# Search results for query: *

1. ### Density of bar submerged in water

Homework Statement A bar suspended in air weighs 1.75 N The same bar weigs only 1.4 N when suspended in water. Calculate; a. upthrust b. density of the bar Homework Equations D = m/v F = m*a The Attempt at a Solution a[/B]. Upthrust is 1.75 N - 1.4 N = 0.35 N b. Assuming density of water...
2. ### Dichromatic(?) prisms

I found what was called a "dichromatic prism" on ebay and bought it. It is a cube and although described as "not perfect" it splits light extremely well and is quite a cool object to show those in my staff room. I understand that dichroic is different but not sure how. Question is, how does it...
3. ### Charge density in a cylinder

∫s ρ(r) dV = Q ∫ Cr2 * 4πr2 dr = 4Cπ ∫ r4 dr so Q = 4CπR5 / 5
4. ### Charge density in a cylinder

∫ Cr2 dr C ∫ r2 dr (0 < l < L) CL3/3
5. ### Charge density in a cylinder

Homework Statement A cylinder of radius a and length l has charge distribution ρ=Cr2 where C is a constant and r is radial distance in cylindrical coordinates. Derive an expression for the average charge density within the cylinder. Homework Equations Well, charge density given is within the...
6. ### Fluid Statics - net upward force

I have looked at the question and unscrewed the bits of paper I attempted my answers on and the h0 seems to be a complete typo, it all says just plain h. I will try to work through with that in mind, as you say.
7. ### Fluid Statics - net upward force

Homework Statement Find magnitude of net upward force on a cube in terms of l, ρ (rho), g and h. then show that h = m/ρl2 A solid cube, mass m, side length l, is placed in a liquid of uniform density, ρ(rho), at a depth h0 below the surface of the liquid, which is open to the air. The upper...
8. ### Force exerted by fluid

Homework Statement A solid cube, mass m, side length l, is placed in a liquid of uniform density, ρ(rho), at a depth h0 below the surface of the liquid, which is open to the air. The upper and lower faces of the cube are horizontal. Find the magnitude of force, Fs, exerted on each vertical...
9. ### Renewed career in science for age 50+ (UK)

Having been a junior design engineer 15 years ago and moved out of that field I am looking to get back into a science based career. I am taking a "Natural Science" degree with a maths/physics bias (lots of that) and can't seem to get any sensible job before I get this in place. I used to use...
10. ### Relationship between elements and speed of light

I recently watched a documentary presented by Professor Brian Cox called Human Universe. In it he was wondering how much leeway he would have, if he was "the creator of our universe", to alter the physical constants. He then went on to say that if he altered the speed of light one way then...
11. ### Partial differential - difference of opinion

oops sorry. Just me being lazy
12. ### Partial differential - difference of opinion

I have a difference of opinion with 2 calculation engines. equation to solve is; d/dx (a(x^2 +y) Wolframalpha of course is a very trusted source but I also use symbolab. Here is a screenshot of the differential I want from both sites and associated answers. . . . and the wolfram solution...
13. ### Free surface charge density (not bound density)

Sorry, I should have said. D1 ┴ is the electric displacement in media 1 perpendicular to a boundary with a different media D2 ┴ is the electric displacement in media 1 perpendicular to a boundary with a different media They form the boundary condition could have been D1 = D2. and P is the...
14. ### Still struggling with physics, but trying, trying.

Still struggling with physics, but trying, trying.
15. ### Free surface charge density (not bound density)

Homework Statement a parallel plate capacitor has 2 plates separated by a dielectric of rel. permittivity 5.0 are separated by 0.20mm and have area of 5.0 cm2. Potential difference between the plates is 500V. I need to be able to calculate the free surface charge density...
16. ### E, D and P inside a material

A problem involving E, D & P. I have the answer but not sure why for the P bit.A bit of clarification if someone could, please. A thick sheet of polystyrene, of relative permittivity ε = 2.5, is placed normal to a uniform electric field of magnitude 500Vm−1 in air. Find the magnitudes of...
17. ### Bound surface charge - hollow tube

Aaaaah! Details. Please ignore this. Yes, the wrong bit is me not looking proplerly. radius is a/2 not a. Sorry.
18. ### Bound surface charge - hollow tube

