Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding [tag:energy-conservation] if it is specifically about its conservation.

learn more… | top users | synonyms (1)

1
vote
1answer
26 views

Calculating Dark enegy, dark matter and the rest [duplicate]

I have read that Dark energy composes roughly 68% of the Universe and that Dark matter composes about27% and everything else is less than 5%. Where and how did they come up with these figures?
1
vote
0answers
28 views

Electron energy from beta decay

I read in my IB-physics book that the average energy for an electron in the beta decay of Potassium-40 is 0.44 MeV. However this would imply the electron have a velocity of 3.9E8 m/s, i.e. faster than ...
1
vote
2answers
65 views

Elevator Cab Problem

I faced this problem in Fundamentals of Physics (Halliday and Resnick): A $0.250\ \mathrm{kg}$ block of cheese lies on the floor of a $900\ \mathrm{kg}$ elevator cab that is being pulled upward by ...
1
vote
1answer
51 views

Can we measure the energy of one of several identical particles?

Suppose we have a many-particle system described via a many-particle wavefunction that involves single-particle states $\lvert\lambda_{a}\rangle$, $\lvert\lambda_{b}\rangle$, ...
1
vote
0answers
26 views

Simulating Debris Burn Up Upon Atmospheric Reentry

For work I need to model Debris (1 gram to no more than 1000 km) burn-up upon reentry. I can't seem to find much information regarding this. I feel like I am looking for the wrong terms or something. ...
1
vote
0answers
32 views

Can you grow more Biomass in a brickhouse only powered by solar cells than in a greenhouse? Will we ever be able to? [closed]

Right now I read that 1m² of solar cells generates roughly 100Watts of energy. There are really efficient Grow LEDs that use only two spikes in the lightspectrum to make plants grow. I have seen some ...
1
vote
0answers
27 views

Energy contribution due to matter transfer

I have a very important question that lingers in my mind for a long time: In the fundamental relations of Gibbsian Thermodynamics, I simply don't understand how the last term appears. I have ...
1
vote
0answers
17 views

Why are heavy elements used in nuclear power plants? [duplicate]

I've seen that the energy produced by breaking an oxygen atom's nucleus is equivalent to burning a LOT of coal, 4.6 million metric tons if I remember correctly. (Crash Course Chemistry) If that is ...
1
vote
0answers
25 views

How can I determine maximum efficiency of a process?(like Carnot did) [closed]

I was thinking about developing an "engine" for energy production from "waste". And then I thought why not develop it like Carnot did. So, is there any way I can develop the "most efficient ...
1
vote
0answers
38 views

Calculating torque needed to climb a graded hill [closed]

this is my first post but I'll try to give my best understanding of the problem. I've been designing an electric skateboard which is driven by only one of the rear wheels (for the moment). As I was ...
1
vote
1answer
47 views

Help with my bouncy ball lab (I know the factors just not how to approach them) [closed]

In my physics lab we need to determine the factors that account for the energy "loss" during a high bounce ball bounce. I know that energy is "lost" (not really) to heat, air resistance, and sound. ...
1
vote
0answers
42 views

Does it take energy to destroy the surface of a liquid jet? [closed]

Imagine we have a collision of a liquid jet with an obstacle Does it take time for the surface of the jet to be destroyed while collision?? Or it'll be destroyed immediately?
1
vote
0answers
12 views

Solving question from a energy perspective [closed]

Im having some trouble with a question i was given for homework. Ive already solved it from a motion perspective but now i need to solve it from a energy perspective. The question states that from a ...
1
vote
0answers
64 views

Theta-parameter vacuum energy

Suppose we have $\theta$-field in QCD (in a special case of constant $\theta$ it reduces to ordinary $\theta$ parameter): $$ \tag 1 Z_{\theta} = \int D[\psi_{QCD}]e^{iS}, $$ where $$ \tag 2 S = \int ...
1
vote
0answers
81 views

Will the block #3 move? [closed]

Hey, On my test I've been assigned to calculate acceleration of the box with mass = m3(I'll call it block #3). The problem is, I can't even imagine the block moving, as I cannot find any force ...
1
vote
0answers
34 views

Turning an engine over with engine compression [closed]

When you compress a bicycle pump and seal the opening, the pump will automatically return to it's initial position when you release it. I suppose this is how air suspension works as well? if you ...
1
vote
0answers
53 views

