Energy density Definition and 186 Threads

Hi, How do I calculate the energy density of either a main-spring (like in a clock) or a motor-spring? Can someone show me values put into the correct formula for something like steel? I did find some material properties like this...

Hi all. I was wondering, are lithium-ion batteries the best type of batteries for electric vehicles? I also remember watching a video of Elon Musk (from Tesla Motors and SpaceX) claiming that lithium-ion batteries are increasing their energy density by 8 to 9 percent every year. I googled...

Homework Statement It has been estimated that a total weight of 4x10^7 kg of ungulates die in the Serengeti each year (Houston, 1979). We assume that these have an mass-specific energy content of 7x10^6J kg^-1 (Peters, 1983), and that the Serengeti stretches over 25,000 km^2 (Sinclair and...

An electromagnetic plane wave has an electric field and a magnetic field. Each component contributes equally to the energy density. Mathematically it is very straight forward to show this is true. The question is, "Fundamentally, why is this true?" Again, I'm not looking for a derivation...

Homework Statement In what units is energy density measured? 1. Watts 2. Watts per cubic meter 3. Newtons per meter 4. Joules per cubic meter Homework Equations E = cB (as derived by Maxwell) The Attempt at a Solution This question is so basic yet I'm confused. I thought...

dealing with absolute functions that are limited always throws me off so let's consider this f(x)=|x| for -∏ ≤ x < ∏ f(t)= f(t+2∏) it's not too bad however finding the energy density is throwing me off a little.. the questions tend to be generally phrased as below: Find the energy...

Hello, I am new to this, if somebody could help me: The energy density in stress-energy-tensor is the amount of energy stored in a given system per unit mass. The T00=energy density. In a 4x4 matrix, we generally start with 1,1. Why it is 0,0? Secondly, what does energy density mean...

I know that energy density of Electric and Magnetic Fields are (ε/2)*E2 and [1/(2μ)]B2 respectively. But I can't understand how we get those expressions. Would be really happy if you could clear my problem. Thanks a lot

I will just paste the link, cause its faster this way: http://scienceworld.wolfram.com/physics/PlanckLaw.html The last equation for spectral energy density if you look at the units you get eVs/m^3 or Js/m^3, yet energy density is supposed to be J/m^3. What am I missing here?

Hi, Been doing some cosmology revision and have a few questions owing to my terrible notes. One of the past papers asks the question "The density parameter of the Universe today has the value Ω = 0.25. Using the value of Hubble’s constant from (b), calculate the mean density of matter...

Homework Statement ##u(\omega) d\omega \propto \frac{(\hbar \omega) (\omega^2)}{e^{\hbar \omega \over k_B T}-1} d \omega ##Homework Equations The Attempt at a Solution ##\hbar \omega ## is the energy of a photon ##\frac{1}{e^{\hbar \omega \over k_B T}-1} ##and this is the density of states...

Hi all, I have a bit of a variation of a question I asked some time ago. Webb et al. continue to publish results which suggest that the fine structure constant may vary over space and time. I realize the results are controversial - what I'm wondering: If the fine structure constant...

Given a cylindrical wire of radius r, length L, carrying a current I, find the total energy stored inside the wire. From griffiths, uem= εE2/2 +B2/2μ and the tot energy is ∫uem dVI have my E and B fields, but my B field is a function of x where x<r, (E is uniform) B=kx/r2 (k=all the...

Is there a way to get energy density from watts, with the resultant energy density not involving time, as watts do? Thanks, Jake

I hope this is the right forum for a first year undergraduate problem! The problem I've been working on is here: http://i.imgur.com/IhTtL.png I think that I have the correct answers, but I'm not sure. I think that the energy densities of the E and B field components will be (0.5)(ε0)(E^2)...

Not really homework, just practice for a midterm, I also have the correct answers; but I guess this is the correct section. Homework Statement In the Earth's atmosphere we have an electric field with a vertical direction down towards the earth. The lower part of the atmosphere has a typical...

I'm trying to calculate the energy density of a given magnetic field (0.03 T). I'm supposed to use the equation u=B^2/2μ right? Well I'm getting 358.1 A/m (?). Something is wrong here. Thanks in advance.

I understand that the the Higgs field has a vacuum expectation value of 246 GeV. I think that means that the expectation of the Hamiltonian energy operator applied to the vacuum state is 246 GeV. What does this imply for the energy density of the Higgs field in the vacuum (i.e. in Joules /...

Are there articles written on exotic, ultimate batteries? I'm not referring to those for hybrid cars, but those that have energy densities up to that of nuclear fuels or higher. Rather than batteries that operate via chemical oxidation and reduction, is there a theoretical type of battery...

Dear all, I would like to know from you the solution about this problem (which is not a homework, but a topic of my Master thesis!): I need the strain energy density related to a circle of radius r0 centered in an arbitrary point of a square plate, under the boundary conditions described in...

I'm looking for a conceptual explanation of a formula in Quantum Mechanics Demystified introduction. They introduce you to the theoretical black body radiation experiment, where demonstrated how a classical approach leads to the ultraviolet catastrophe. In the explanation they have the...

I propose a formula for the dark energy density (dark energy formula): ΩΛ = ΩbAeh/t2 In this equation: ΩΛ is the dark energy density, with the estimated value from 0.712 to 0.743, according to WMAP seven-year data release [1]; Ωb is the baryon density, 0.0456 ±0.0016, according to WMAP...

Why is photon gas pressure = photon energy density (per volume) divided by 3? Thank you

I have never been formally trained in GR, and have a question regarding the basics of how to calculate the energy density from a metric. This question arises from thought experiments involving a field with a negative energy density. This is important only because I expect the energy density...

