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

5
votes
2answers
1k views

Does space have to be filled with charged particles to carry electromagnetic waves?

I'm a newbie here so have mercy. I'm studying electromagnetic waves. This is the propagation of energy via the vibration of charged particles, as I understand it. A charged particle could be like ...
2
votes
1answer
138 views

potential energy of an object due to other two objects

consider object A with mass $m_{A}$ and positional vector $\overrightarrow{r_{A}}$ object B with mass $m_{B}$ and positional vector $\overrightarrow{r_{B}}$ object C with mass $m_{C}$ and positional ...
4
votes
5answers
9k views

How does ATP transfer energy to a reaction?

This is a question for which I've found it surprisingly hard to find a good answer. Biology texts talk mystically about the ATP->ADP reaction providing energy to power other reactions. I'd like to ...
0
votes
1answer
310 views

calculating of g force in fun ride

we vistited yesterday a fun park and saw a ride which was a big centrifuge only in vertical. this one had a speed meter but it was broken and i also wanted to know how much g-force is on there... so ...
6
votes
2answers
887 views

What is the highest energy position for a double pendulum? And for which energy positions is it chaotic?

Math/physics teachers love to break out the double pendulum as an example of chaotic motion that is very sensitive to initial conditions. I have some questions about specific properties: For a ...
1
vote
2answers
1k views

how does human brain compare to a modern CPU in energy per bit?

Can someone compare the energy efficiency of human brain as a computer ? What is the energy in joules / flop ? may be some reasonable assumptions on the computational load of common tasks such as ...
9
votes
4answers
722 views

fraction of magnetic energy stored outside a solenoid

If I have a long solenoid, e.g. length $l$ and radius $r$ with $l = kr$, where k >> 1, with a nonpermeable (e.g. air) core, how much of the magnetic energy is stored outside as compared to inside? If ...
1
vote
2answers
4k views

Finding the max height of a ball launched as a projectile using work-energy

A ball is launched as a projectile with initial speed $v$ at an angle $\theta$ above the horizontal. Using conservation of energy, find the maximum height $h_\text{max}$ of the ball's flight. Express ...
3
votes
1answer
615 views

Why is E85 less efficient than straight gasoline?

Why is straight gasoline (or whetever the mixture was before the introduction of ethanol) more efficient (ie, more miles/gallon) than E85? I've known since it's introduction that E85 was less ...
2
votes
1answer
419 views

whats the rate of energy increase required for constant acceleration between 0.0c and 0.99c?

I was wondering how much energy would be required to accelerate 1000kg to 0.99c at 1G. What I don't understand is what the rate of increase of energy is required as velocity increases. I was looking ...
2
votes
6answers
1k views

What's the problem with solar energy?

What "breakthrough" from a theoretical point of view is needed for solar energy to become feasible energy alternative?
5
votes
1answer
377 views

What type of solar energy technology has the most future potential?

In terms of dollars per watt, using theoretical efficiency limits, what technology holds the most promise to become the primary solar energy capture technology? My hunch is carbon-based modules, ...
2
votes
2answers
707 views

Limit on geothermal energy that could be extracted before the earth's magnetic field collapsed?

This is more of a theoretical thought-experiment question. Basically, how much geothermal energy can we extract before the loss of the magnetic field makes it a terribly bad idea? Will the ...
0
votes
1answer
117 views

People eating and gaining energy

Let's say we have 100 grams of chocolate which has 571 kilocalories (so it's about 2.4 megajoules) and we have barbell which weights 100 kilograms and need to lift it to 0.5 meters height. So how ...
1
vote
1answer
294 views

Energy storage and battery banks [closed]

Why is it not possible to store windfarm energy in battery banks? A lot of energy isn't being used when wind farms are at peak and we have no way of storing it. This is wasteful. So why don't we use ...
2
votes
2answers
2k views

In summer are the upper storey flats more hot or the lower storey flats?

I have often heard neighbours talking things like in a multi storied apartment, the upper flats are more hot in summer then lower flats (or vice versa?) and similarly for some comparison in winter? ...
2
votes
1answer
4k views

calculating thermal energy [exam prep]

I'm preparing for a grade 11 physics exam but having trouble with a question: a 19.9kg mass slides in a straight 28.8m line experiencing friction with a coefficient of 0.105, how much thermal energy ...
2
votes
1answer
629 views

Energy produced by a swing?

Please can someone help me with a equation for how much energy a person generates when swinging on a swing?
3
votes
0answers
109 views

What is the energy conversion efficiency of a computation device like a modern CPU? [duplicate]

Possible Duplicate: How efficient is a desktop computer? I believe the energy conversion efficiency of a modern CPU is extremely low, because it dissipates pretty much all of it as heat, ...
3
votes
1answer
279 views

If 100% of the energy from the sun is reflected back into space

100% of the energy from the sun is reflected back into space, it's just shifted from a low-entropy state to a high-entropy state, and from a high frequency (ultraviolet) to a low frequency (infrared). ...
2
votes
4answers
3k views

Can heat be transfered via magnetic field in a vacuum?

