The law of conservation of energy, which states that the amount of energy in a system is constant. For questions about Earth's environment, see the climate-science tag instead.

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2answers
381 views

What is changing in latest light bulb technologies?

I'm confused with the latest home lightning bulbs. Understanding filament bulbs was easy. For example take 220V, 100W filament bulb: Power = $V^2/R$ Filament gets heated and emits energy in the form ...
2
votes
1answer
2k views

How to calculate calorie expidenture from lifting a weight?

Am I correct to say that when a human lifts a dumbbell from, say, 2 feet off the ground to 6 feet off the ground, he will have increased the potential energy of the weight and thus will have burned ...
6
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2answers
351 views

Conservation of energy in a non-linear oscillator

I have a homework question about a "non-linear oscillator". I actually have an answer to this question, but the answer I get is stronger than what is needed according to the question. The question ...
2
votes
1answer
66 views

stress work of uniformly deforming continuum

I have a volume which is deforming (using explicit time-integration scheme) uniformly with velocity gradient $L$ and stress tensor $\sigma$. I would like to determine work done by the volume ...
8
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0answers
504 views

Would Portal-style portals transmit gravity? [closed]

In the video game Portal, there are often puzzles which must be solved by gaining a large amount of momentum. Typically, this is accomplished by putting one portal on the ground and another directly ...
8
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3answers
1k views

photons in expanding space: how is energy conserved?

If a photon (wave package) redshifts (streches) traveling in our expanding universe, is it's energy reduced? If so, where does it go?
1
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1answer
272 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 ...
1
vote
3answers
539 views

The “binding energy” of bonded particles adds mass?

This is a follow-up my previous question. Several of the answers indicated that the mass of a particle (atom, proton, etc.) increase with the "binding energy" of it's component particles - the energy ...
3
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0answers
106 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
273 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). ...
8
votes
4answers
22k views

What is the relationship between kinetic energy and momentum?

I can't seem to figure out the relationship between $E_k$ and $p$ or $F$. I understand that the units are pretty different. But for example: A bullet with a mass of 10.0g is moving at the speed of ...
6
votes
6answers
633 views

Why is global conservation of energy not considered a tautology?

This question is in reference to my downvoted answer to this active physics.SE question. More than one user has indicated that it is simply wrong and I am having trouble understanding why. My point ...
5
votes
5answers
2k views

Conservation law of energy and Big Bang?

Did the law of conservation of energy apply to the earliest moments of the Big Bang? If so, what theoretical physics supports this? I hear that Einstein's theory of relativity disputes the law of ...
2
votes
1answer
135 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 ...
2
votes
2answers
366 views

Mechanical energy problem - please help [closed]

Quite new to all this - simple question. "A ball starting from rest rolls down a hill and reaches 10m/s at the bottom. If the same ball starts rolling down the hill from an initial velocity of 5m/s, ...
0
votes
1answer
598 views

Lawrence Krauss' Theory of Origin

I don't understand how a vacuum, the absence of matter, can hold energy. How can it hold energy when Einstein proved that matter is energy? And a second related question; how does the energy in a ...
1
vote
1answer
490 views

Kinetic energy loss when a rigid body falls into water

Assume you have a rigid body falling into the ocean at terminal velocity. Also assume that the rigid body does not break on impact. How could you figure out how much kinetic energy would be lost in ...
6
votes
1answer
395 views

Where does the energy go, when light is blocked by polarisation

I've been looking around about LCD monitors, and how they polarise light. When a pixel needs to be black, the light is "twisted" so that it can't go through the polarising sheet in front. What happens ...
16
votes
2answers
817 views

If energy is only defined up to a constant, can we really claim that ground state energy has an absolute value?

Sorry if this is really naive, but we learned in Newtonian physics that the total energy of a system is only defined up to an additive constant, since you can always add a constant to the potential ...
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9answers
2k views

How to explain independence of momentum and energy conservation in elementary terms?

