# Search results

1. ### Lead oxide + base

what is the product of the reaction: PbO + NaOH -> ? or PbO + NaOH + H2O -> ?? is there any difference?:frown: thanks
2. ### Demagnetization factor

i have little questions which make me banging my head against the wall over the last few days. now, before too many walls collapsed:yuck: : 1. can anyone direct me to an easy reference which explain how to calculate demagnetization factor N for simple geometries (such as long cylinder...
3. ### Molecular assemblies question

i have been reading articles on molecular assemblies and dna design. my question is: if we take one simple living cell, and using molecular assembly we can create exactly the same copy of that cell (precisely to the atomic level), then will the cell be alive?:bugeye: thanks.
4. ### Help in superparamagnetism devices

does anyone know some useful devices or applications which make use of superparamagnetism phenomena? or a website which discuss these devices. thanks.
5. ### Kondo effect

does anybody know a good website on kondo effect, start from simple introduction step by step. because i don't know anything about it. thanks.
6. ### Superconductor question

change the radius of the ring.
7. ### A simple calculus question

thanks. i just want to clarify again, does it mean dr/dt is small, i.e. small velocity?
8. ### Superconductor question

in deriving quantization of flux in superconductor ring, the momentum of cooper pair p: p=\hbar\nabla\theta=e^*(\Lambda J_s + A) then integrate around the ring, \hbar\oint\nabla\theta dl=e^*\oint(\Lambda J_s + A)dl using stoke's theorem and integrate sufficiently deep in the ring where...
9. ### A simple calculus question

i just want simple explanation of this. velocity v is a function of r and t. then my professor, in order to derive relationship of something, he wrote: \frac{dv}{dt}=\frac{\partial v}{\partial r}+\frac{\partial v}{\partial t} then assuming small spatial variation, \frac{dv}{dt} \approx...
10. ### Superconductor q's

1. is MgB2 brittle? can we draw it like wire and wind it up make a coil/electromagnet with it? 2. liquid nitrogen boils at 77K and freezes at 63K. most superconductor experiments use liquid nitrogen, so does it mean most applications use this temperature range (eg. for squid)? what about...
11. ### Magnetic cooling question

in a book says cooling by demagnetization can be explained in terms of reduced entropy. As far as I know, entropy is associated with order. More order we have (i.e. more spin alignment), then less entropy S. Demagnetization results in disorder, since spins are more disorder, thus higher...
12. ### Copper nickel magnetism

why adding copper to nickel result in reduced magnetism of nickel? :confused:
13. ### Units for susceptibility Xm

i think magnetic susceptibility xm=M/H is contant and independent of the units we are working with. in different refferences, they sometimes quote mks or cgs units which gives different value of Xm. It confuses me. Is Xm units dependent? for example Xm for oxygen is about 10^-4 in cgs...
14. ### What is the proper name for H?

in electrodynamics, we have magnetization M. the other quantity associated with free current is H. i haven't seen a book that gives the name to H. what is the proper name assign to H?
15. ### Magnetism question

paramagnetism is a many body effect. iron when heated up to its crtitical temperature will become paramagnetic of course. paramagnetic is in fact due to orbital overlap! when one unpaired orbital of an atom overlaps with its neighbouring atom, these two unpaired orbital will merge into...
16. ### Magnetism question

i thought paramagnetism might be many body effects?? diamagnetism is clearly not. i read a book which says the orbital 3s2 and 3p2 can form hybrid orbital. This orbitals (when Si atoms are put together in crystal) can either form bonding or anti-bonding configuration. i include the image...
17. ### Magnetism question

silicon atom has 14 electrons with 2 unpaired electrons in 3p orbital. why silicon is not magnetic? :confused:
18. ### Free particle wave equation

for a free particle, the wave equation is a superposition of plane waves, \psi(x,0)= \int_{-\infty}^{\infty}g(k)\exp(ikx)dk and g(k)= \int_{-\infty}^{\infty}\psi(0,0)\exp(-ikx)dx one is the Fourier transform of the other. some cases to solve this is when we assume a small delta k, so...
19. ### QM integration question

if i want to calculate mean square radius by using: \rho^2=\frac{\int \mid\psi\mid^2 r^2 dr}{\int |\psi|^2 dr} with \psi=\sum_{k} a_k exp(ikr). How do I solve the numerator part?
20. ### 4 superconductor questions

1. why, instead of cooper pair, don't we have a group of N electrons coupled for superconductivity? 2. Has cooper pair been observed experimentally? 3. What kind of thermometer do we used to measure near 0 K temperature? 4. Has Josephson tuneling for oxyde high Tc been made? Because...
21. ### Simple superconductor question

suppose we have a superconductor speciment above Tc, and we put it in magnetic field. then we cool it down to below Tc, then a screening current is produced. where does the electron get the energy to initiate the current on the first time?
22. ### 'escape' velocity

really? i thought it's about initial momentum or kinetic energy? in fact krab says smthg different about the atmosphere. i still think the escape velocity is medium dependent, by looking at the way the integral was performed. :confused:
23. ### What Engineering to study

that's no surprise for biomedics.
24. ### Moment of force and tension

i made a lazy sketch here. maybe this is what you meant?
25. ### Mechanics Of solids

when the centre of mass of the system remains static. subsystem mass are not necessarily static.
26. ### Excellent *teachyourself* Calculus books?

