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My Physics Questions
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by kudoushinichi
on 2007-08-27 08:19:32 (edited 2007-08-27 08:45:35)
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I'm a humble pre-u Physics student with lots of questions and eager to learn. So here I'm going to post my questions. 1. I'm having a lot of trouble in understanding the concept of Internal Energy... The question is; Which of the following system is the Internal Energy increasing? a) water freezing at constant Temperature b) a stone falling under gravity in a vacuum c) water evaporating at constant Temperature d) stretching a wire at constant Temperature My thinking: I know that Internal Energy is defined as the sum of the microscopic Kinetic Energies and Potential Energies of the system... but I don't quite understand fully the concept yet... In situation a) water molecules come closer to each other... so does that mean work is done on the system (hence, there is an increase in the Internal Energy according to the 1st Law of Thermodynamics)? But there the system is at constant Temperature and according to my understanding, Internal Energy only depends on Temperature and the amount of atoms (U = 3/2nRT). So does this mean that the Internal Energy is constant? In situation b), the stone is falling in vacuum, which means there is no air resistance. So the stone is accelerating, velocity is increasing, hence kinetic energy is also increasing, right? Does this Kinetic Energy counts as Internal Energy? If it is counted, then it means the Internal Energy of the system is increasing, right? In situation c), water molecules are moving away from each other, hence doing work against its surrounding. So according to the 1st Law of Thermodynamics, the Internal Energy should be decreasing, right? But the system is at constant Temperature... does that mean the Internal Energy is not changing? In situation d), stretching a wire means we are increasing the distance between the atoms in the wire. Does this mean we are increasing the Potential Energy of the system or are we increasing it? But the system is at constant Temperature... so no change in Internal Energy?? This thing is giving me a big headache. I am inclined to believe that options a) and b) are correct. But I am not sure... I hope someone would clarify my understanding of Internal Energy and help me with this... 2. Here's a question about electromagnetic induction... We have a transformer and the current in the primary coil is varying according to the equation I = I(max) sin ωt. If we plot the graph of current against time, we will get a nice sine graph. Now, if we were to plot the graph of magnetic flux, Φ in the core against time, we will also get a sine graph. But why is this so? Can anyone explain this to me, mathematically or otherwise? Cos when I used the equations B = F/(IL) and Φ = BA, I ended up with a csc graph... If we were to plot a graph of emf, E induced in the secondary coil against time, then we use Faraday's and Lenz's law (E = dΦ/dt) to differentiate the Φ against t graph and we will get a -cos graph. Is this correct? 3. The value of acceleration of free fall, g at the equator depends on the Gravitational Force of the Earth and the Centripetal Force due to the rotation of the Earth about its axis. The Gravitational Force on a mass of 1kg at the equator is about 9.80N and the Centripetal Force due to the rotation of the Earth is about 0.0337N. My question is, the the value of g 9.83 m/s^2 or 9.77 m/s^2 and why?
Shinjitsu wa itsumo hitotsu!
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Re: My Physics Questions
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by
on 2007-08-27 10:33:32 (edited 2007-08-27 10:43:10)
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Ok, first off, let's be honest: this is your homework assignment. Now, I'd feel dishonest just doing your homework for you. 1. Internal energy is a neat way of thinking about a system. A system is a closed off bunch of stuff doing things. Like, water boiling or a rock falling or whatever. I will remind you how to evaluate the energy in each of the examples, which will be left up to you. A) Freezing is a phase transition. It's easiest to think about this problem in reverse, what do you have to do to zero degrees F ice to turn it into zero degrees F water? B) This is a Newtonian gravitation problem. Again, lets think in reverse: What do you have to do to put the rock back up where it was? C) Another phase transition. D) What happens when you let go? 2. I don't understand your question. You've done a poor job translating the wording of the assignment into first person. Electromagnetic induction is awesome stuff, try using a real coil to help satisfy your uncertainty of the problem. Empirical solutions rock. 3. What happens to people standing on a marry-go-round that's rotating real fast? We know imperially they get thrown off. The centripetal force is an outward force. The force of gravity is an inward force, cause that's where it pulls your face when you trip and fall. Ouch! They point in the opposite direction (outwards is the opposite of inwards). So, lets take the difference: -9.80m/s2 + 0.0337m/s2 = -9.77m/s2 rounded to two decimal places. I like to orient so that inwards is negative, cause it sucks to fall on your face, but you can orient your access upside down from that, and call it 9.77m/s2. 
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Re: My Physics Questions
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by kudoushinichi
on 2007-08-29 00:54:10
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This isn't an assignment. These are actually past-year questions from the exam I am taking at the end of this year. I have the answers to these questions, the problem is; I don't understand why the answers are as they are. I have consulted my lecturer and his explanation is unsatisfactory. So I came here to seek clarification and to better my understanding of the problems. I am not seeking shortcuts to finish an 'assignment'. In case you didn't notice, I wrote down my line of thoughts on how the question should be tackled and not merely writing the problem down. I want to know whether or not my lines of thought are correct. I have much respect in your Physics knowledge, Gendou-san, but I feel quite insulted with your accusation. However, your hints helped and I appreciate it very much. So on to the questions; 1. For your hints, these are what I am able to surmise a) To turn ice into water without change in temperature, we have to supply heat. Since the situation is in the reverse, heat is lost. Hence the internal energy of the system is decreasing. Is this correct? b) To put the rock back where it was, we need to do work on it. In the reverse situation, if the stone falls freely, gravity is doing the work. However, according to Wikipedia, 'Internal energy does not include the translational or rotational kinetic energy of a body as a whole'. So I'm thinkin that the internal energy of the system is unchanged. Am I right? c) To change water to vapour, heat is supplied. Hence, according to the first law of thermodynamics, the internal energy of the system is increased. Correct? d) So, with reference to your hint, stretching the wire is really doing work on the system. Hence, again, according to the first law, the internal energy is increased. Is this right? If I am correct for all of the above, this means options c) and d) are correct. Please correct me if I've got anywhere wrong in my explanation. So I think I have a better understanding of internal energy now. I had the initial impression that internal energy only depends on temperature. I realise now that it is actually the internal energy of an ideal gas that is only dependent on temperature. 2. Forget about this question for awhile, I'll need to show figures... 3. According to my understanding, centripetal force is always directed towards the centre of the circle. So why in this case the centripetal force is an outward force? I am aware that a standing on a fast-spinning merry-go-round will make us feel as if we are about to be thrown of. So what is wrong with my understanding of the direction of the centripetal force?