Finding the distance a spring travels [closed]

So you hold a spring in your hand. You know that if you attach a 0.100 kg object that it will stretch the spring 0.05 m. The spring is launched off your hand(1.25 m off the ground) at a 35 degree ...
1
vote
1answer
40 views

Relation of Force and Energy, from a biophysics point of view

Considering us humans practically, does energy require force, ie we do apply some internal and external forces to do work, or does force require energy. ie we can only apply a force on an object if we ...
1
vote
0answers
21 views

Speed of an antimuon in a pi meson decay [closed]

I'm trying to confirm the speed of an antimuon in the $ \pi^+ \rightarrow \mu^+ \nu_{\mu}$ decay through the laws of conservation but it doesn't add up. I'm using $m_{\pi} = 139.6 \ \frac{MeV}{c^2}$, ...
1
vote
1answer
50 views

Is there a principal difference in acceleration from an impulse and the free fall of a body in the orbit around a gravitational mass?

Especially taking in account that acceleration need a force which leads to some conversion of energy. This is not the case for orbital motion, such a acceleration does not convert energy. That seems ...
1
vote
1answer
43 views

Understanding energy and its transfer

Basically the concept of energy is confusing because in all other cases like velocity or mass I can see and clearly imagine whatever changes happen but when somebody says energy got transferred from ...
1
vote
0answers
27 views

FD Statistics - average energy derivation in terms of fermi energy [closed]

I need to derive an expression for $\langle \epsilon^n\rangle$ of an electron in terms of $\epsilon_F,$ for $T\ll T_F.$ Would appreciate some help. Thanks!
1
vote
0answers
41 views

Earth to LEO, gravity drag and potential energy

A lot of references indicate takes about 9.4 km/s of delta-v to reach LEO. https://en.wikipedia.org/wiki/Low_Earth_orbit#Orbital_characteristics The delta-v needed to achieve low Earth orbit ...
1
vote
1answer
57 views

How would the angular velocity of the rod change if it slipped on the table?

I wanted to consider a second case of my homework assignment. We were asked to solve the question: A uniform rod of length b stands vertically upright on a horizontal plane in a position of unstable ...
1
vote
0answers
74 views

How can we produce gasoline with nuclear fusion and without fossil fuels? [closed]

If all fossil fuels were suddenly to run out tomorrow, but we also were to discover an easy way to produce energy from nuclear fusion, how might we produce gasoline? Could we do it in a way that is ...
1
vote
0answers
25 views

How to compute the speed of a cosmic ray from its energy? [duplicate]

How can you compute the speed of a cosmic ray from its type and measured energy? For example, if a cosmic ray were said to be a proton of 10 GeV, how would you compute the velocity of that cosmic ...
1
vote
1answer
42 views

Energy from 10% of stars in Galaxy over their lifetime [closed]

I heard an assertion that referred to the 'energy from 10% of stars in our Galaxy over their lifetime' on Radio 4 Inside Science and wondered if it is possible to estimate/calculate the energy value. ...
1
vote
1answer
53 views

What mechanism can we use to convert waves energy?

In term of renewable energy, can we use waves as another energy that can be stored and used instead of solar, wind and etc ? What I mean here is not sea waves but another kind of waves such as radio ...
1
vote
0answers
146 views

What is the relationship between the velocity of an object and its Sound Intensity Level in an inelastic collision?

Assuming the mass, acceleration, and height of the dropped object are constant and the area of the measured sound wave is constant, would the relation be linear, exponential, logarithmic, square root ...
1
vote
0answers
53 views

Mass, Spin, Internal Energy and 1-Particle States in Galilean Quantum Mechanics

I have been reading an article discussing the unitary representation of Galilean group and non-relativistic quantum mechanics. The link to the article is given below. http://arxiv.org/abs/1107.2442 ...
1
vote
0answers
25 views

Minimal possible energy to make calculation [duplicate]

Are there any restrictions to energy that must be consumed to make calculations? Can calculations theoretically be done without consuming it?
1
vote
0answers
62 views

How to solve this energy problem using trends related to impulse and momentum? [closed]

The question runs a 1kg block moves with a speed of 10 m/s. It strikes a spring that has a spring constant of 25 N/m. Determine the maximum compression of spring. The energy way of solving ...
1
vote
1answer
24 views

Gas Cooled Fast Reactor Coolants

I know that helium and CO2 are used as fast reactor coolants, but how about other inert gasses with low neutron absorption, like argon, why do I see no papers on that? Oh and bonus question. Why is ...
1
vote
1answer
62 views

How to calculate the potential energy of an $H_2$ molecule

From left to right, electron $e_1$, $e_2$ and proton $p_1$, $p_2$. $r_0=0.529nm$ The total energy is sum of energy require to bring each particle to its place. Take the place of $e_1$ is zero ...
1
vote
1answer
146 views

Why do electrons occupy in discrete energy states?