just borrow the books from the library. save your bucks.
27. ### What Engineering to study

majority of rich people and successful businessmen did not even go to college.
28. ### 'escape' velocity

suppose we have a stone with infinite melting temperature, what is (numerically) the escape velocity if we consider the AIR FRICTION? has anybody seen the figure yet? how do we calculate it?
29. ### Jobs in Theoretical Physics

that's interesting question by kinsemc. i want to know the same thing. suppose some one with phd degree in theoretical physics, specializing in ... let's say string theory? is there any conceivable job beside teaching?
30. ### If there is no atmosphere,

what will be the colour appearance of the sea on earth?
31. ### Aluminium compound causes cancer?

whenever i use deodorant (any brand of deodorant), i always see similar aluminium compound- i cannot remember the name. is this metal compound dangerous for the skin? :frown: i am not chemist, and i know very little about chemistry.
32. ### Can Anti-Matter be classed as

mmm... i am not used to american style.
33. ### Can Anti-Matter be classed as

i remember one of my best professors said, "i sometimes ask my phd students some silly questions, and i mean it. we often have gaps in our understanding, no matter how good we (think we) are." and he is so damn good. yet he is so humble.
34. ### Stuck on Thermodynamics

ooo... you correct. atmospheric pressure! thanks for mukundpa so resultant pressure is 1.56*10^5 - 1*10^5 = 0.56 * 10^5 Pa. the force on each surface is 0.56 * 10^5 * 0.115=6400 N (2 significant figures). the last bit is ok, but what about multiplying with 3/2, as i said in the formula...
35. ### E is an irrational number

there may not be a single line of truth there in the process. e is not rational. you use the taylor series inappropriately, what is theta? actually the exact definition of e is e=\lim_{h\rightarrow 0}(1+h)^{\frac{1}{h}}=\lim_{h\rightarrow \infty}(1+\frac{1}{h})^h=2.718281828 . . . ..
36. ### Programs Omg, why do I have to take these classes for a math degree?

you mentioned database? here's a good lesson from my life experience: "you will never regret to learn database!" i believe the same thing for other comp sci modules. programming skills is vital for physicists and math-man.
37. ### Do Physicists Have To Work From Big Cities?

some particle accelerators are located in remote areas. astronomical observatory too.
38. ### Stuck on Thermodynamics

force = pressure x area. force on each side = pressure x area on each side of cube. in kinetic model, T is proportional to KE. precisely KE = (3/2)NkT for total N particles. an e.coli consists of water and other molecules, as you said. your job is to find N. mass of water in one e.coli =...
39. ### Lenses/Mirrors, [1/f = (1/do+1/di)] sign help

focal point is no problem, as you said. becareful with do and di. now, if i use light rays coming from left to right, and the mirror/lens is on the right, for mirror: do is positive when object is on the left, in front of mirror, i.e. real object. do is negative when the object is on the...
40. ### Prove of refractive index

i derived this equation before. the formula is just an approximation for small angle case, where sin\theta=tan\theta for small angle. i.e. it is for the case where you almost look vertically down to the object from above. you can derive it easily by drawing slender triangles. very easy. if...
41. ### Not too hard tension question

what do you mean? i use limiting equilibrium assumption
42. ### Not too hard tension question

Vertically: Tsin\theta+N-mg=0 where N is the normal force by the floor upwards, so N = mg-Tsin\theta horizontally: Tcos\theta - \mu N = 0 substitute for N from the first equation, express T in terms of theta, T=\frac{\mu mg}{cos\theta + \mu sin \theta} minimum T is when \frac...
43. ### How blue-tack sticks?

suppose we have a very smooth and hard surface such as mirror, and we stick a blue tack on it. there is no chemical reaction. what force keeps it stuck? is it electromagnetic?
44. ### Stopping particle spin

i know, i read this thing long ago. one qm book says electron dimension is smaller than any physical meaning, and if the physical spin applies, the speed of the circumference will exceed speed of light etc etc. spin is just intrinsic angular momentum, it's the most basic mathematcal concept in...
45. ### Stopping particle spin

is it possible (just in theory) to reduce the spin of a particle to zero? take a lepton for example - to reduce complications. if it is, what will happen? ?
46. ### Simple dirac delta question.

probably this problem will never arise in physics. i never see it myself in any physics textbooks.
47. ### Colonization Of The Milky Way Inevitable

our knowledge is probably only 0.000000000000000000000001% of the truth.
48. ### Colonization Of The Milky Way Inevitable

They communicate with dark energy signal ! . . . that's why we cannot contact them. :bugeye:
49. ### Simple dirac delta question.

just curiosity, if you integrate dirac-delta from exactly zero to infinity, will you get one or a-half? since it is symmetrical about zero, i think it is a half. is it? i mean: \int_{0}^{\infty} \delta(x) dx=\frac{1}{2} or is it 1? thanks.
50. ### Planet magnetic field

ehm,... not lava. but LARVAE ... he he heh...