Questions tagged [energy]

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.

Filter by
Sorted by
Tagged with
0
votes
2answers
34 views

How does a quasar convert mass to energy?

I'm trying to understand where the energy for a quasar comes from and specifically if it comes from converting mass to energy. I know that as matter falls into a black hole gravitational energy is ...
9
votes
4answers
441 views

How do we calculate energy of food?

Here is the problem, according to me: In a classic hamburger, according to my internet research, approximately 906 kJ are contained. Now take a car of 1 000 kilograms, and push it to 30 m/s. It has ...
0
votes
3answers
73 views

What has entropy to do with internal energy?

I'm trying to get an intuitive understanding of what entropy is and, frankly, I'm starting to get desperate for the click in my mind that allows me to connect all that stuff. This is my current ...
2
votes
1answer
39 views

How to convert $\rm kg/m^3$ to $\rm J/m^3$ for natural gas and biogas?

I want to convert from mass density to energy density: Natural gas has a density of $0.8\: \rm kg/m^3$. Biogas has $1.15\:\rm kg/m^3$. I want to convert them into $\rm J/m^3$ so that I can ...
3
votes
0answers
63 views

Can bound states have zero energy?

Suppose a system is under the influence of a potential which vanishes at $\pm \infty$. Now we know that if the energy of the system is negative ($E<0$) then the system is in a bound state and the ...
0
votes
1answer
24 views

Is their a theoretical minimal amout of energy to keep some information?

The goal here is to keep a bit of information $b$ at the temperature $T$ during a time $\delta t$. I don't know if this question makes a lot of sense, considering the diversity of ways to encode ...
2
votes
0answers
69 views

Doesn't electrical energy gets lost due to slackening of local electrical wires hinged between street electric poles?

I am talking about the wires which get somewhat loose or somewhat like parabolic shaped due to earth's gravity. What I think is due to slackness of wires there is a net increment in resistance which ...
0
votes
0answers
22 views

Why does the phenomenon of localized energy producing particles and antiparticles lead to the creation of an infinite number of particles?

Penrose says the following (paraphrased) in "Road to Reality" on pg 611 of the first American edition A particle and anti particle may combine to produce energy, as given by general relativity. ...
0
votes
3answers
40 views

Why is there a 1/2 in the expression for electrostatic energy U?

The expression for electrostatic energy is $$U= \dfrac{1}{2} \times \int \rho\,\phi\, dV$$ where $\rho$ is the charge density and $\phi$ is the potential at that point in ($dV$) Let me explain what I ...
0
votes
1answer
46 views

First law of thermodynamics clarification on internal energy

Starting from the experience of joules we know that the mass before falling has a potential energy $mgH$ where $H$ is height from floor. Once we reach the height $0$ we observe that the kinetic energy ...
0
votes
2answers
105 views

Why are certain quantities so fundamental to physics? [closed]

Apart from having a qualitative description of quantities such as momentum, work and energy, why are these quantities considered so fundamental? What is the reason to define them in the first place? ...
1
vote
0answers
24 views

Why is this way of deducig the 1 dimensional Maxwell-Boltzmann Distribution wrong?

I asked before how the 1D Maxwell-Boltzmann distribution can be derived. However, I did not get or find an answer to my own method explaining why it is wrong, which I am quite curious about. It is a ...
0
votes
0answers
35 views

Work a medium of energy conversion? [duplicate]

Is work a medium to convert one form of energy to other? i.e. to say without work you can't convert type of energy? Similar type of question pertains to heat.heat is also medium of conversion of ...
1
vote
1answer
50 views

Complex Scalar Fields and Killing Vectors

In a stationary and axisymmetric spacetime, there are two Killing vectors, say $\zeta^\mu$ and $\xi^\mu$, one timelike and one space like. I understand that for a real scalar field, $\phi$, that ...
1
vote
1answer
82 views

Volume of a speaker needed to boil an egg [closed]

I have a fun question to ask that me and my friends couldn't figure out. If I was to boil an egg using a speaker, how loud would that speaker need to be? (Yep, this is clearly what keeps us physicists ...
1
vote
2answers
40 views

Is Kinetic energy always conserved in an elastic collision/impact?

While working out through some problems I encountered this problem : A ball moving with a velocity $v$ hits a massive wall moving towards the ball with a velocity $u$. An elastic impact lasts for ...
0
votes
1answer
19 views

Why potential energy is neglected in this problem for calculating the energy difference between two stationary Bohr Orbits?