Homework Statement A thick-walled cylindrical tube of dielectric material has internal radius a/2 and external radius 2a, and its polarization is given in cylindrical coordinates by the expression P = (A/r2) er. Derive expressions for the net charge on the inner and outer surfaces of a...
19. ### Wavlnumber in media

Yes, I see. And yes, typo's. Ta.
20. ### Wavlnumber in media

Homework Statement Glass has a permeability of ε=2.0 and relative permeability μ=1. Electromagnetic radiation of angular frequency ω=2.8x109 s-1. travels through the glass. Calculate a) refractive index b) speed of radiation c) frequency of radiation d) wavenumber of radiation a, b...
21. ### Vector multiplication

Thank you. A simple thing becomes clear again.
22. ### Vector multiplication

Homework Statement Still having a little trouble so here's the problem. (ex + ez) x (3ey - 4ez) The Attempt at a Solution (ex * ez) + (ex * (-4ez)) + (ez * 3ey) + ( ez * (-4ez) now, these are all orthogonal to each other, so, for example, if I have ex * ey then I should end up...
23. ### Lorrentz force law & vector notation

Homework Statement A current element is 3.0mm long, centerd on the origin of cartesian coordinates, and carries a current of 2.5A in the direction ez. What field does it produce at the point (3.0, 0, 4.0) Homework Equations This is obviously a Lorentz force law question so . . . ∂B =...
24. ### Permanent dipoles in molecules

Thanks. Perhaps I can make this a thesis for my Doctorate (if I ever get over A-Levels =)))
25. ### Permanent dipoles in molecules

As I understand, when an electric field is applied to an atom then the nucleus and electron cloud will be shifted out of balance giving rise to an electric dipole moment. Some molecules have permanent dipole moments though, e.g. Hydrogen Chloride, ". . . when a hydrogen chloride molecule is...
26. ### Coulomb's law three points on a line

Homework Statement Three point charges, q1, q2 and q3 all on x-axis (i.e y=0 and z=0) +q1 at x = 0 -q2 at dist from origin x = a +q3 at dist from origin x = -2a where q1, q2 and q3 are magnitudes of the charges. Assuming q1 = q2 derive magnitude of q3 in order for there to be zero net force on...
27. ### Reversal of Earth's magnetic field

I understand that the sun changes magnetic polarity far more often than Earth does (correct me if I'm wrong as it's just what I read). Apparently it is statistically likely that the Earth's magnetic field will change polarity at some time, so what sort of effects will this have in practcal...
28. ### Laser pointer and CD

Hi. When I point my red laser pointer at the playable side of a CD at an angle the spot becomes diffracted, a bit like the fringes you can get if you put a hair across the beam. Could you keep to the CD phenomenon, when trying to explain and whether it really is related to Youngs Fringes or...
29. ### Total energy of Coulomb model of Hydrogen atom

Ah! n = principal quantum number m = mass of electron L2 = square of magnitude of orbital ang momentum. so . . . . E = 22 ∏2 hbar2 / 2(9.1x10-31) 2 hbar2 E = -2∏hbar/m er! surely too messy and probably wrong.
30. ### Total energy of Coulomb model of Hydrogen atom

Hmmm! Well, En = (n + 1/2) hbar ω0 and En = n2 ∏2 hbar2 / 2mL2 where principal quantum number n = 0, 1, 2 . . . but I can't evaluate ω0 and I don't have the mass for the other one . . do I?
31. ### Total energy of Coulomb model of Hydrogen atom

Homework Statement Hi, my question is regardng a Coulomb model of an H atom specified with Hamiltonian operator, Hhat, by spherical coordinates of energy eigenfunction ψ2,1,-1 (r,θ, ∅) =(1/ 64∏a02)1/2 r/a0 e-r/2a0 sinθ e-iθ Principal quantum numer n = 2 orbital an mom l = 1 magnetic...
32. ### Total spin quantum number

I have a pair of non interacting, identical 1/2 spin fermions in a one dimensional infinite square well with walls at x=0 and x=L. One particle is in ground state, the other in first excited state. This two-particle system has total spin quantum number S=0 I have normalized energy...
33. ### Hamiltonian matrix and eigenvalues

OK. An example I have has me stumped temporarily. I'm tired. General spin matrix can be written as Sn(hat) = hbar/2 [cosθ e-i∅sinθ] ...... [[ei∅sinθ cosθ] giving 2 eigenvectors (note these are column matrices) I up arrow > = [cos (θ/2)] .....[ei∅sin(θ/2)] Idown arrow> =...

