# Search results for query: *

1. ### Molar heat capacity in metals and ionic solids

I am doing my revision and noticed that metals all have a molar heat capacity ~25 J/mol/K = 3R. Ionic solids such as NaCl and CaF2 however have different molar heat capacities. (~51 and 72 respectively) Why is this? there is no explination that my lecturer gave and I can't find it online but...
2. ### Resolution on my detectors

i am calibrating a detector set up using alpha particles to find resolution. i have 2 silicon detectors a thin and thick one where the alpha is expected to pass through and lose energy in the first and be stopped by the second. My handbook says i can find the energy resolution in the second...
3. ### Photon counts, finding background

Homework Statement I have been given some data, which is 20 successive 500second bins containing raw photon counts.(ie signal+data) The xray source is known to show flaring and from optical properties one suspects that it may show eclipses lasting ~1000sec. I need to estimate the...
4. ### Showing that a model is not a good fit

thanks for replying so this is what i have got from your response: so if my area calculated is the prob of getting more than the chi squared (216) is 0.0345, then this means that at a 5% significance level it is unlikely that we will get a result of more than 216. but I am not quite sure...
5. ### Showing that a model is not a good fit

ok so i have some data (d) of star counts (N=181), and a model (m = b-Fo where b=5 and Fo-constant flux) I have found the chi squared value = 216 I know that the number of degrees of freedom here is N-parameters = 181-1 = 180 my question is: "show that the model is not a good fit to the...
6. ### Minimum chi squared estimation

hi i have some data (star counts) and i have a model and i want to perform min chi squared so if i call my data di, and my model mi with std dev = \sigmai = 1 then \chi^2 = \sum \frac{(di - mi)^2}{\sigma i^2} no my model is this mi = bi - Fo where bi is the background which has been...
7. ### Mode of a probability distribution

Homework Statement I have a luminosity prob distribution that i want to plot and find the expectation value and mode for. Homework Equations p(L)dL = A \frac{L}{i}^aexp(-\frac{L}{i}) \frac{dL}{i} A=const a= -0.7 i= 1.4e10 solar luminosity units lower limit = 1e9 solar luminosity...
8. ### Estimated probibility does not match results, why? (galaxies)

Homework Statement my book states that approx 75% of galaxies in the universe are spirals, but then it says that out of a sample of 14 from the core of a large cluster of galaxies only 3 were found to be spiral. it was suggested we could use binomial distribution to look at the prob of...
9. ### Linear attenuation coefficient

Homework Statement sphere r=1cm contains same density of H2O as surroundings with additional N=10^20 iodine atoms/cubic cm linear attenuation coeff (u) for water at 50keV = 0.2 /cm crosssection for interaction of 50kEV photon with iodine = c = 3.6x10^-22 /cm^2 a) calculate u for the...
10. ### What is meant by density in units of critical density?

what is meant by density, such as the baryonic matter density in cosmology, in "units of the critical density"? its mentioned in a question i need to do but i don't really know what it means.
11. ### Taylor expansion around z=0

Homework Statement I have the function (a(1+z)3 + b)-1/2 and i need to taylor expand it around z=0 to the first order, a and b are constants, there sum is equal to one. I have the answer: 1 - (1+q)z where q = a/2 - b This is in my physics book but it does not explain the...
12. ### Ratio of number density of photons to baryons

Homework Statement photon/baryon ratio = Np/Nb Np, is the number density of the CMB photons Nb, is the number density of baryons Evaluate this ratio at the redshifts z=0 and z=100. From my understanding, if interactions are negligable, particles cannot simply disappear. Therefore...
13. ### Fourier transform on an image

Homework Statement Okay so i am applying a FT to an image of particles that are forming a lattice, and i need to find the average distance between the particles because its not a perfect lattice, I am getting an airy pattern and i believe that the distance to the first ring is the average...
14. ### Errors on velocities and distance.

Homework Statement A galaxy has velocity due to Hubble (Ho) expansion, and in addition a peculiar velocity (AN this is the velocity of the galaxy in its local enviroment/gravitational potential) this vel. is 450Km/s I need to estimate the the minimum distance at which a galaxy must be in...
15. ### Positron-electron collision can someone check my answer?

wait is it just zero?...thats what i got from doing some rearrangement (too much to write out)
16. ### Positron-electron collision can someone check my answer?

ok so on the left the Ecm is 1000GeV, is pcm2c2 = Ecm2 - m2c4 ...wait i don't think that helps... my lecturer didnt really go over how to get the momentum. the only thing he metioned ws the 4momentum but i don't think that is what i use here
17. ### Positron-electron collision can someone check my answer?

well i set it to m2c4 cause this is the invariant mass and so i can find this in the lab frame and it will be the same in the cm frame. when i found the relation for this i went to the cm frame to use its Ecm to find the energy i need for the beam. now while writing this i think i should...
18. ### Positron-electron collision can someone check my answer?

as in that i should have put (2m)2c4 on the left hand side?
19. ### Positron-electron collision can someone check my answer?