Why can't there be any continuous energy band in an atom?
1
vote
0answers
65 views

question regarding work energy theorem [closed]

The question says A smooth track in the for of a quarter circle of radius 6 lies in the vertical plane. A ring of weight 4N moves from $P_1$ to $P_2$ under forces $F_1$,$F_2$ and $F_3$. $F_1$ is ...
1
vote
1answer
78 views

How much energy is saved when using luggage with wheels? [closed]

How much energy is saved when using luggage with wheels as opposed to carrying the luggage? Thanks.
1
vote
0answers
33 views

Internal heating effects of a circuit element?

The heat-up of a solid circuit component in an electric circuit (for which no chemical reactions, phase changes reactions or other stuctural changes take place) may include: Joule heating $\dot ...
1
vote
0answers
109 views

What is the difference physiologically between running uphill vs. downhill? [closed]

In pure potential energy terms a person needs positive energy (work) to lift their body to a higher gravitational potential. When they turn around and run back down, the same transaction is reversed ...
1
vote
0answers
44 views

Specific heat at constant volume

Me and my friend came across this derivation is some lecture notes of our thermal physics module. We have been trying to calculate the partial differential of the internal energy but cannot get the ...
1
vote
2answers
286 views

Fan Speed Formula

Is there any formula for computing fan speed by using air mass,air flux, air density or specific heat? I have computed air mass and air flux, and found the values for air density and specific heat, ...
1
vote
0answers
79 views

Is shaking a phone enough energy to charge a smartphone?

Is shaking a phone enough energy to charge a smartphone? For example I'd like to use some kind of power generator to charge smartphone's battery but I'm wondering if shaking (by hand, or motion when ...
1
vote
0answers
31 views

What does it mean to perturb the electromagnetic energy with respect to a change in a Lorentz frame?

I am perturbing the energy of an electromagnetic field using \begin{align} \dfrac{\partial U}{\partial \zeta} = \dfrac{1}{\mu} \boldsymbol{H}\cdot \dfrac{\partial \boldsymbol{H}}{\partial \zeta} + ...
1
vote
0answers
62 views

Energy density question

I have looked for these formulas everywhere but cant seem to find them... What is the formula for the energy density of the electric current ,and the energy density of the mechanical wave?
1
vote
0answers
13 views

Vertical wall at 3 different temperatures - do the same natural convection coefficient correlations apply?

I have an vertical enclosure that consists of 3 shelves, each with its own temperature and are thermally isolated from each other. We will call the shelves A, B, and C with A being the base shelf. The ...
1
vote
1answer
1k views

Do Waves have mass? [duplicate]

Do any form of electromagnetic waves like visible light have mass? It is known that all electromagnetic waves have energy. According to Einstein's $$ E^2=p^2c^2 + m^2c^4 $$ formula, the energy of a ...
1
vote
1answer
63 views

How do you define the total rotational energy of an object?

This problem arose when I was applying a conservation of energy argument to a mechanics problem, (a spinning coin on a table) and wasn't sure how to define the total rotational energy of the coin. At ...
1
vote
3answers
71 views

Closed-system power-generation using waterwheels

I'm not a physicist, and this question is bugging me .. :) Is it possible to create a closed power-generation system, using upper and lower water-reservoirs where a continuous flow from the upper to ...
1
vote
0answers
50 views

Calculate energy-density of known plasma in microwave?

Let's assume I want to create a plasma in a regular household microwave similar to this home-made experiment. Although I am dealing with a small amount of mass, I assume that the addition of microwave ...
1
vote
0answers
41 views

How much energy does a PV cell need to work?

How much energy does a $1m^2$ photovolatic/solar cell need to work? Can it work using a bunch of Laser lights? Edit: Ok so this PV cell is $1m^2$ is dimesions. The N-type has silicon doped with ...