essay that talks about the different types of diffusion across a cell membrane. Specifically I need to compare a neurone cell, with either a blood cell, or a epithelial cell. My tutor has given some strong hints that she would prefer a comparison of a neurone with a blood cell though. (I'm not sure if she means red or white however?) as both of these apparently show the full range of difference membrane transport methods. I seem to be doing OK with Neurones as there appears to be plenty of information out there, however I'm having a terrible time sourcing any information on the full range/function of membrane transport in blood cells. Can anyone please point me to some sources where I might be able to find out something about this?

Hi can anyone recommend any good books by respected authors on the subject of behavioural modernity? (Or what is otherwise known as 'the great leap forward'). I'm attempting to learn more about this topic, but I don't want to waste my money on poor purchases of inappropriate titles. I also would be up for reading any online books, or watching any videos. Thanks! Merged post follows: Consecutive posts mergedOK, for those not quite familiar with the term: http://en.wikipedia.org/wiki/Behavioral_modernity

I think a lot of poles had their own distilleries for this and did make very high grade alcohol. There is quite a tradition of preserving food this way in Poland. Alcohol that is especially pure 95% + is said to be especially good at preserving the flavours of certain types of food (particularly fruits). Anyway what better way to enjoy one's s-self at a social gathering than to present them with a bowl of fruit that has been preserved in alcohol. Fruit that gets you drunk... It's a pity nature didn't think of it first.... ;-)

So can pure ethanol be bought legally in the UK or not and if so where? I have some Polish recipes for preserving food and fruit (which is my main area of interest) and these require ethanol of the highest possible purity. (So no dangerous additives to make it unfit for human consumption).

Erm.. the Economist is an extremely right wing British establishment rant mag - and this guy is largely critical of Blair and Bush for not being 'right wing enough', not becuase he personally leans towards the left in any way. The far right in British politics are generally admiring of the American economic system, and they would very much like to see it transposed here to the UK. More private enterprise involvement in government, social services and health care, less government regulation, a severe curtailing of the welfare state and so on - so you have to read this in context. The guy has an agenda, just like most people in publications of this nature do. GJ

Hi, what are the chemical properties of the most popular brands of car tires? (Specifically I'm interested in how they are constructed, what they are made of, what the chemical properties of the stuff they are made of are and so on....). Also what happens if you heat these tires up? Do they melt at a specific temperature? Is there any similar kinds of materials that melt at a higher temperature - and if so what are they? GJ