35. ### Particle in a potential well of harmonic oscillator

Homework Statement I have a similar problem to this one on Physicsforum from a few years ago. Homework Equations Cleggy has finished part a) saying he gets the answer as Ψ(x, t) = (1/√2) (ψ1(x)exp(-3iwt/2+ iψ3(x)exp(-7iwt/2) OK classical angular frequency ω0 = √C/m for period of...
36. ### Expectation value in the ground state

Homework Statement . Hi, could someone look at the attachment & comment on whether I'm anywhere near getting the expectation value correct, please. In the grnd state; 1. terms such as AA†A†A, with lowering operator on RHS has zero expectation value, 2. terms such as AA†A†A† with uneven...
37. ### Expectation value at ground state

I realize that at ground state of a harmonic oscillator the energy will be at zero. I'm assuming that the expectation value will also be at zero. Could someone confirm this & possibly explain just a little more. Thank you
38. ### What are raising & lowering operators

A little behind in this subject, but I understand raising & lowering operators to almost be factors of the Hamiltonian operator. raising - Ahat dagger = 1/√2 (x/a - a*(δ/δx)) lowering - Ahat = 1/√2 (x/a + a*(δ/δx)) I also have the Hamiltonian as; Hhat = (Ahat dagger * Ahat + 1/2)...
39. ### Deriving wave eqn. from Faraday & A.M.

Thank you. Nose - Face - well known phrase.
40. ### Deriving wave eqn. from Faraday & A.M.

Homework Statement I'm asked to start from Faradays Law & Ampere-Maxwell law then derive the equation for the magnetic field, B, in the form; del2B = 1/c2 * d2B/dt2 Homework Equations Faraday: curl E = -dB/dt A.M : curl B = ε0μ0 dE/dt The Attempt at a Solution taking curl of...
41. ### Polarised (polarized) light; EM explanation.

When an EM wave encounters a dielectric some of the wave is transmitted & some reflected, as with a metal, too. When encountering a metal the normal part of the EM wave will absorb a small amount of energy, up to the skin depth, but a dielectric will allow the normal incidence of the wave...
42. ### Polarised (polarized) light; EM explanation.

Facets of a rock specimen sparkle brightly In terms of electromagnetism how can I show that a linearly polarised light source can be used to determine that the facets are dielectric rather than metallic? Should have to do with scattering, but not sure whether it could be explained with a...
43. ### Energy stored in capacitor

Homework Statement A parallel plate capacitor with plates of area, A & separation of d is filled with LIH dielectric of relative permittivity ε. By considering energy associated with the electric field derive an expression for electric energy stored in the capacitance when a voltage of V is...
44. ### Long straight wire: Ampere/Biot Savart

L . . . oops!
45. ### Determine whether vector field is magnetic or electrostatic

OK, OK I've kicked off a bit early. I will be using the equations booklet & make sure I look at it to complete problems in the way I've been taught (although coming back to it from a while ago dulls the mind if you're not using it).
46. ### Determine whether vector field is magnetic or electrostatic

1. divergence should be proportional to the density of magnetic "charge" (div B = 0 - no monople law) 2. div E = ρ / E0 (and for a conservative (electrostatic) field the curl should be zero. (Faradays law - curl E - -∂B/∂t)) The difference is that I "get" 2 and can show this by the matrix I...
47. ### Determine whether vector field is magnetic or electrostatic

Homework Statement Three vector fields are listed below. Determine whether each of them is electrostatic field or magnetic field. Homework Equations F1(x, y, z) = A (9yz ex + xz ey + xy ez) F2(r,∅,z) = A [(cosx/r)er + (sinx/r) e∅] F3(r,θ,∅) = Ar2 e(-r/a) er The Attempt at a Solution Used...
48. ### Long straight wire: Ampere/Biot Savart

Homework Statement A long straight wire carries a current of 2.0A. An electron moves parallel to it at a distance of 0.01m (I'm assuming id doesn't matter which way it moves) at a velocity, v = 100ms-1 Calculate magnitude & direction of force acting on it. Homework Equations B = μ0 L / 2∏r F...
49. ### Lorentz force - e- in EM field

Again, I have to go "Doh!" Thank you.
50. ### Lorentz force - e- in EM field

Ah! Is this to do with ex X ey = -ey? They're orthogonal?