Shinjitsu wa itsumo hitotsu!
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Re: My Physics Questions
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by none123456
on 2007-08-30 15:28:01
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I'll attempt to answer #2 - Im assuming that your getting a nice sine graph for current because of an AC current. Question though to help you - how exactly does a transformer work? electric current induces a magnetic field which in turn creates an electric current on the other side. Thus the electric current directly affects the magnetic field. ... I get completely lost on the last part of ur 2nd question... but I don't know graphs that well :p #3 - By Definition F(c) is always a force directed outwards of the circle. There is something else forcing inwards, depending on the situation. In your example, the conflicting forces are Fc and Fg. |
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Re: My Physics Questions
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by
 
on 2007-08-30 15:54:24 |
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Wow, physics questions in a Anime forum... nice. I'm a IB student so I can help out a bit. #1 (A is not correct, using the formula U=1.5nRT, you are given that temp is constant and water freezing. The amt of H2O molecules is never stated so it's safe to assume that no extra molecules are gained or lost and "R" is a constant number. Using that info, you have a constant times 3 others making it the same internal energy until the either the temp is changed or some molecules come in or go missing. C is basically the same exact thing. D did not say anything about a spring. If it was, then that would be an answer but of course, no. B never stated that you had to put it back up. As far as I'm concerned, the way the problem is worded means it's asking whether the system is increasing at THAT specific point in time. Not over a long run. Now figure the rest out for #1 2) I forgot this one, I must admit, it's been 2 years since I took another physics course. But I am a calculus 3 student so if you give me all the formulas, maybe I can figure out a relationship. 3) Centripetal force is always pointed outward, I don't know where you got inwards. You're on earth, it's spinning, so you should be flying into space, but some other force counteracts Fc which is gravity. The guy before me did the math. Anyways, admin, what classes did you take >.>; and grade level?  |
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Re: My Physics Questions
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by kudoushinichi
on 2007-08-30 21:06:40 (edited 2007-08-30 21:09:20)
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@none123456 Okay, I'll try to reword my question. An ac current is supplied to the primary core. So, the function for the current in the primary coil is I = I(max) sin ωt. Thus, if we plot a graph of current in the primary coil, I against time, t, we will get a sine graph. Now, the varying current creates a varying magnetic field around the coil. My question is, what will the graph of magnetic flux, Φ in the iron core against time, t look like? According to the answer I have, the graph of magnetic flux, Φ against time, t is also a sine graph... Why is that? Here's how I initially thought it should be... We have the formulas B = F/(IL) ...(1) Φ = BA ...(2) Where B = magnetic field strength F = force on wire I = current in wire L = length of wire Φ = magnetic flux A = area So equation (2) can be written as, B = Φ/A ...(3) which I substituted in equation (1), F/(IL) = Φ/A So I solved for Φ, Φ = FA/(IL) and then I put in the ac current function from above, I get, Φ = FA/(LI(max) sin ωt) = FA/(LI(max)) x csc ωt ... (5) So from this derivation of mine, the graph of magnetic flux, Φ against time, t should be a csc graph. But obviously, this is wrong be cause this isn't what is said in the answer... I want to know why is this wrong and why does the graph of magnetic flux, Φ in the iron core against time, t gives a sine graph. @SkyL For 1., I don't agree with you about situation A. Because the formula U=1.5nRT only applies to ideal gases. I still think that the internal energy in system A is decreasing... Like Gendou said, situations A and C are phase transitions. Hence, internal energy should change. For situation D, you should know that all material are to some extent elastic, right? It doesn't matter if it is a spring or just a straight wire, both are elastic to some extent. Now, if we were to stretch the spring, the distance between the atoms in the spring should increase a little; which means we are doing work on the wire. Since the wire stays at constant T, there is no heat loss or gain. Hence, according to the first law of thermodynamics, the internal energy of the system should be increasing. That's how I think it should be. For question 3, I still don't understand why in this situation the centripetal force in an outward force. Because according to Wikipedia, 'the force is directed inward, toward the center of the circle.' My textbook says that too.
Shinjitsu wa itsumo hitotsu!
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Re: My Physics Questions
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by
on 2007-08-31 17:19:40 |
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Best way to answer your question 3, Ride the merrygoround or a car going in circles and see where you start flinging off to. Wikipedia is great for general knowledge, but not so much technically. Best way to understand physics is to actually experience. Well, Newtonian physics I mean. As with #2, I need to go read a physics book again. I forgotten a lot of stuff. Yes, I know about #1. But your question still seems to me that it's asking for that specific point of time. Shouldn't you ask your teacher for help? I thought they are the first person to be asking. |