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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.

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49 views

Can you anti-dot? (9-22 from Marion Thorton)

I'm solving a question out of the textbook and it reduces to the following. a particle of mass 2m with velocity $v_0$ collides with a particle of mass m at rest. The collision is elastic. So using ...
2
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3answers
55 views

Wave and relativity

From Wikipedia, "In physics, a wave is a disturbance that transfers energy through matter or space, with little or no associated mass transport." But Einstein said that energy equals mass so if a wave ...
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1answer
18 views

Interface between two phases minima of the energy and interface between a minimum and a “vacuum”

Cahn and Hilliard define the energy of an interface: the difference per unit area of interface between the actual free energy of the system and that which it would have if the properties of ...
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4answers
71 views

In $E=hf$, can $f$ assume any positive value? (Beginner) [duplicate]

The energy of photon is given by the equation $E=hf$, where $h=$ Planck's constant, and f=frequency of radiation. Is f quantized, or can it assume any value? If it can assume any value, then wouldn'...
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18 views

Electromagnetic propulsion

Is it possible to convert energy in magnetic field of a solenoid into acceleration of a magnet going through that solenoid ? My experiment was to look at the effect of current intensity on the ...
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1answer
44 views

On Joule's free expansion experiment

While reading through an introductory textbook in Thermodynamics, I've come across a series of arguments that seem somewhat arbitrary/vague, on which I'd like some clarification. To begin with, the ...
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0answers
18 views

Conservation of momentum and energy question [on hold]

A small wooden block with mass 0.800 kg is suspended from the lower end of a light cord that is 1.60 m long. The block is initially at rest. A bullet with mass 12.0 g is fired at the block with a ...
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0answers
13 views

Heat capacity of mixtures [on hold]

I have some fish balls with a heat capacity of 192 J per degree Celsius, and 800g of water (which has a heat capacity of 3360 J per degree Celsius). So is the heat capacity of the mixture = (3360+192) ...
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0answers
21 views

Energy of Landau Levels in a 3D Rectangular Slab

I'm currently going through Weinberg's Lectures on Quantum Mechanics, and I have some confusion as to where to start in one of the problems. In Chapter 10, Section 10.3, Weinberg goes through the ...
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0answers
30 views

Why can bound energy eigenstates be written as real functions? [duplicate]

Found this statement in Quantum Mechanics by David McIntyre and was wondering how this is proven.
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1answer
38 views

If a box is carried 2 flights of stairs to an attic, is more work done than opposed to go straight up with a ladder? [duplicate]

The box has the same weight. I think that the stairs does more work because it travels a further distance as it is a less direct path opposed to going straight up.
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38 views

Stars with different Speed of light [on hold]

Is the speed of light changes for each star depending upon the diameter or the speed is constant irrespective of the diameter in vacuum
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1answer
59 views

Is it possible to modify solar panels to work off of nuclear radiation instead of solar radiation?

On my understanding of how solar panels work, solar rays enters the cell, and a chemical reaction turns that into power. Nuclear radiation is a flow of extra electrons i beleive, and solar rays are ...
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1answer
29 views

Phase transitions and Helmholtz vs Gibbs Free Energy

Gibbs phase rule for Gibbs free energy says that phases during a phase transition must be in mechanical, chemical, and thermal equilibrium, e.g. $T_1=T_2$,$G_1=G_2$, and $P_1=P_2$, where the ...
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1answer
43 views

What additional percentage of the suns energy is being trapped via green house gases?

Every year we release more greenhouse gases which causes the atmosphere to trap more energy and thus warm the planet. What additional percentages of energy are we talking about here? I would assume ...
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2answers
77 views

How is it possible that the earth has the concept of energy equilibrium: the same amount of energy comes in and out?

