# 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 the [energy-conservation] tag if it is specifically about its conservation.

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### The distance covered when 22 MeV gamma rays travel through air

Both alpha and beta radiations can only travel short distances through air as they're not as penetrating as gamma radiations. How long gamma radiations with 22 MeV energy can travel in air? Is it ...
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### Can gravity and hydro power be utilized in a closed system to generate efficient electricity? [closed]

This is my second post on this topic. The first one had some flaws but I resolved them with this new design. This is more of a thought experiment - which I don't think is that outlandish given ...
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### Calculating Local Energy Dissipation Rate (EDR or epsilon)

I am simulating the flow in a stirred tank reactor using a software for computational fluid dynamics (palabos) and would like to calculate/estimate the local energy dissipation rate (EDR or epsilon) ...
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### Explain to a mathematician how SK model comes into being? [closed]

In particular, how does one understand Hamiltonian is defined as $$H=\sum_{i<j}g_{ij} \frac{1}{\sqrt{N}}\sigma_i \sigma_j+h\sum_i \sigma_i~?$$ Is it correct that Hamiltonian is just energy of the ...
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### Need to find Stopping Distance [closed]

I'm given these following. Mass of the car $= 1800 \text{ kg}$ Friction coefficient $= 0.6$ Velocity $= 8.3 \text{ m}/\text{s}$ I need to find the stopping distance and deceleration using this and I ...
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### Thermistors vs Thermocouples Heat Capacity

Just a simple question, does a thermistor or a thermocouple have a greater specific heat capacity? By this I mean, which thermometer absorbs the most energy from the system it is measuring?
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### Energy neither created nor destroyed. Then where energy comes from? [duplicate]

We are stating that energy neither be created nor be destroyed. Then where does it come from?
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### Energy in hamiltonian formalism from phase space evolution

The hamiltonian for a free falling body is $$H = \dfrac{p^2}{2m} + mgy$$ and since we are using cartesian coordinates that do not depend on time and the potential only depends on the position, we know ...
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### Why is the kinetic energy for non-relativistic velocities not described by $KE=mc^2$?

Today, I learned that the kinetic energy of an object can be described by $$KE=\gamma mc^2=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}mc^2.$$ I have been taught in school that the kinetic energy of an object ...
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### Lagrangian of a free particle in Special Relativity and equivalence between mass and energy

I am a bit confused on the way Landau derives the Lagrangian of the free particle in SR (L. Landau, E. Lifshitz - The Classical Theory of Fields) and his conclusions about the equivalence between mass ...
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### Deriving magnetic field energy per unit volume for a wire

The magnetic energy per unit volume for an inductor can be derived from the formula: $U=\frac{1}{2}L{I}^{2}$ But how to derive formula for energy per unit volume for a (long) wire. Since there is no ...
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### Does it always take the same amount of energy to get from A to B?

Imagine you have a point A and a point B. Does it takes the same amount of joules to travel from A to B no matter how you do it (if in neutral conditions e.g. no wind)? For example, walking, riding a ...
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### Won't decrease in kinetic energy decrease the momentum as well?

My doubt stems from the question attached. If some kinetic energy gets converted into spring potential energy, shouldn't the velocity of be reduced and subsequently momentum? I also considered the ...
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### How an inductor lose energy when pulling iron

I'm confusing with a problem. Consider an ideal inductor with a shorted circuit and no resistance and having a current through it. we know this current is constant while the inductor has no resistance ...
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### COM frame for photons

We know that there is no frame in which the energy of a photon is zero and hence it has zero momentum. But can we find a frame in which the energy of two head on photons is zero, or the center of mass ...
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### Kinetic energy of a rolling disc when slipping

To derive kinetic energy expression for a rolling body, we consider the instantaneous center of rotation and write the angular energy about that point. After some algebraic simplification, we conclude ...
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### Can hypervelocity impacts be an efficient EUV source?

Imagine that two small pieces of rock hit each other at a very high velocity (for example, small meteoroids moving at orbital velocity in opposite directions). How much of the impact energy could be ...
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### What actually is a force?

