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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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1answer
51 views

Relation of Hydrogen orbitals to its spectral series?

I'm looking for the link between the Rydberg formula for hydrogen spetcral series $$\frac{1}{\lambda_{\mathrm{vac}}} = R\left(\frac{1}{n_1^2}-\frac{1}{n_2^2}\right)$$ and this image. Is it right to ...
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3answers
60 views

Converting potential energy to kinetic energy

I know how potential and kinetic energy interrelate when you drop anything. But is it a general understanding in the community that a change in potential energy is always kinetic energy? I am ...
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3answers
63 views

Example of both energy being/not being a monotonically increasing function of entropy and vice versa?

In (https://theoreticalminimum.com/courses/statistical-mechanics/2013/spring) Dr. Susskind says we can assume that in most of the cases energy is a monotonically increasing function of entropy and ...
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5answers
255 views

Conservation of mass in relativistic collisions?

It's stated in my textbook that relativistic mass is conserved in collisions, even in inelastic ones. So if you have a particle with rest mass $m$ moving with speed $u$ (considerable fraction of the ...
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0answers
42 views

Calculating spring constant [closed]

We have a vertical spring positioned on the moon. When compressed by $17m$, this spring sends $1500kg$ packages into escape velocity $(2400m/s)$, and then back to Earth. What is the spring constant? ...
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3answers
61 views

A doubt related to Sound [closed]

According to laws of conservation of energy energy can neither be created nor be destroyed. We all know that sound is energy. But according to law of conservation of energy- sound can neither be ...
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1answer
36 views

Probability of Photoelectric and Campton decrease as photon energy increases, WHY? [closed]

Why Probability of Photoelectric and Campton decrease as photon energy increases??
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2answers
49 views

Why is the net work of a falling object impacting the ground zero?

In my physics textbook, I found the following problem: "A 2,100 kg pile driver is used to drive a steel I-Beam into the ground. The pile driver falls 5.00m before coming into contact with the top of ...
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2answers
49 views

A question about magnets, energy, and forces [duplicate]

Following my first lecture on magnetism today, I'm left with just one question my lecturer couldn't answer. Imagine a 1 kg magnet supporting its own weight stuck to a vertical surface, resisting ...
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1answer
46 views

Time rate of change (the time derivative) of the internal energy.

I am reading in an article that according to the first law of thermodynamics, the time rate of change of the internal energy obeys the following relation, $\dot{E}+\int_A \mathbf{q} \cdot \mathbf{n} \...
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1answer
53 views

Finding G.P.E. of a system of particles

we want to calculate the P.E(potential energy) of a system containing 3particles p1,p2,p3.the point of observation is P.so now we should add up the P.E at P due to p1,p2,p3 to get the net potential ...
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3answers
105 views

Why does it take so much energy to keep the protons and neutrons in an atom together

I heard on the net that the reason there is so much more energy released during the breakage of an atom, i.e. the sun, than from breaking a chemical bond is because the particles are pushing away so ...
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1answer
72 views

If energy can't order itself, how did energy get ordered or made? [duplicate]

In the universe we see that hydrogen (bound up energy) is found in a highly concentrated ordered state in some places and it's highly dispersed in most other places. There is a natural progression ...
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3answers
143 views

What is the Hamiltonian in the “energy basis” for a simple harmonic oscillator?

My textbook says that for a simple harmonic oscillator the Hamiltonian can be expressed in the "energy basis" in this way: $$\hat H=\hbar\omega\bigg(\hat a^{\dagger}\hat a + {1\over 2}\bigg).$$ I ...
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0answers
28 views

How can I draw the energy bands for the first and second zones of Brillouin? Is it conductor or insulator?

I want to draw the energy ($E$) diagrams for a simple cubic cell of parameter $a$, where each atom provides two electrons for the almost free electron levels for planes [100], [110] and [111]. I ...
2
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1answer
54 views

What happens to the gravitational potential energy of the objects at the singularity of a black hole?

As they are 0 distance from the center of the gravitational body, do they have no potential energy? Does this mean that the singularity is at the lowest energy state possible? Sorry if this sounds ...
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0answers
40 views

Trying to get one step closer to the understanding of free energies $F$ and $G$

I find it quite hard to get a good grasp of the notions of Helmoltz and Gibbs free energy, because they can be introduced in so many different ways, and embody so many different ideas at once... Here,...
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2answers
188 views

In quantum mechanics, why is $\langle T\rangle=\frac{\langle p^2 \rangle}{2m}$ rather than $\langle T\rangle=\frac{\langle p \rangle^2}{2m}$?

I'm a newbie reading quantum mechanics from "Inroduction to Quantum Meachanics" by Griffiths and in the early pages of the book the author defines: $$\langle x\rangle =\int_{-\infty}^{\infty} x|\Psi(...
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3answers
78 views

Does a spring's $k$ affect the acceleration of a coupled mass?

Suppose I have a spring of spring constant $k$ attached to a mass $m$, with a constant force $F$ applied to the free end of the spring thus accelerating the mass. What happens to the acceleration of ...
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1answer
48 views

In vertical circular motion of a mass attached to a rope, where does the elastic potential energy stored in the tension goes?

