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)

11
votes
1answer
2k views

Does light lose energy in transit?

Consider a photon is an energetic particle and therefore has a gravitational field. When a photon passes a molecule or particle of dust in space it will pull the dust towards it and deform the dust. ...
0
votes
2answers
414 views

Conservation of energy in transformers

How does energy remain conserved in a transformer if emf is increasing, or decreasing? Does the current decreases to accomodate? Does Ohm's law still hold here? Although we know, Ohm's law is not ...
4
votes
2answers
1k views

Is the E-cat by Andrea Rossi et al. for real?

Does this thing really do what they say? http://www.youtube.com/watch?v=EhvD4KuAEmo If it does, it looks like this will probably be the biggest breakthrough in science ever :)
8
votes
2answers
7k views

What is the relationship between Energy, Entropy, and Information?

What is the relationship between Energy, Entropy, and Information? I read this - What Is Energy? Where did it come from? - and the top answer says that 'energy' is an abstract number that is a ...
3
votes
2answers
210 views

How can water be liquid and gas in the same environment?

Think about it. If there's water in a gaseous form, how can there also be water in a liquid form? The gaseous water would have to have more energy than the liquid, but if it's the same temperature, ...
3
votes
2answers
170 views

Food calories vs gravitational potential energy [closed]

Imagine you could use all of the energy you have ever extracted from your food as a source of propulsion. Is the energy contained in the food you’ve eaten over the course of your lifetime enough to ...
3
votes
2answers
15k views

What happens to the absorbed light energy?

When light comes across with a solid material, some of it is reflected, some of it passes through and some of it is absorbed. I understand the reflection and passing through, but I don't understand ...
1
vote
1answer
2k views

Why doesn't the potential energy of any object equal 0

Consider a particle on the ground. This particle is raised by a force of magnitude $mg$ to a height $h$ above the ground. At this point, the work done on the particle by the force is $mgh$, which is ...
0
votes
4answers
50 views

High power short duration from low energy source?

1MW, can 1000Joules source of energy deliver 1M of power in 1 millisecond? $Energy =$ $P$ $x$ $t$ , so does it make sense to have small amounts of energy in large pulses of power? In an extremely ...
0
votes
2answers
585 views

Particle in a Box: Energy Less than the Potential Energy

I am reading quantum mechanics from Shankar's Principles of Quantum Mechanics. On page 157 he defines the box potential $V(x)$ as $$ V(x) = \left\{ \begin{array}{rl} 0 &\mbox{ if $|x|< L/2$} ...
2
votes
1answer
1k views

Energy Stored In A Capacitor (Slowly Moving Parallel Plates Together)

I think this is a canonical problem with capacitors, but I'm having difficulty understanding the paradox. In Purcell's Electricity And Magnetism, problem 3.16 reads: Calculate the electrical ...
1
vote
0answers
238 views

What is the best way to heat air? [closed]

I want to know which way we could create hot air most efficient, how can we convert all energy input into air temperature and decrease air density as much as possible? Is the plasma (such as fire) is ...
1
vote
1answer
67 views

How to find the maximum height with a loss of 5 J of energy due to air drag [closed]

Can someone please help me to figure out how to use the 5J of energy into my equation for the maximum height. I know that the maximum height is given by, $$h_{max}={(v_f-v_i)^2 \over 2g}$$ but I'm ...
-1
votes
1answer
64 views

how to use the energy work kinetic theorem [closed]

A single force acts on a $3.4~\text{kg}$ particle-like object in such a way that the position of the object as a function of time is given by $x = 4.2t - 2.1t^2 + 2.5t^3$, with $x$ in meters & $t$ ...
0
votes
1answer
42 views

Relevance of this equation

The book 'The variational princples of mechanics' by Lanczos was recommended to me and I am reading it now. So far it is an enjoyable journey, but I don't see the significance of this equation: $t = ...
1
vote
1answer
76 views

What's diamond's melting point?

I've been looking around but I've found no clear answer. The most I could find was: "around 4000K" Has it not been determined and it's just an estimation?
0
votes
2answers
372 views

Why do we use less fuel by driving at constant speed?

