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• Users: Flucky
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1. ### Metal sheets of various mass left out in the Sun

Yes that makes sense, thanks again. You've been very helpful so I am going to push my luck and ask if you have any criticism of my logic for part b)?
2. ### Metal sheets of various mass left out in the Sun

Ok great, thanks. That has cleared it up for me - but just to fully make sure I have another hypothetical for you if you don't mind: Now we have one thin sheet and one cube, again the same type of metal. The front surface areas of both are equal which means the same amount of radiation is...
3. ### Metal sheets of various mass left out in the Sun

So let's assume they have been left in the sun for a long time and are in equilibrium. Each sheet will be at the same temperature right? But the amount of heat hitting each sheet (therefore the amount of heat lost to the surroundings) is also the same. So the air on the surface of each sheet...
4. ### Metal sheets of various mass left out in the Sun

Actually thinking about it a bit more, for a) if the sheets have the same dimensions therefore the same exposed surface area, would the amount of heat Q hitting the sheets be constant? Because the same amount of radiation will be hitting each one? Edit: I think I'm confusing myself. The...
5. ### Metal sheets of various mass left out in the Sun

These are questions I've made up to try and wrap my head around the topic, so the theory within the questions themselves might be flawed. 1. Homework Statement There are a number of metal sheets that are similar in every way except they each have slightly different masses. The temperature of...
6. ### Which of the SEMF terms account for low A binding energy?

Homework Statement Which of the semi-empirical mass formula terms accounts for a drop in stability at low A and why? Homework Equations B = avA - asA\frac{2}{3} -acZ(Z-1)A\frac{-1}{3} -aa(A-2Z)2A-1 ± apA\frac{3}{4} The Attempt at a Solution I'm struggling getting my head around this one. The...
7. ### Equation for neutron proton-fusion, and deuterium photodiss.

Forgot I had posted this question. Thanks!
8. ### Equation for neutron proton-fusion, and deuterium photodiss.

Homework Statement Write the equations for neutron and proton fusion to form deuterium and for deuterium photodissociation. Homework Equations The Attempt at a Solution There is plenty on proton-proton fusion to form deuterium, but not much on proton-neutron. Is it simply p + n -> D ? Is...
9. ### Using nuclear shell model to determine parity and spin

Great, thanks for the help.
10. ### Using nuclear shell model to determine parity and spin

I'm pretty sure we'd only be working with the first one, the Nilsson diagram looks more advanced. Going off the first diagram it looks like our lone neutron would be in the 1f\frac{7}{2} shell where nuclear spin j = \frac{7}{2} and parity = (-1)3 [to the power 3 because it is in the f shell] =...
11. ### Using nuclear shell model to determine parity and spin

Homework Statement Use the shell model to determine the parity and spin assignments for all the stable isotopes of calcium. Homework Equations n/a The Attempt at a Solution The stable isotopes seem to be 40Ca, 42Ca, 43Ca, 44Ca, 46Ca, and 48Ca I believe all of those isotopes except 43Ca are...
12. ### Can't get my head around reduced mass (particle physics)

I will create a new thread with proper formatting, and pose the question as a whole - so could you delete this thread please? Edit: properly formatted thread at https://www.physicsforums.com/threads/calculate-the-wavelength-of-the-n-4-3-transition-in-he.791525/
13. ### Can't get my head around reduced mass (particle physics)

Hi Orodruin I'll give you the full question to give a better understanding- Calculate the wavelength of the n = 4 → 3 transition in 4He+ to an accuracy of 4 significant figures. (R∞=109 737 cm-1.) (Fine structure effects can be neglected.) Now the equation that I'd use for this is...
14. ### Can't get my head around reduced mass (particle physics)

I know it's probably simple but I just don't understand reduced mass. I am trying to work out the reduced mass of 4He+. m = memN / me + mN Can somebody please just explain step by step what I do. This is only a segment of a 1 mark question and I'm getting my knickers in a frustratingly twisty...
15. ### Lennard-Jones Potential: Derive an expression for equilibrium constant