(E+mc2) is the sum of the energies of the 2 particles. the e- is stationary so i thought that its energy is just mc2 in the lab frame p2c2 i didnt put a p(e+) +p(e-) because the momentum of e- is zero so its just the mom. of the e+
20. ### Positron-electron collision can someone check my answer?

ok so i tried this instead mass of e+ and e- => m2c4 = (E+mc2)2 - p2c2 = E2 - p2c2 + 2Emc2 + m2c4 = 2Emc2 + 2m2c4 and as mc2<<E m2c4 = 2Emc2 now this is where I'm abit unsure: can i say: m2 = (1000GeV)2? cause this would give (1000GeV)2 = 2E(0.51MeV) so E = 1.96x1018eV
21. ### Positron-electron collision can someone check my answer?

where is it that I've gone wrong? what would you suggest to try and get the right answer?
22. ### Positron-electron collision can someone check my answer?

okay then thanks for that... also could you just look at this as well? i wanted to try it before i put it up if the same energy in the cm were to be achieved by colliding a beam of positrins with a target of stationary electrons, what energy of the positron would be required? in cm frame...
23. ### Positron-electron collision can someone check my answer?

Homework Statement a positron and electron travel in opposite directions each with energy 5ooGeV and collide head-on. what is the energy in the centre of mass (cm) of the collision? The Attempt at a Solution E(cm) = E(e+) + E(e-) = 500GeV + 500GeV = 1000GeV is this right? thanks
24. ### Using stokes theorem to find magnetic field

Homework Statement magnetic field is azimuthal B(r) = B(p,z) \phi current density J(r) = Jp(p,z) p + Jz(p,z) z = p*exp[-p] p + (p-2)*z*exp[-p] z use stokes theorem to find B-filed induced by current everywhere in space Homework Equations stokes -...
25. ### Energy density of photons and matter

thats ok, its just that it doesn't seem right. is it correct to get the temp by saying T~1/(z+1)? cause this just gives values i would have thought as being too small. or is this the overall temp of radiation and matter? and i only want the CMBR temp
26. ### Energy density of photons and matter

but surely erad/em = 1 [cause I am looking for equality] and so erad/em = 1 = erad0(z+1)/em0 so erad0/em0 = 1/(z+1) therefore z+1 = em0/erad0 which is what i put
27. ### Energy density of photons and matter

ok so em0/erad0 = 1+z z= (1.8x10-4)^-1 - 1 = 5526.0066027 this gives a temp of the CMBR => T=1/(z+1) = 1.809x10-4 K this doesn't seem right. surely temp should be larger than now?
28. ### Energy density of photons and matter

ok i just tried z+1 ~ (1.8x10-4)-1 so z is ~5526 i know this doesn't take into account 3 types of neutrinos do i times 1.8x10-4 by 1.68 to take them into account?
29. ### Energy density of photons and matter

ok thanks for that the reasoning behind that makes more sense now btw the next part says to calculate th redshift at which the energy density of matter = that of radiation (i.e. the CMBR photons) i tried: e = energy density e[rad] / e[matter] = 1 when equal and this is also...
30. ### Energy density of photons and matter

oh from the E=mc^2 relation of mass to energy.
31. ### Energy density of photons and matter

i am thinking that i times the baryonic mass density by c^2 this would give 1.44Gev/m^3 so the ratio would be 0.261Mev/m3 / 1.44Gev/m^3 = 1.8125x10-4
32. ### Energy density of photons and matter

hi im doing a question at the moment and am having issues understanding the question, and i can't ask my lecturer as he is stuck abroad with no internet. the question asks to calculate the ratio of current energy density of CMBR photons to that of baryonic matter. the present density of...
33. ### Entropy change in an isolated system

hi I am having a few conception problems with entropy problem: a solid with constant heat capacity and initial temp To, is brought in contact with hotter reservoir temp T1 and allowed to come into equilibrium. This is all isolated. a) is the change in the entropy of the universe negative...
34. ### Probability of occupancy in energy level as T->0 or inf.