Say you want to store hot coffee in a container surrounded by a vacuum. To remove all sources of conductive energy loss the container is suspended in the vacuum by a magnetic field and does not have a ...
0
votes
1answer
196 views

My computer turns on when it “sees” me [closed]

Before asking, I swear that, for how it may seem strange, this is all true. Very briefly, when my pc is off and I am near it, it turns on. I do not touch anything. Nobody else have the same effect. I ...
0
votes
1answer
458 views

Uses and interpretation of the 'Bowen Ratio' ($B_o=SH/LE$)

The Bowen Ratio is the ratio of sensible heat flux to latent heat flux, so presumably it gives some information about the relative importance of these processes. But it is not clear how this ...
4
votes
1answer
4k views

What energy is transformed to heat when a candle is burned?

What energy is being transformed to heat when you burn a candle?
2
votes
2answers
374 views

How does my air-cooling unit work?

During the winter, my office gets a little cold. I have a unit (essentially a space heater) that heats the area around it to help keep me warm. Now that it's summer, I realize the unit also has a ...
2
votes
2answers
1k views

Degeneracy of states in mixed infinite square well, harmonic oscillator

I'm trying to determine the degeneracy of states given by $g(\epsilon)=g_{0} \epsilon$ for a system that is trapped in a quite specific potential. In two dimensions, the particle has a potential as ...
2
votes
2answers
2k views

How do electron configuration microstates map to term symbols?

I am trying to understand energy levels of electron configurations. I visited the NIST web site and discovered that the notation used here are called term symbols. After reading corresponding ...
6
votes
3answers
377 views

How can you tell if a critical energy density is actually a black hole?

Here's a question inspired by Edward's answer to this question. It's my understanding that the average energy density of a black hole in its rest frame is $\rho_\text{BH}(A)$, a function of surface ...
2
votes
2answers
768 views

When matter and anti-matter collide

Do they create energy? Or do they just disappear with zero energy? If they create energy when disappearing, that means it takes energy to create them, right? If they disappear into zero energy, ...
1
vote
0answers
152 views

Will earth rotation be slowed down? [duplicate]

Possible Duplicate: Are tidal power plants slowing down Earth's rotation? Since we have various energy acquiring facilities that harvest energy from earth rotation. (e.g. wind, tidal ...
5
votes
3answers
711 views

Kinetic Energy vs. Potential energy with regards to creating particles

So I know that when you collide particles with high enough kinetic energy, (kinetic energy = at least the rest mass of the particles you are making), you get particles. How come potential energy ...
0
votes
1answer
406 views

what does 'same energy as a ton of TNT' mean?

Very often the energy released in large-scale events (e.g., earthquakes, meteor impacts, etc) is described in terms of some number of 'tons of TNT'. This may be all well and good for those with an ...
0
votes
2answers
215 views

Internal Energy Terminology

I am interested in finding out whether the term 'Internal Energy' can be used in both of the following cases: In the case of a macroscopic body upon which light is incident, the internal energy ...
1
vote
3answers
839 views

Why isn't pressure a measure of energy?

Hey guys, I'm having a problem in understanding the 1st Law of Thermodynamics. If i would increase the pressure of a closed system by compressing a gas in a cylinder isothermal, the 1.Law states, ...
0
votes
1answer
184 views

Mass into energy

The mass of a nucleus if less than the mass of the protons and nucleus. The difference is knows as binding energy of the nucleus. This nuclear binding energy is derived from the strong nuclear force. ...
3
votes
2answers
3k views

How does electron excitation relate to atomic orbitals?

Taking off from the Rutherford-Bohr model, Figure 1. Rutherford-Bohr model. when an electron absorbs energy (from light, heat, or otherwise) it jumps to a higher energy level, or shell. Then when ...
0
votes
5answers
768 views

Energy transfer to a driven oscillator

Why is the amount of energy transferred to a driven mechanical oscillator largest when the drive frequency is the same as the natural frequency of the oscillator? Why are they exactly out of phase by ...
8
votes
2answers
412 views

Shaking a jar of balls

A jar is filled with two types of balls, red and green. Red balls have radius $r_1$ and mass $m_1$, green balls have radius $r_2$ and mass $m_2$. If initially the balls are randomly placed throughout ...
4
votes
6answers
1k views

Can gravitational potential energy be released in a fire?

If one takes a bundle of wood up high to the mountains so its potential energy increases, would there be obtained more heat by burning it?
1
vote
1answer
310 views

Energy, power and action

Through unit analysis, one can identify the following relationship linking energy, action and power: $energy^2 = action \times power$ Alternatively, we rewrite this expression as: $power = ...
7
votes
2answers
192 views

Why did this glass start popping?

I remember a while ago my father dropped a glass lid and it smashed. It looked something like this. When that happened, for about 5 minutes afterwards, the glass parts were splitting, kind of like ...
3
votes
2answers
631 views

How much does it cost every time your car stops for a traffic light?

Every time we have to stop the car, it is costing us extra money, because we have to then accelerate to full speed again. I would like to know how much. In order to simplify the situation, we can ...
1
vote
1answer
268 views

Is there a theory about kinetic energy “particles”? [closed]

We have a model of electricity which says electrons flow from one place to another. We have a model of optics which says that photons go from one place to another. As I understand, there is currently ...
3
votes
2answers
692 views

What is an “Idea” in terms of time space and matter? [closed]

If the physical world we know is made of matter stretched in time and space and some forces affecting it, what would be an Idea, pure Information or something similar. Does those things don't exist ...
1
vote
1answer
252 views

2 Electricity generator connected

I'd like to know (my knowledge at physics are realy poor) what would happen if I've got a generator connected to another generator and both of them generating electricity for a house or whatever. ...