It is common practice to drive at constant speed as much as possible to reduce fuel consumption, or at least to avoid "pumping" on the throttle (at frequencies high enough that the lowpass filter does ...
1
vote
0answers
55 views

Energy of a system

I'm considering a ring of mass $m$ sliding along a cardioid with equation $r=a(1+\cos\theta)$. We let the angle between the downward vertical and the radius vector be $\theta$. The ring is attached to ...
0
votes
1answer
58 views

Motivating the importance of energy [closed]

I'm working on a project right now and part of the introduction is about energy, and I'm trying to introduce the notion of energy in an unconventional way. Right now, I am looking for examples of ...
1
vote
0answers
37 views

Energy in a string [closed]

A nylon string of length L = 50 cm and under a tension of T = 70 N is plucked with an amplitude of 5 mm. What is the energy in the string? Please give me the relationship between energy of the ...
2
votes
4answers
6k views

Which is more efficient, heating water in microwave or electric stove?

So our propane tank in the kitchen ran out again today. Which is more energy efficient, boiling water in a microwave on an electric stove? All things being equal i.e. starting temperature and mass ...
1
vote
1answer
51 views

Joules To Move A Kilogram In Space [closed]

If roughly ten joules lifts one kg on earth one meter, how many in space? Meter / joule/kilogram Kilogram/meter/joule
2
votes
1answer
136 views

A few questions about photons

I have a few questions about photons. I know the emitting and absorbing of photons is related to quantum leaps. Does a quantum leap cause emitting/absorbing, or is it the other way around? Do ...
0
votes
1answer
187 views

How to convert translational energy to rotational energy [closed]

Suppose that I have a body moving at certain velocity in straight line. So it has translational energy. I want to convert this energy to rotational energy of another body, by making the velocity of ...
8
votes
3answers
5k views

Kinetic energy with respect to different reference frames

I'm having problems understanding the following situation. Suppose two 1-tonne cars are going with the same orientations but opposite senses, each 50 km/h with respect to the road. Then the total ...
6
votes
1answer
235 views

Steam above meal

When I fry something on the pan and the fire is big and intensive (the flame over the pan), there is no stean above the pan or just a little. But when I turn off the fire, suddenly the steam appears, ...
4
votes
2answers
664 views

What does it mean to “convert energy into time”?

In a recent article about creating electron-positron pairs by colliding photons in a laboratory, Andrei Seryi, director of the John Adams Institute at Oxford University, was quoted to said: It's ...
1
vote
1answer
75 views

Time energy uncertainty principle [duplicate]

$ \sigma _{H}\sigma _{Q}\geqslant \frac{h}{4\pi }\frac{d\left \langle Q \right \rangle}{dt}$ $\Delta E = \sigma _{H}$ $\Delta t = \frac{\sigma _{Q}}{d\left \langle Q \right \rangle / dt}$ $\Delta E ...
0
votes
0answers
225 views

Temperature dependence for specific thermal diffusivity in the diffusion formula

I recently found this answer about the diffusion equation (nice one actually), but have one doubt about the temperature dependence of this formula. If the "packet" of energy (terminology suggested ...
5
votes
1answer
5k views

Why does compressing a piston increase the internal energy?

When we compress a piston, its total internal energy increases, however I don't understand why. As the piston compresses, the temperature should change, as the total energy density increases. As a ...
1
vote
0answers
31 views

How can a photon exist on its own without a mass? [duplicate]

For example, thermal energy exists and has no mass, but is carried by particles which have mass. A photon is described as a particle - how can a photon exist on its own, travel in space and even push ...
1
vote
0answers
495 views

Energy of RLC circuit

If you are given a general differential equation for an RLC circuit, for example, $$L\left(\frac{d^2 Q}{dt^2}\right) + R\left(\frac{dQ}{dt}\right) + \frac QC = V\cos(\omega t),$$ which is a driven ...
0
votes
1answer
104 views

Why isn't the electric field just a mathematical tool?

I'm limiting my question to this field because it is the only one I know of with a certain degree of knowledge. I doubt they really exist because of the following reasoning: Coulombs law was stated ...
5
votes
1answer
144 views

Energy usage in different reference frames

Imagine a moving object at constant speed (like a car). This object is, then, accelerated for a brief moment. In different reference frames (at rest and moving along with the object), the variation of ...
3
votes
1answer
191 views

Thermal emission cathode electron velocity distribution

I can't find any experimental data (or theoretical expression) on what is the velocity (or energy) distribution of thermal emission cathode electrons emmited from the cathode at approximately 2000 K ...
1
vote
1answer
165 views

What is the inverse of Energy?

