# ahyaa

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21

1. ## Enthalpy of the Universe

My professor posed a theoretical question today and I can't seem to figure this out. The prompt was this: Calculate the enthalpy change of water ΔH when 18 grams of water ice at 230 K are converted to liquid water at 350 K. Also calculate the enthalpy change of the heating block as a result of this process and the enthalpy change of the universe as a result of this process. I don't have a problem with the calculation of the enthalpy change of water etc., what I can't seem to figure out is how to prove that the enthalpy change of the universe will be zero. I was curious if anyone has been able to prove this? The best I got was ΔHsurr + ΔHsys = ΔHuniv = 0 because ΔHsurr = -ΔHsys.
2. ## Defining Speed = Distance/Time, or ||Displacement||/time?

Well said, thank you
3. ## Defining Speed = Distance/Time, or ||Displacement||/time?

Hello, Given that distance is path-dependent and displacement measures only the net change, how do we define speed? I have seen speed defined as 1) distance over time, and 2) the magnitude of velocity. I recognize that these are two different things because distance is path-dependent while velocity = displacement/time and displacement measures net change. Thus, which definition is the accepted definition for speed?
4. ## BigDye Terminators

Hello all, I just started working in a genomics lab and we use Sanger sequencing. The sequencing protocol utilizes the BigDye 3.1 Terminator Kit and a BigDye XTerminator for clean-up. I did some online searching but to no avail couldn't really figure out what was going on during the BigDye Terminator reaction. Can anyone tell me what the primary agents are in the buffers and what they do e.g. Taq polymerase and MgCl2 or something of that sort? I'm just curious as to what is actually going on in these steps.. I am also a bit confused as to how chain termination works. Won't ddNTPs prevent DNA elongation?
5. ## Extraterrestrial life

I'm going to agree with CharonY on those points... as for my own opinion: I'm no astronomer, so I don't know exact numbers of how many stars and planets are out there; but I imagine from a numbers-standpoint there is definitely extraterrestrial life out there. I think the sole most important thing for "life" to exist is self-replication. Nature also prefers the efficient, so the most efficient self-replicators will proliferate, and the most of efficient of those will go on, and so forth. Then you have physical factors that affect how these self-replicators are selected. Physical effects such as gravity, temperatures, and chemical composition of planets (or whatever environment these self-replicators might arise), are the factors that determine the "characteristics" of these self-replicators. So yes, if you had a planet full of rock, in which some self-replicating molecule (maybe even atom) arose, then you might very well see organisms that "eat" rock due to its abundance. These physical 'factors' really limit what kind of life you might find, which is actually very good if we're trying to picture what kind of life might exist, because it narrows the possibilities. So even if you have a planet full of rock, the rock may not be a suitable energy source so perhaps life would not arise on its own there for an infinitely large amount of time. Before you can extrapolate to very extraordinary life, I think it's necessary to start with the simplest of them all. The simplest organisms are often the most efficient, and from simple organisms you can determine what kind of complex extraterrestrial life you might find. And these simple organisms are efficient because nature prefers the efficient, so conversely if we understand nature (the sciences) we could understand how simple organisms might arise, and ultimately how complex life might arise. As Ophiolite said (I'm going to kind of.. extrapolate from what he said ), if we only understand physical laws as they govern Earth, we'd only understand how life could arise on Earth. So what we really need to do is understand the physical laws of other planets, star systems, etc.
6. ## Ideal Heat Engine w/ Non-Zero Power

You're misunderstanding here, I said because I understand how zero power is possible, the answer that "nonzero power is impossible" seems wrong. I have not heard of the zeroth law being re-stated in such a way. I wouldn't disagree that it may be out of my scope, so if you would like to enlighten me I would be highly interested in how this restatement is arrived at. (Actually, looking back and thinking about this I agree completely, but I just haven't seen the law restated in this way, but it is good to know). Your heat pump model does make sense and I find it very interesting. But would you not say that heat flows from the compressed working fluid to output medium, and from the environment to the expanded working fluid, due to an increase in entropy?
7. ## Ideal Heat Engine w/ Non-Zero Power

Okay, I'll follow along with you. My answers to your questions (elaborated further from my previous post): 1) Heat can flow from a cold body to a hot one in a cooling process. This particular heat pump would be a refrigerator. If we were to look at an energy diagram for a refrigerator, we see that heat is "pumped" from the cold reservoir (e.g. inside of a refrigerator) to the hot reservoir (e.g. your kitchen). The cold reservoir is simply a 'sub-system' that has a lower temperature than the hot reservoir. The reason you can't have 100% efficiency is because you'd need a cold reservoir with 0K, and to my knowledge that is not possible. 2) Heat flows from a colder region to a hotter region via work input. Otherwise, heat flow from a colder to hotter would violate the second law (that entropy of a system must increase or remain the same). 3) I suppose the answer to #1 gives the general idea here. Also... The second law doesn't say anything specific about cycles. My confusion stems from how they relate power (energy/time) to this. I understand how a "zero power" would arise, that is making time infinitely large would give you a zero power. But what makes an infinitely large time impossible, is what I don't understand.
8. ## Ideal Heat Engine w/ Non-Zero Power

Thank you for your pointers, yes the fourth answer is indeed with relation to the 1st law rather than the 2nd law. However in trying to prove my misreading of the question you didnt really answer the question of why it is impossible for an ideal engine to have a nonzero power, according to the 2nd law. It did not specifically ask for a scenario in which the engine was at full stop, but in order to consider something impossible we must more or less consider all possibilities no? I think I'm going to bury this problem as a poorly worded one in the back of my mind somewhere. Thank you both for your input.