I was reading this article on the Earth's Energy Budget, as well as a few other articles that basically contained the same information: That there must be an equilibrium of energy coming into the ...
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2answers
84 views

Does an electron's drop from $2s$ to $1s$ produce the exact same type of photon in different atoms and molecules? [closed]

Does an electron's drop from $2s$ to $1s$ produce the exact same type of photon in different atoms and molecules? Also, does a change from a higher principal quantum number to a lower one always ...
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2answers
48 views

Why the rubber ball bounced higher than the glass ball

For a teaching degree we did the following experiment: Drop a rubber and a glass ball of approx. same size from approx. same height onto laminate flooring. As we expected, the rubber ball bounced ...
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5answers
71 views

Could we harness the energy from room temperature? [closed]

If no atom is at absolute zero, it must have kinetic energy. Therefore, could we harness that energy, (sure for small objects, not much energy would come of it) but surely the earth itself has a ton ...
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0answers
24 views

Percentage increase in wavelength leads to percentage decrease in energy. (Compton effect)

In this question: What percentage increase in wavelength leads to a 75% loss of photon energy in a photon-free electron collision? My understanding of this question is that $0.25\times E_i = E_f$, ...
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1answer
41 views

Regarding the equivalence of expressions for the change in internal energy of a system when the process is not quasi-static

It is well-established that some authors prefer expressing the change in internal energy of a thermodynamic system in terms of the work done ON the system and others will express it in terms of the ...
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0answers
37 views

Impact parameter for particle capture

"Let a particle of mass m moving in a potential $$V(r)= \frac{-k}{r^3}$$ come in from infinity with speed $v_0$ and impact parameter $b$. What is the largest value of the impact parameter ($b_{max}$) ...
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5answers
719 views

Paradox in the definition of work [duplicate]

I'm trying to understand the Oberth effect and came across this paragraph that seems crazy to me: when the rocket moves, its thrust acts through the distance it moves. Force multiplied by distance ...
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1answer
34 views

Derivation of Helmholtz free energy and associated relation

I am quite new in thermodynamics and I have never touched it so far. But now I have some basic questions that I hope someone can kindly explain them to me. I know the Helmholtz free energy is ...
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1answer
73 views

Useful Momentum Relation for Relativistic Particles [closed]

I've seen that $$p = \sqrt{(\frac{E}{c})^2-(mc)^2} = \frac{1}{c} \sqrt{{E_k}^2 + 2 E_0 E_k}$$ for a relativistic particle on hyperphysics and in my modern physics class. I understand that the first ...
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2answers
60 views

Are matter and energy dependent on each other? [closed]

If matter creates energy and energy creates matter can one exist without the other?
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3answers
103 views

Electron falling into proton approaches infinite kinetic energy why?

Foreword I believe this question to be different from the usual "Why don't electrons fall into the nucleus" because this A: asks about the details of explanations for why electrons don't fall into ...
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0answers
36 views

Toughness of material to be not damaged by impact / penetration [closed]

A bullet has mass $m = 0.2 \; \text{kg}$ and speed $v = 700 \; \text{m/s}$. The tip of this bullet is a circle with $S = 1 \; \text{mm}^2$. Now this bullets hits a target which is made from some ...
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1answer
29 views

confusion regarding energy levels of the nucleus of a Sodium atom [closed]

In my textbook I was asked to compute the wavelength of a photon emitted as the nucleus of a Sodium atom pass from the first excited level to the base level. I got the formula itself right, but ...
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0answers
8 views

Microwave radiation levels

what is the strength of microwave radiation reaching the earth's surface on an average day, measured in Watts?
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1answer
23 views

What's Gravitational Potential Energy of a Balanced Mass on a single Point?

Anyone know how to calculate the gravitational potential energy of a balanced Mass such as an upside down 2d triangle balanced on a single point. Is there any way to calculate the gravitational ...
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0answers
34 views

Different energy levels of an atom vs. electron shells

I am confused about the following point: When studying graphs such as the one below, which depicts the different energy levels of an atom (Mercury in this case), I always thought that the different ...
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1answer
38 views

Force input to harmonic oscillator force as function of time or displacement as function of time? [closed]

Question title basically says it. If the governing equations are like: $$x''m_1 = c({x_1}'-{x_2}') + k_1({x_1}'-{x_2}') = f(t)$$ etc... Since all the terms are force terms, shouldn't the input ...
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1answer
29 views