Is there a name for the inverse of Energy $1/E$ or the inverse of the Joule $1/Joule$? It comes up if you divide the mass-energy relation by energy (so you get a constant of 1).
1
vote
0answers
256 views

How much energy does a powerlifter need to perform a session of squats?

So I started the math myself and I'm hoping someone can look this over and improve on my idea. Let's say that a powerlifter performs the following sets of squats: % 1RM = lifted weight in percent ...
1
vote
2answers
85 views

can one measure energy to a finite accuracy?

Can one measure energy to a finite accuracy in bounded amount of time? I don't know much about QM, but someone told me that the energy-time uncertainty principle says that it would take infinite ...
0
votes
0answers
99 views

Energy of moving capacitor

On the following link were a discussion about energy in capacitor moving parallel to its field: Where's the energy in a boosted capacitor? My question is what happen if capacitor is moving ...
2
votes
3answers
321 views

How to combat the black-body temperature of an object?

I'm trying to model the temperature of a large spacecraft for a space colony simulation game I'm working on. In another question, I checked my calculations for the steady-state black-body temperature ...
1
vote
0answers
72 views

Does a pulled rubber band contain as much energy as a twisted rubber band?

lets say I take two similar rubber bands. One of them I pull until it almost reaches its breaking point. The other I twist until it almost reached its breaking point. Do both of these rubber bands ...
0
votes
0answers
89 views

How is Energy Distributed within a Electromagnetic Wave?

Let $$\hat{H}$$ Denote the full energy of the electromagnetic wave. Now is it right to assume that $$\hat{H}=\sqrt{2\hat{H}_y^2+\hat{H}_x^2}=\sqrt{2\hat{H}_z^2+\hat{H}_x^2}$$ If not, what is ...
1
vote
0answers
40 views

What is the fluctuations of the energy of a simple harmonic oscillator? [closed]

$$\begin{align} \varepsilon&=\frac{\vec{p}^{\,2}}{2m}+\frac{K}{m}\vec{q}^{\,2}\\ \rho(q,p)&=\biggl(\frac{\omega}{2\pi k_BT}\biggr)^3e^{-\frac{\varepsilon}{k_bT}} \end{align}$$ where ...
0
votes
1answer
5k views

Maximum length stretch of vertical spring with a mass? [closed]

Having a vertical spring, if we hang a mass to it, what is the maximum stretch length of spring? and how big should be mass to break the spring?
2
votes
1answer
79 views

What's wrong with this temperature-in-space calculation?

I'm trying to calculate the steady-state temperature of a body in space, but my numbers are coming up much too small. For example, for a 1-meter cube, I'm getting a temperature of 194 K (or -81 C). ...
11
votes
1answer
722 views

What exactly does the Hamiltonian operator tell us?

I'm confused about how energy and time are linked. On the one hand, the Hamiltonian seems to describe the time evolution of the system because in the time dependent Schrodinger equation, $$ \hat H ...
2
votes
2answers
132 views

Can a laser be designed to ionize muonic atoms so as to prevent a-sticking?

Muon catalyzed fusion is currently little more than a lab curiosity today in part because of how many hydrogen nuclei can be fused before the muon is carried away by an alpha particle. ...
5
votes
2answers
559 views

Is there a relationship between the energy of a photon and the energy of an electromagnetic wave?

If the energy of a photon $E_{p}=hv$ And the energy of an electromagnetic wave is $E_{w}\propto \hat{\mathbf B}^2$ What is the relationship between $E_{w}$ and $E_{p}$?
1
vote
2answers
650 views

Bound states, scattering states and infinite potentials

I am doing my first semester of Quantum Mechanics and we're using Griffith's Introduction to Quantum Mechanics. As he is introducing the Dirac delta function potential he explains bound and scattering ...
3
votes
1answer
117 views

Why do safety helmets have a softer inner layer nearer the head?

I know that when an object collides onto the helmet, it causes an inelastic collision so that energy is absorbed by the structure of the helmet, so what exactly does the softer inner layer do? Does it ...