First law of motion clearly says that, anything that creates a state change is a force. So, in our thermodynamic systems thermal energy, hence kinetic energy of constituent particles change when ...
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### Thermal Energy and Forces on a particle

Suppose there is a container (filled with vacuum) in which there is a particle (only one).Let the initial temperature be $0K$ so that the particle will have $0$ Thermal energy. Now as we increase the ...
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### PMDC Motor Characteristic Graph for Given Parameters

TL;DR Need to make a graph which showcases all parameters of a PMDC motor against the torque input. I am trying create a graph similar to as shown below. However, I am confused as to how the torque is ...
53 views

### Can gravity be utilized to generate hydropower in a clean, repeatable fashion without rivers or lakes? [closed]

I came up with this idea today and am just wondering if it is feasible. It is much easier to communicate via images so I drew the attached image to convey it. The key concept is rolling a ball in ...
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### Clear up confusion regarding energy dissipation by sliding friction on a rigid body [closed]

Does anyone have a relatively simple approach for evaluating the heating for a solid non-rigid body? For example, in fluid mechanics the mechanical energy equation sometimes includes a friction loss ...
63 views

### Does the universe lose energy when spacetime expands?

When reading about the Big Bang you’ll commonly hear that the universe began “cooling down” soon after. However, if we assume the universe is a closed system, would that thermal energy not continue to ...
68 views

### How much energy is in the Universe as photons? [duplicate]

The mass of Universe in kilograms is about 10 power 53. But how much energy exists in the Universe in form of photons? And if they would all be converted into mass, how much mass that would make? Dark ...
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### Exergy of a vacuum in a cylinder with a piston

So my system cylinder with volume V and rigid walls, except one of them is movable (piston). Outside I have pressure P0 and temperature T0. What is the exergy of my system? So I've been told that ΔB = ...
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### Poynting vector on the surface of an isolated system

We've an isolated system inside which currents and charges reside. If the Poynting vector on the surface of the system is non zero, then there is energy exchange between the system and the ...
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### Work and energy: law of conservation of energy? [duplicate]

According to the law of conservation of energy: Energy can neither be created nor be destroyed Then how did energy come into existence?
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### Can human created heating increase the temperature of the planet surface/atmosphere?

If we ignore the greenhouse effect and imagine all the worlds energy is produced by nuclear or other power plants would the energy needed by humanity to heat be enough to measurably increase the ...
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### Does an electron change its energy when the spin projection is changed?

When an electron changes its spin projection in a magnetic field does it receive or take energy? When the initial spin projection is indefinite (as is according to QM) how much is that energy?
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### Estimate Calories Burned from Arbitrary Limb and Body Movements [duplicate]

I'm looking to create a model using a reasonable physics-based approach to estimate calories burned for arbitrary movement. To keep the scope fairly limited and tractable, I am looking at the ...
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### Estimate Calories Burned from Arbitrary Limb Movement

I'm looking to create a model using a reasonable physics-based approach to estimate calories burned for arbitrary movement. To keep the scope fairly limited and tractable, I am looking at the ...
33 views

### How do I derive this internal energy equation?

So my question is probably quite basic, but I have found this paper online. http://www.astro.lu.se/~david/teaching/SPH/notes/annurev.aa.30.090192.pdf And it defines the rate of change of thermal ...
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### Why do inelastic collisions occur in theoretical calculations?

When solving collision problems related to the conservation of momentum in my applied maths course, the question of whether the collision was elastic or not is often asked. A lot of them time (such as ...
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### Where does the extra energy come during superelastic collison

It is known that in superelastic collison the energy of particles after collison is more than the original particles but how is it possible . I barely could find suitable information on net for a high ...
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### Producing Pressurized Hydrogen Underwater Then Using it To Do Work

This is a bit of a weird one but I didn't know where else to ask this. I was recently talking to my brother and he told me about an idea he had talked with his boss about how to create compressed ...
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### Energy-conservation for exothermic reactions

How do exothermic reactions not violate energy conservation if e.g reactants were given 2200kJ/mol of energy and product released 2300kJ/mol of energy, where has this +100kJ/mol come from?(2200kJ/mol ...
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### Can particles gain energy during inelastic collision?

We know that when two particles collides inelastically, the net energy equation is given by $$E_i=E_f+Q$$ where $E_i$ and $E_f$ are initial and final mechanical energies of the particles and $Q$ is ...