Finally found a similar question in the depths of the internet and looks like it is right, so all is good here.
16. ### Lennard-Jones Potential: Derive an expression for equilibrium constant

Edit: well this is frustrating, not sure why the itex things aren't working.. Edit2: I've attached a picture of the question at the bottom as well. Homework Statement The cohesive energy of a solid noble gas may be written as (See picture below) Derive an expression for the...
17. ### Entropy change of melting ice cube initially at -5°C

Great, thanks
18. ### Entropy change of melting ice cube initially at -5°C

Homework Statement Calculate the entropy change of an ice cube of mass 10g, at an initial temperature of -5°C, when it completely melts. cice = 2.1 kJkg-1K-1 Lice-water = 3.34x105 Jkg-1 Homework Equations dQ = mcdT dS = \frac{dQ}{T} ΔS = \frac{Q}{T} Q = mL The Attempt at a...
19. ### Diffraction with a large array of slits

I've had a look all round but can't seem to find anything that helps, unless I'm missing something obvious. I've played around with a few applets as well. The things I found focused on the first equation, and the questions would often be "find the #th maxima" with slit width etc given. The 90...
20. ### Diffraction with a large array of slits

Hi all, exams soon and I'm stressing out over this small question. If anyone could guide me through, explaining why you're doing what you're doing that'd be beyond great. Homework Statement Light of wavelength λ is incident normally on a screen with a large array of slits having equal widths...
21. ### [Statistical Physics] Microstates in a large system/Boltzmann entropy

Ah that's good then. I just thought it was an unreasonably large number. JK^{-1}
22. ### [Statistical Physics] Microstates in a large system/Boltzmann entropy

Maybe a mod could move this into the Advance forum?
23. ### [Statistical Physics] Microstates in a large system/Boltzmann entropy

Shameless bump
24. ### [Statistical Physics] Microstates in a large system/Boltzmann entropy

Homework Statement For a box containing 1m^{3} of nitrogen at S.T.P., estimate the number of microstates which make up the equilibrium macrostate. Homework Equations S = Nk_{b}(ln\frac{V}{N} + \frac{5}{2} + \frac{3}{2}ln\frac{2πmk_{b}T}{h^{2}}) where the entropy of a volume, V ...
25. ### [Statistical Physics] No. of dice rolls so prob of getting a 6 is >90%

"At least"... such a rookie mistake missing something like that. It's unreal how much time I've just wasted. Thanks, I've got it now.
26. ### [Statistical Physics] No. of dice rolls so prob of getting a 6 is >90%

This question is really wearing me out, any ideas? Homework Statement How many times must I throw a fair dice in order to make the probability of getting at least one ‘6’ greater than 90%? With this many throws, what is the probability of getting three ‘6’s? It's the first part I'm...
27. ### [Statistical Physics] Probability of finding # photons in the mode

Ooh thanks for pointing that out, I accidentally put a minus in there.
28. ### [Statistical Physics] Probability of finding # photons in the mode

Ok think I've got the relevant equation for a) and b) now. For any future readers: P(n) = \frac{1 - exp(-\frac{ħw}{kT})}{exp(\frac{nħw}{kT})} where n is the number of photons in the mode.
29. ### [Statistical Physics] Probability of finding # photons in the mode

You've just made me realize the equation should have a -1 on the bottom. Can't edit my original post for some reason.
30. ### [Statistical Physics] Probability of finding # photons in the mode

Ah ok, so the mean number of photons might still be ok. How do I go about answering a and b?
31. ### [Statistical Physics] Probability of finding # photons in the mode

Homework Statement A cavity contains black body radiation at temperature T = 500 K. Consider an optical mode in the cavity with frequency ω=2.5x10^{13} Hz. Calculate a) the probability of finding 0 photons in the mode b) the probability of finding 1 photon in the mode c) the mean number...
32. ### Electrons bombard gaseous hydrogen - photon energies emitted?

Thanks maajdl
33. ### Electrons bombard gaseous hydrogen - photon energies emitted?