an atom has 3 non-degenerate E levels, = 0, e, 2e What is the prob of occupancy of each energy level as T-> zero and infinty and why? for E=0 I imagine that the prob for T->0, is 1 is one or approaching one as as the temp approaches absolute zero electrons occupy the lowest state as...
35. ### Distance travelled by wave at whichamplitude decreases

ok so I've tried this: E=Eo*exp[i(wt-kz)] =Eo*exp[i(wt-krz)]exp[-kiz)] - shows amp falls by exp[-1] so =Eo*exp[i(wt-krz)]exp[-5kiz)] - shows amp falls by exp[-5] therefore absorption coeff = 5ki absorption length = 1/5ki = c/5niw ki = niw/c w = 2pi*c/n2L...L = wavelength =...
36. ### Distance travelled by wave at whichamplitude decreases

okay so the question involves a boundary between 2 dielectrics. one with n1=1.5 the other n2=1.4 incident angle of wave I= 80deg critical angle i found to be C= 69deg given wavelength in air is 700nm 1) prove component of wave vector parallel to boundary is = ky = k2*(n1/n2)*sin(I)...
37. ### Potential due to electric dipole

oh yeah... of course:rolleyes:should have realized that earlier... thanks for the help, that's another revision topic i can tick off btw one quick question unrelated - I'm doing a question at the moment that simply states that there is a laser beam with power = 15MW/m^2 and i need to give...
38. ### Potential due to electric dipole

okay s if i use 1/r*d/dtheta then Etan becomes = -(-psin(theta) / 4(pi)(epsilon)r3) okay i get that now...thanks btw just so i can get the physical pic in my head, is the tangental component of V perpendicular to the moment 'bar', or is it simply horizontal? cause i pictured the diagram to...
39. ### Potential due to electric dipole

i'm not sure on how to do a tangential derivative...i may have done it in maths but didn't know that's what it was called. is it related to spherical coords?
40. ### Potential due to electric dipole

well E = -(grad)V so could if i use d/dr? and get E = = -psin(theta) / 4(pi)(epsilon)r3 would i then just take E[tan] = Esin(theta) if we assume the dipole is at an angle theta to the E field? but what about the - sign
41. ### Potential due to electric dipole

potential due to electric dipole is V = p.r / 4(pi)(epsilon)r3 show the tangential component of the electrical field is = psin(theta) / 4(pi)(epsilon)r3 what I've tried: i assumed there are 2 charges separated by distance r potential difference U = qV = -qE.r so can i...
42. ### Show eigenfunctions are orthogonal

hi one of my past papers needs me to show that if 2 eigenfunctions, A and B, of an operator O possesses different eigenvalues, a and b, they must be orthogonal. assume eigenvalues are real. we are given \int A*OB dx = \int(OA)*B dx * indicates conjugate
43. ### Wavefunctions and boundaries of a 1D-potential well

for the first thing your right i did write it down wrong t=it was meant to be b<x. but with the the reason i wrote that the first derivative of the wavefunction is not continuous at the boundary x=-a, and b was cause i thought that when on the otherside of a boundary the potential->infinity...
44. ### Wavefunctions and boundaries of a 1D-potential well

for the first thing your right i did write it down wrong t=it was meant to be b<x. but with the the reason i wrote that the first derivative of the wavefunction is not continuous at the boundary x=-a, and b was cause i thought that when on the otherside of a boundary the potential->infinity...
45. ### Wavefunctions and boundaries of a 1D-potential well

for the first thing your right i did write it down wrong t=it was meant to be b<x. but with the the reason i wrote that the first derivative of the wavefunction is not continuous at the boundary x=-a, and b was cause i thought that when on the otherside of a boundary the potential->infinity...
46. ### Wavefunctions and boundaries of a 1D-potential well

|.........| |.........| Vo < E |.........| |......__________| |......| |......| |......| |......| -a......0....b V(x)= infinity --- x<-a --- region 1 0 --- -a<x<0 --- region 2 Vo --- 0<x<b ---...
47. ### Wavefunctions and boundaries of a 1D-potential well

the attachment here shows a potential well and i am practicing writing down wave functions in each region and boundary conditions (for this problem I suppose the boundaries would be x= -a, 0, b. i think that for this problem i need 4 functions and i thought that they may be: when x<-a...
48. ### Degeneracy in energy levels

what do you mean I'm using the hydrogen quantum number that i shouldn't be?
49. ### Degeneracy in energy levels

okay this isn't exactly me asking how to solve a question but just to verify the theory in my head. i need to calculate the 4 lowest energy levels for a 3d box (that has sides (2a,a,a)) and show all the quantum numbers (i imagine it means the x,y,z components of n, l, ml) and give the...
50. ### Fiinite potential well V E

so after thinking on it believe the ground state would stay the same as is the zero point energy which doesn't change, but then would the 1st excited state have enough energy to become unbound from the potential well