Quark-Gluon Plasma vs QCD binding energy

The binding energy of nucleons is on the order of around 900 MEV. Yet, Quark-Gluon plasma is observed at around 175-300MEV. This doesn't seem to make any sense - how can only 300 MEV worth of energy ...
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2answers
25 views

Displacement using work energy principle

How do you use the work energy principle to find the displacement of an object. With of course the mass, speed and friction forces given?
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0answers
19 views

Heisenberg Uncertainty Principle for energy-time [duplicate]

I have just studied the Heisenberg Uncertainty Principle, after which there is one paragraph dedicated to a similar principle related to energy vs. time. However, the explanation is very short, and I ...
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2answers
47 views

Energy that can control itself (A unit that can process information without body) [closed]

I read a book called The future of the mind. There it suggested a conscience that exists in the form of energy without body. Ideally, I thought the energy without body may take the form of light. ...
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1answer
22 views

Breaking Stuff and Putting it Back Together

I was wondering if you could put something back together exactly the way it was before. It takes a certain amount of energy to break something, so theoretically could you put it back together exactly ...
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0answers
40 views

How much energy is stored in the charge when electron and positron are created?

I had asked "The energy of electron formation becomes mass, charge, spin and momentum. How much becomes the charge? We know how much becomes mass: E = mc^2. What is the equation for the charge?". ...
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0answers
12 views

Enthalpy extraction vs thermal efficiency?

These are just two terms I've found in the literature with regards to energy systems but I cannot appear to find a definition of enthalpy extraction, perhaps because it's so obvious. But just to ...
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3answers
49 views

What does the unit “kcal/mol” mean?

I recently encountered units of energy written as $``\mathrm{kJ}/\mathrm{mol}"$ or $`` \mathrm{kcal} / \mathrm{mol} "$. I'm aware of the meanings of $`` \mathrm{kJ} "$ and $`` \mathrm{mol} " , $ ...
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4answers
91 views

Energy vs. mass in photons

I am confused about the following: Mass is condensed energy. A photon has no mass, but it has a degree (of quantised) energy. Does that mean it does not have enough energy for it to be condensed ...
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0answers
44 views

The relation between mass, inertia and energy

I have trouble understanding the following concept: I learnt that mass is a measure of inertia, and that seemed logical enough. Yet separately I learnt that mass is a form of condensed energy. If ...
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2answers
61 views

when the current flow through a resistor, what does it lose?

We all know that in a series circuit, the current is the same for all of the elements. But when the current flow through a resistor, what does it lose? The current is the same because it doesn't ...
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1answer
16 views

UV Preconditioning test [closed]

I wanna build a solar panel with a new material and I wanna to test the endurance of the material against UV light. I've found in internet a Standard for UV tests. It says it has to be irradiated with ...
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1answer
46 views

When we push on a wall, why isn't there any work done? [duplicate]

I have seen a similar question on the site, but that was kind of different in its essence — it was aimed at asking the solution of a particular problem, while I am having trouble conceptualizing the ...
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1answer
23 views

Capacitor, stored energy versus reactive power

When an AC voltage U is put onto a loss less capacitor C the capacitor will require a certain reactive power Q (current 90 degress out of phase with the voltage): $Q=UI = \omega C U^2$ in VAr (Only ...
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0answers
13 views

Why the eigenvalues of hamiltonian are discrete in bounded systems (only discrete energy levels)? [duplicate]

In one dimensional motion the general potential is given as in the figure above When energy is between V min and V 1 the energy levels are discrete like in the potential barrier and well example or ...
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0answers
39 views

Hydrogen energy state transition times and probabilites

How correlated is the energy absorbed to the later energy emitted, and hence to the transition type? What is the scale of time it takes to make a downward transition in energy level? In a very cold ...
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0answers
17 views

What happens when a photon does not excite the eletron in hydrogen [duplicate]

Let's say the energy carried over (photon) is not enough to change energy level of electron in hydrogen, or, it is a tad too much and we instead refer to the extra energy as the insufficient carrier ...