10. ## Ideal Heat Engine w/ Non-Zero Power

The difficulty I'm having is with understanding the explanation for the answer they provided. It was a multiple choice question, basically: of the four choices, which one would be impossible? The underlined answer is the 'correct' answer. The explanation is provided by the text the problem was selected from so it is the explanation of the correct answer, given by the book. I do understand why the other answers are incorrect - it is possible for 1) heat to flow from a colder body to a hotter body, as in a heat pump, 2) 99% efficiency, just not 100%, 3) a physical process to yield more energy than what is put in - this is kind of a misleading statement but I believe heat pumps satisfy this - that is the work input (typically electrical work) is typically smaller than the heat transfer to the hot/cold reservoir (thus COP's tend to be larger than 1). My question is why can an ideal engine NOT have non-zero power? If the engine isn't running, does it not have non-zero power? Or as they put it: if the work is being done at the infinitely slow rate, the power of such an engine is zero. The "explanation" seems to contradict itself.
11. ## Ideal Heat Engine w/ Non-Zero Power

According to the second law of thermodynamics, it is impossible for: heat energy to flow from a colder body to a hotter body an ideal heat engine to have the efficiency of 99% an ideal heat engine to have non-zero power. a physical process to yield more energy than what is put in ^As taken from a homework problem, the explanation: The ideal engine follows a reversible cycle--therefore, an infinitely slow one. If the work is being done at the infinitely slow rate, the power of such an engine is zero. An alternative way to state the second law of thermodynamics is as follows: It is impossible to construct a cyclical heat engine whose sole effect is the continuous transfer of heat energy from a colder object to a hotter one. This statement is known as Clausius statement of the second law. Note the word "sole." Of course, it is possible to construct a machine in which a heat flow from a colder to a hotter object is accompanied by another process, such as work input. Is anyone able to explain this in layman terms, i.e. to an first-level college physics student...
12. ## Friction, Normal Force

After looking for this answer online I was surprised I couldn't find it.. so naturally I came here Anyways, I was taught in class that friction = coefficient*normal force. My question is what happens if we have an angled normal force (with respect to our axes), would the normal force in the friction equation be one of the components, or the magnitude of the entire normal force?
13. ## IQ heritability -a question to knowledgeable users

I'm pretty late on this topic... but in response to the OP I completely agree with this. Even if you were to choose a mate with all the "smart" genes there's a good chance they may not be turned on because as Delta1212 said, genes are highly regulated by environment and upbringing of an organism. So even if you have the capability of producing these "smart" proteins by having the genes, they may not be expressed and so you may not ever end up having a "smart" kid. Human heredity is so complicated, I'd think if people truly understood it we'd have taken measures by now to leave the best working genotypes for our offspring.
14. ## RNA Primers & Okazaki Fragments

So I've asked a couple professors and they all say that RNA primers are not part of the "Okazaki fragment," they're separate things by definition. Apologies for the seemingly trivial topic!
15. ## Conservation of momentum and energy

I was wondering if there are cases in which momentum or energy are not both conserved together ie energy conserved, but momentum is not. The source of my confusion stems from the explanation that momentum is conserved in a "closed system" (no matter exchange, no external forces) while energy is conserved in an "isolated system" (no mass or energy exchange with environment).
16. ## Limit to how much our bodies can adapt?

Whether you're talking about adaptations or acclimatizations (which are not hereditary) there's going to be some upper limit based on the amount of energy needed to grow and sustain life.

18. ## Work and Gravitational PE

Ah, I figured it out. My main problem was 1) improperly using the dot product, which for vector(A) dot vector(B) = ||A||*||B|| cos(theta). Instead I used the vector for F as -mg and there should be no minus sign as you said. Extending this to the more general form of work W = Fd --> F and d are always positive. 2) The F in work is the external (not internal) force. As you said it's the force (and work from this force) to overcome gravity not due to gravity. Thanks!
19. ## Work and Gravitational PE

Let's say we use an axis with positive pointing up. Let's represent the stored energy due to its position (grav. potential energy) re-written as work (from work-energy theorem = delta E = Work). If the object moves from h1 to h2, where h1 > h2 (that is the object is falling in this case) then the stored energy = F(parallel) * delta h * cos(0) = F(parallel) * (h2-h1) = -mg * -h (because h1>h2). We thus have grav. potential energy = mgh (h = h2-h1) for an object falling from h1 to h2. I guess my question is, why is mgh positive for a falling object, shouldn't the object be losing potential energy, therefore mgh should be negative?
20. ## RNA Primers & Okazaki Fragments

Hi, I have a simple semantics question. I have been reading some literature that seems to consider RNA Primers as part of Okazaki fragments, but this should not be so because Okazaki Fragments by definition are just the DNA portions synthesized on the lagging strand. Any opinions/clarifications on this?
21. ## Work and Gravitational PE

Hello, I was wondering how signs are assigned to gravitational potential energy. Ug is positive when an object starts at rest and falls straight down. This makes sense to me because the gravitational pull of the earth (Fg = mg) pulls the object a distance of h (thus = mgh). This fits the work equation = Fdcos(theta). But if this is case, shouldn't Ug be negative, because Fdcos(theta) = mg*(-h)cos(theta), where -h is from the object moving along an arbitrary negative axis, and cos(theta) is 1 because the force and displacement vectors are parallel. Any enlightenment is appreciated.
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