Question An electron in a hydrogen like atom makes a transition from a state in which its de-Broglie wavelength is $\lambda_1$ to a state in which its de-Broglie wavelength is $\lambda_2$, then ...
1
vote
2answers
83 views

Why is particle physics called high energy physics? [duplicate]

The highest energy accelerator till date is the LHC which operates at an energy scale of perhaps 10-100 TeV. In SI units this is about $\sim 10^{-6}-10^{-5}$ Joule which is several orders of magnitude ...
0
votes
1answer
29 views

What is the “energy associated with torque?”

I'm told that when a magnetic dipole is placed in a magnetic field $\vec{B}$, it experiences a torque $\vec{\tau} = \vec\mu \times \vec{B}$ and an associated energy $ H = -\vec{\mu} \cdot \vec{B} $, ...
0
votes
1answer
30 views

Energy consumption by body

i think the energy consumed by vertical jumping is too high when compared to the "2000 calorie a day requirement". Say i weigh 60kg, i have studied my centre of mass from a video with wall markings, ...
6
votes
2answers
110 views

Can I use the expansion of the universe to create or destroy energy?

Imagine I have a setup like this: (The objects are not attached to each other. The red objects are much heaver than the black one, and the setup is balanced.) This setup has a non-zero ...
2
votes
2answers
107 views

Is vacuum energy transient or permanent?

Vacuum energy is based on Heisenberg's energy–time uncertainty principle which says that $\Delta E$ can exist for $\Delta t$. This $\Delta E$ can produce electron positron pair for $\Delta t$ by ...
0
votes
3answers
131 views

Is kinetic energy relative? [duplicate]

Assume there is a rocket with 10 kg of fuel in a large empty space without any external forces such as gravity. Rocket burns 1 kg of fuel and gets a $v_1$ velocity gain. Now it is moving in $v_1$ ...
0
votes
1answer
58 views

How much kinetic energy does a helicopter use in a hover? [duplicate]

A helicopter just circulates air in a hover and maintains a stable altitude. So, how much energy is used to do this? Using the standard equation $KE = \frac12 mv^2$; then the kinetic energy used would ...
-1
votes
0answers
44 views

If energy exists today, wouldn't it have had to be created somewhere? [duplicate]

Energy exists today in different forms, and the 1st law of thermodynamics states that energy cannot be created or destroyed, only changed into different forms. However, since there is energy today, ...
2
votes
2answers
116 views

Newton's Universal Law of Gravitation doubt

The Universal Law of Gravitation states that the module of the force, $F$ is $$F = \frac{GmM}{r^2},$$ where $m$ and $M$ are the mass of the two objects and $r$ is the distance between the two objects....
1
vote
0answers
30 views

Physics of Athleticism for Dunks

I was curious of the athleticism I'd need to dunk a standard rim at $10$ ft above the ground. Using an online calculator, given that I am $5$ feet $6$ inches tall with a standing reach of $7$ feet $1$ ...
-1
votes
3answers
70 views

Is “physical damage” proportional to momentum or kinetic energy? [duplicate]

If a rock is thrown at someone's head, or a car drives against a wall, or a meteor hits a planet, is the physical damage done to the object proportional to the projectile's momentum, or to its kinetic ...
1
vote
0answers
40 views

Canonical or mechanical 4-momentum to measure energy?

I've often seen that the energy of a particle $p$ with 4-momentum $p^\mu$ measured by an observer $O$ with 4-velocity $u^\mu = dx^\mu/d\tau |_O$ is given by $$E=-p_\mu u^\mu.$$ In general, what ...
0
votes
1answer
27 views

How to reproduce free energy in canonical ensemble in stat mech

Given a quantum mechanical density matrix $\rho$, the internal energy and entropy of the system is given by: \begin{align} E &= \text{Tr}[H \rho] \\ S &= -k_B \, \text{Tr}[\rho \ln \rho] \end{...
0
votes
1answer
69 views

Total energy of a two particle system [closed]

Question: Velocity of center of mass of two particles is $~v~$ and the sum of the masses of two particles is $~m~$. Kinetic energy of the system is: A$)~~=\frac{1}{2}mv^2~$ B$)~~\ge \frac{1}{2}mv^2~$...
4
votes
2answers
170 views

How general is the relation between entropy and energy?