When a bob is attached to a rope where the rope has negligible mass, the bob undergoing circular motion will have its KE1+PE1 at any point of the circular motion equal to KE2+PE2 at another point of ...
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2answers
36 views

In a box, two flashlights face each other. How does the light which hits the side of the box vary with two lights in it instead of one light? [closed]

Does the light hitting the light affect the light which ultimately hits the side of the box?
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3answers
52 views

Contradiction on Oscillating Mass [closed]

A bead is oscillating in horizontal direction as shown in the figure, our aim is to find the angular frequency of the oscillating bead First, we can write the potential as: $$V(l)=\frac{1}{2}k\cdot l^...
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1answer
112 views

Doesn't the “Mexican hat” potential give a misleading impression that the barrier height between two vacua is finite?

For $\mathbb{Z}_2$ symmetry breaking in a Classical Field Theory described by a potential $$V(\phi)=\lambda(\phi^2-v^2)^2,\tag{1}$$ there is a finite energy barrier of height $\epsilon=\lambda v^4$ ...
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2answers
143 views

Physical Significance of $U$ (Internal Energy ) , $H$ (Enthalpy) , $F$ (Free Energy) and $G$ (Gibbs Free Energy)? [closed]

I know their mathematical definitions and how these terms are interrelated (mathematically) but I fail to understand the physical meaning of none but one which is INTERNAL ENERGY . It seems ...
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2answers
31 views

Thermal energy transfer + combined energy question

In thermal energy transfer, why does the total combined energy of all particles remain constant?
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4answers
102 views

Momentum of an ultrarelativistic electron

I am aware of the relativistic equation: $E^2 = (pc)^2 + (mc^2)^2$ And if we are dealing with a massless particle then $E = pc$ However I am doing some work in Astrophysics and have been told that ...
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1answer
37 views

Is the ground state energy always larger for the system with higher potential energy?

Say we have two Hamiltonians $\hat{H}_1$ and $\hat{H}_2$ that differ only in their potential energies and $$V_2(x) > V_1(x)$$ for all $x$. Is the energy of the ground state of system 2 necessarily ...
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0answers
24 views

Free Particle With Zero Energy (No Relativity) [closed]

Relativistic reasoning would lead one to conclude that a free particle with zero energy does not exists since it cannot have any mass given that the total energy $E = mc^2$ for any given particle at ...
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2answers
46 views

Gain in electric potential energy or loss?

When a unit positive charge (say A) is in the field of a unit negative charge (B) , it attracts A towards it. But when it moves towards B its potential energy changes to kinetic energy and by the time ...
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3answers
366 views

Does light lose energy when it reflects from a surface?

When light reflects from a surface, at least the direction of its momentum changes. Since the total momentum must be conserved, there has to be something going on within the atoms of the surface. So ...
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3answers
100 views

In an adiabatic process, why is it that if work is done very quickly then there is no time for energy transfer to the surrounding?

I don't seem to understand the logic in adiabatic process of how 'if work is done on the system very quickly e.g. compress a piston containing n number of gas particles, there is no time for energy to ...
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1answer
49 views

Where does the energy from tidal power plants come from? [duplicate]

Tidal power plants taps into the energy in the motion of the oceans due to the gravitational interaction between the earth and the moon, so much is clear to me. This question is about where would this ...
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0answers
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Energy in magnets [duplicate]

If I have a magnet and an iron object, then the object would move towards the magnet, and the energy required to do so is stored in the magnet itself, but there can't simply be an infinite amount of ...
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1answer
56 views

What is negative energy as used by Dirac to explain pair production? [duplicate]

While reading about pair production, I came across the concept of negative energy. What is negative energy exactly? And can there be negative mass corresponding to this energy?
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1answer
40 views

Why do we prefer to write free energies as functions of particular state variables? [duplicate]

Anyone who's studied thermodynamics knows the fundamental relations for the various free energies: \begin{align} \mathrm{d}U &= T\:\mathrm{d}S- p\:\mathrm{d}V\\ \mathrm{d}F &= -S\:\mathrm{d}T- ...
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1answer
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Why is Kinetic Energy in Energy-Distance graph linear, but parabolic in Energy-Time?

Specifically, the projectile (falling off a table) problem. The graph of the Energy-Distance is linear, but I don't understand why. Isn't $KE = (1/2)mv^2$?
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1answer
91 views

Is mass equivalent to potential energy? [closed]

In other words, do we really need the concept of mass? If so, why?
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2answers
47 views

Is it possible to survive the destruction of a planet (earth)? [closed]

I recently watched Star Trek VI: The Undiscovered Country again. The main story evolves around the destruction of a moon of the klingon empire which is their main ...
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2answers
66 views

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

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 ...
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3answers
87 views

Wave and relativity [closed]

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
30 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
95 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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0answers
26 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
145 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
43 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
45 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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1answer
62 views

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

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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2answers
88 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
45 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
97 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 ...