I need some help finding out the energy density of different heavy water reactors. In IAEA research reactor database they only list the thermal output of some HWR - RR, but they lack information about the dimensions of the core, thereby making it difficult to figure out their energy density...

From an entropy argument it is concluded that the CMB neutrino temperature is (4/11)1/3 of the CMB photon temperature. This assumes massless neutrinos. Although neutrino contribution to the present energy density is therefore very small, it was significant must earlier, such as at the time of...

I'm looking for an equation and I am not quite sure where to start, and am hoping someone could direct me to a good reference to some similar type equations, or at least kick me over to the proper forum. I am looking for an equation that describes the energy distribution within the space...

I was going through my notes preparing for finals (this is my last one hooray!), and I saw that the Number Density of radiation and matter changes at the same rate (N=Na^-3) while the energy density is different (E=Ea^-3 for matter, E=Ea^-4 for radiation). I think I get why the energy densities...

Hello, consider a 1D elastic wave which have the amplitude: A=cos(x) What is the energy density: \frac{dE}{dx} of this wave? I seem to recall that the energy of a wave is proportional to the square of the amplitude: E \propto A^2 That seem to mean that \frac{dE}{dx} \propto cos(x)^2...

Homework Statement Monochromatic energy density (Uλ)(i.e., per unit volume) of the radiation of a blackbody is proportional to (where λ is wavelength) (1) T4 (2) λ -5 (3) λ5/2 (4) T5/4 The Attempt at a Solution A slight modification of Stefan-Boltzmann's law suggests that the...

Homework Statement If the ratio of the energy densities of matter and radiation was much higher than it is today (by several orders of magnitude), in a few sentences describe what effect this would have on the redshift (or scale factor) when the universe switched from being...

The energy density function for the electric field in vacuum is u=\dfrac{\epsilon_0}{2}E^2 And the cited textbook result for the energy density inside a dielectric is: u=\dfrac{\epsilon_0 \epsilon_r}{2}E^2 Now, one way to reach the upper formula is to look at the energy as...

Is the vacuum energy density, in case it exists which is the mainstream view, supposed to be continuously varying (decreasing) as the universe volume expands or is it really a constant (wich would be rather odd judging by what density and volume usually mean in physics)?

Hi yall, I was just wondering, seeing as how photons are massless, would there be a limit to the amount of electromagnetic energy you can fit in a given (finite) area? Of course excluding the practical reasons that would stop you. And if there is a limit, how come? Because black holes are...

Hey I am studying electromagnetism at the moment and one problem I'm having is the energy density. Instead of typing everything out, this pdf has everything. http://www.physics.sfsu.edu/~lea/courses/ugrad/360notes11.pdf There are some things I don't understand. Why is the surface integral 0...

Homework Statement Find the potential energy density of a hanging string of mass density m/L that has been displaced from equilibrium at a point a distance d up from the bottom of the string. This point is displaced a distance X in the x direction, and a distance Y in the y direction. The...

Homework Statement An elliptical galaxy has gravitational boundaries defiend by 9x^2+16y^2+144z^2=144. A black hole at the center of the galaxy is interacting with dark matter producing a radiation envelope defined by the region inside the cone Z^2=27x^2+48y^2. If the energy density in this...

Hi, I've not posted on here before but I'm trying to keep on top of work over the summer and I'm having some real problems with this question Homework Statement Consider the plane polarised EM wave in a source free vacuum with magnetic field B = (1,1,0)B0cos(kz-wt) where B0 = 0.001T. Find...

Does a photon have energy density?? can anyone help me what a photon behaves when talking about energy density? it seems that photon just travel through time but not space ,or I am wrong? can anyone help? thanks so much~~

In section 8 of Einstein's On the Electrodynamics of Moving Bodies Einstein uses the expression A(sq)/8 pi. He states this represents the energy of light per unit volume. Does anyone know how he derived this?

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...

This might sound like a dumb question but I wanted to know if energy density has an effect on the gravitational field, for instance say there are two masses of equal density, one say is a low temperature neutron star and another is a white dwarf (please exclude for now any impossibilities...

http://farside.ph.utexas.edu/teaching/em/lectures/node56.html By equation 595, electric energy in the universe caused by an electric field is derived as http://farside.ph.utexas.edu/teaching/em/lectures/img1261.png. However, how come you can differentiate with respect to volume and say that...

Hello. This is question for my course work, I was wondering if I could get some insight, here is the question: Assume that the vast majority of the photons in the present Universe are cosmic microwave radiation photons that are a relic of the big bang. For simplicity, also assume that all the...

and what is kinetic energy spectrum?

The electromagnetic energy density is well-defined through the square of the Maxwell Field tensor. Why cannot such a quantity be defined for the grav. field?

Hi, Energy density of the ith annular section of a Gaussian pulse is written as F(r_i, r_i+1)=2*totalEnergy/[(r_i+1^2-r_i^2)*w^2]*integral(r*exp(-r^2/w^2),from r_i to r_i+1) where r spans from r=0 to r=r_n (theoretically infinity) in n steps, w is the waist size of the beam. This...

Why is there no satisfactory way to define the energy density of a gravitational field? Why must we use global energy concepts? I'm currently reading proofs of the Positive Energy Theorem, but none of the references explains why this is so. Can anyone explain it or point me to a good reference?

Hi, new guy here. I once saw a graph on the development of the energy density of the universe, with the X axis measuring time (I believe it was in billion years or so) while the Y axis meansuring the density. There were three lines in that graph, energy density of empty space, energy...

Does anyone have an up-to-date reference for the best value of \Omega_{\nu}, given our current understanding of neutrino masses? Dodelson's cosmology book has \Omega_{\nu}\approx 10^{-3}, but something more recent would be nice.