I am not a physicist, and my understanding of entropy is based on information theory and probability theory, rather than on physics. I am somewhat confused by how physicists think of entropy, and ...
0
votes
0answers
27 views

Confinement energy and energy conservation

When particles are confined, they gain confinement kinetic energy. Where does this energy come from in terms of energy conservation?
2
votes
1answer
35 views

Determine/measure energy of neutral particles

I wonder that nobody came up with this question in advance - maybe it is too obvious? However.. How can neutral particles be measured (with respect to their energy in a particle detector)? I know ...
1
vote
2answers
38 views

Where does the energy go when punching a ball in the air causing it to “float”?

Here is a link to a video of a people punching a ball so it appears to float: https://www.youtube.com/watch?v=v7wqrUFREK0 When the arms are continuously punching the ball, it does not change the ...
2
votes
2answers
123 views

How can the 1 dimensional Maxwell-Boltzmann Distribution be derived?

In terms of energy, the 3 dimensional MB Distribution is giving the probability for a particle to have an energy $E \geq E + dE$ is: $$f(E) = \frac{2}{\sqrt \pi} \cdot \bigg(\frac{1}{k_BT}\bigg)^{\...
0
votes
1answer
51 views

In electrostatics, why can't we tell how much energy is located in a volume inside the $E$-field?

In Purcell's Electricity and Magnetism, page 32, it is stated that the potential energy $~U$ of a system of charges is given by (using cgs units): $$U=\frac{1}{8\pi}\int_{\mathrm{Entire \\field}} E^{...
0
votes
2answers
22 views

Does the kinetic energy always decrease with gain in potential energy even if the system is being provided energy externally

A car accelerates uphill does its kinetic energy increase or decrease?
0
votes
1answer
27 views

Why and in what cases we use $E$ instead of $\Delta E$ in energy-time uncertainity relation?

sometimes in problems of energy or time uncertainity we use the value of energy of the particle/system to calculate the delta t. Now what are those cases specific examples, where we can apply this?
1
vote
2answers
50 views

Observing a system in an energy eigenstate when the eigenstate is not normalized

In the following notes from an MIT OCW course, Zweibach claims that energy eigenstates are not necessarily normalized. https://ocw.mit.edu/courses/physics/8-05-quantum-physics-ii-fall-2013/lecture-...
1
vote
1answer
49 views

Can an exothermic reaction be modified to alter how its energy is released?

As I understand it, exothermic reactions release energy. This energy can be released through the generation of heat, sound, light, or electricity. Questions: Is it true that exothermic reactions ...
2
votes
3answers
94 views

Why is more than one ripple created when a rock is thrown onto the still surface of a pond?

I have tried making an analogy with a simple pendulum: if you give it a push it will probably make several oscillations. But why isn't there just one ripple that would carry the energy of the several ...
0
votes
0answers
40 views

How to find an equivalent for the partial derivatives of internal energy?

Actually, I am trying to relate force-temperature results from one test (hydrothermal isometric tension) to heat-temperature results from another test (Differential Scanning Calorimetry). I started ...
0
votes
3answers
99 views

What is free energy in the context of a quantum field theory?

I was reading the papers Large $N$ behavior of mass deformed ABJM theory and New 3D ${\cal N}=2$ SCFT's with $N^{3/2}$ scaling. These papers talk about the free energy in the context of quantum field ...
1
vote
2answers
49 views

Solving a lever problem using Newton's laws, without energy conservation?

Context: I have no background in physics, and I'm trying to understand the role of the concepts of energy, momentum and other invariants/symmetries in physics. In particular, I'm trying to get a feel ...
2
votes
2answers
65 views

Does all energy of moving car converts to thermal energy?

I argue with my friend and cannot say that he is wrong although I feel it somehow. But I don't have good arguments against him. I want to know who is right and why. The problem is Let's imagine we ...
-3
votes
1answer
66 views

Energy is in vacuum or not?

Is energy present in vaccum is zero, infinity or particular value depending on volume, length, height etc of container containing vaccum or some other parameter.
2
votes
1answer
69 views

What is the most general conception/formalization of “energy”? [duplicate]

(disclaimer: I am not a physicist). I have been taught about the concept of energy in a fairly ad-hoc way. i.e. I was simply "told" that kinetic energy equals $\frac 1 2 m v^2$, and similarly I was ...
0
votes
0answers
6 views

Why is a grand partition function defined as the sum of total energies?

In Eq. 22.19, why is this partition function defined as not the sum of internal energy, $U_i$, but the sum of total energy, $E_i$?