Homework Statement A beam of 13.0 eV electrons is used to bombard gaseous hydrogen in ground state. What photon energies will be emitted? Homework Equations ΔE = -13.6 (\frac{1}{n^{2}_{f}} - \frac{1}{n^{2}_{i}}) However hydrogen is in the ground state therefore n_i= 1: ΔE = -13.6...
34. ### Thermal physics - entropy change (latent heat, specific heat etc)

Hi Chet thanks for pointing that out, missed it completely
35. ### Thermal physics - entropy change (latent heat, specific heat etc)

Hi all, could somebody have a look over my answers for this question please? The value I got for the second part seems quite feeble. Homework Statement The Attempt at a Solution Part a) Key m = mass cs = specific heat bronze cm = specific heat molten bronze Tfus = melting...
36. ### Luminosity & flux equations confusion

I didn't really think about it so automatically assumed "ah it must be velocity"
37. ### Luminosity & flux equations confusion

Hi phyzguy thanks for replying. Seems to be going for £88 on Amazon so I might be giving that a miss. Am I right in saying the following: --------------------------------------- Monochromatic Flux is the energy falling on a unit area, per unit time, at a given frequency. As no light...
38. ### Luminosity & flux equations confusion

Hi folks First off I have only just figured out that v is used as bloody frequency in loads of astrophysics equations. Not fun. I've got monochromatic flux as: Fv = dE / dt·dA·dv now I'm happy with this. But now the lecture notes I'm looking at describes monochromatic luminosity as...
39. ### Kirchoff's rules & resistors in parallel

Thanks for replying but we're back to the beginning now as that's what I had already got, it's the next bit I'm having trouble with - getting the equation 1/R = 1/R1 + 1/R2
40. ### Kirchoff's rules & resistors in parallel

That's what I got for my loop 1, so it must be my loop 2 that's incorrect. If it's not I2R1 - I3R2 = 0 is it E - I2R3 = 0 ?
41. ### Kirchoff's rules & resistors in parallel

Sorry I must be missing something, I can't see what's wrong with the 2nd law bit
42. ### Kirchoff's rules & resistors in parallel

Homework Statement Use Kirchhoff’s junction rule and loop rule (twice) to find the effective resistance of the two resistors connected in parallel as shown, and demonstrate that this agrees with the expression 1/R = 1/R1 + 1/R2 (poorly drawn picture of the circuit) The Attempt at a...
43. ### Revolutions per minute, angular deceleration

Just as a check did anybody get the answer 363 revolutions?
44. ### Revolutions per minute, angular deceleration

Right OK just found an equation θ = ωit + 0.5αt2 which looks very useful. With that I think all I need to do is find the time it takes to completely stop, use that in the equation I've just stated then convert θ into revolutions and Bob's your uncle. I'll give that a go but I think I've got it...
45. ### Revolutions per minute, angular deceleration

Thanks for the reply. For the car thing I'd just use a kinematic equation - but surely distance doesn't relate to revolutions? Are there any equations that are cirular motion specific?
46. ### Revolutions per minute, angular deceleration

Hi all, Homework Statement The motor driving a large grindstone is switched off when a rotational speed of 360 rpm has been achieved. After 15s the speed has decreased to 210rpm. If the angular deceleration remains constant, how many additional revolutions does the stone make before coming to...
47. ### Separation between lowest energy states

No idea there was a wiki article on it, much appreciated
48. ### Separation between lowest energy states

Sorry for the delay I went for lunch. I'm a little confused as there is only one length-related variable in the equation yet we are given, and asked for, another length (or does it simply mean the difference between energy values?). Also what is η? I'm trying to teach myself this topic...
49. ### Separation between lowest energy states

Could somebody point me in the right direction on how to go about starting this question please? In need of some guidance on where to begin. Homework Statement Calculate the separation between the two lowest energy states for an electron confined in an infinite potential well of width 1nm...
50. ### Interference - Q done, is it right?

Thanks a lot, big help