I'm trying to explain to someone learning elementary physics (16 year old) that linear momentum and energy are conserved independently. I'm not a professional physicist and haven't tried to explain ...
14
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4answers
2k views

Energy conservation and interference

I have a problem with energy conservation in case of interfering waves. Imagine two harmonic waves with amplitudes $A$. They both carry energy that is proportional to $A^2$, so the total energy is ...
2
votes
2answers
742 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, ...
18
votes
7answers
1k views

If the universe is expanding, what is it expanding into?

If the universe is expanding, what is it expanding into? When the big bang happened where did it occur? When the big bang happened how did it occur? Where did the energy come from? Energy can not ...
15
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4answers
712 views

Redshifting of Light and the expansion of the universe

So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light ...
5
votes
4answers
2k views

Recovering energy from a modern, magnetic-levitated flywheel

A modern flywheel rotor is suspended in a vacuum by magnetic bearings. This means that nothing touches the rotor as it spins. When time comes that we need to recover that stored kinetic energy, how do ...
2
votes
1answer
454 views

How much energy can be extracted from hydrogen?

http://en.wikipedia.org/wiki/B-L says that the difference between baryon number and lepton number is conserved. Ordinary hydrogen has one of each, but turning it into helium releases only the binding ...
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8answers
3k views

Can a car get better mileage driving over hills?

Two towns are at the same elevation and are connected by two roads of the same length. One road is flat, the other road goes up and down some hills. Will an automobile always get the best mileage ...
2
votes
4answers
2k views

Will a CFL light bulb and an incandescent light bulb produce exactly the same amount of overall temperature increase over time?

Will a CFL light bulb and an incandescent light bulb, in separate respective closed systems, produce exactly the same amount of overall temperature increase over time? Assume you have two identical ...
3
votes
3answers
308 views

It is possible to extract energy from black holes or..satellites?

When something falls down, potential energy is transformed into kinetic one. Furthermore, you can attach a pulley and extract some energy (like in an aljibe), for example with a dynamo... If the ...
5
votes
3answers
683 views

Conservation of energy in quantum teleportation

Consider the quantum state teleportation protocol of Bennett et. al. How does one prove that this protocol would never violate the conservation of energy? At the face of it, it doesn't seem to be ...
3
votes
1answer
490 views

Does the potential energy for a given photon increase or decrease in quanta?

As a photon leaves a strong gravitational field, it loses energy and redshifts. Is the exchange in potential energy of a photon characterized by energy quanta?
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3answers
707 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, ...
14
votes
2answers
978 views

Redshifted Photon Energy

A photon emitted from a receding source (Doppler redshift) has less energy when detected at an observer's location. Please explain the energy loss from the perspective of energy conservation.
5
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2answers
624 views

Half wave plate and angular momentum

Given: A half wave plate freely floating in space. Circularly polarized light, falling perpendicularly to it. The plate changes polarisation of the beam to the opposite one. Therefore it ...
12
votes
3answers
2k views

Popular depictions of electromagnetic wave: is there an error?

Here are some depictions of electromagnetic wave, similar to the depictions in other places: Isn't there an error? It is logical to presume that the electric field should have maximum when ...
8
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6answers
1k views

Energy conservation and quantum measurement

Consider a particle in a potential well. Let’s assume it’s a simple harmonic oscillator potential and the particle is in its ground state with energy E0 = (1/2) ℏω0. We measure its ...
5
votes
2answers
2k views

Capillary action and conservation of energy [duplicate]

When I dip a paper towel in a cup of water the water gets drawn up due to capillary action. How is this reconciled with conservation of energy, as it would seem on the surface that the potential ...
19
votes
6answers
10k views

Newton's cradle

Why, when one releases 2 balls in Newton's cradle, two balls on the opposite side bounce out at approximately the same speed as the 1st pair, rather than one ball at higher speed, or 3 balls at lower ...
1
vote
1answer
20k views

Laptop self charge cable? [closed]

Ok this is a really silly question, but I was really curious as to why this wouldnt work. I'm just starting my electricity and magnetism course, so I really dont know much about electricity. I know ...
8
votes
2answers
848 views

How can neutrinos oscillate though the lepton flavors have differing masses?

Since the total mass-energy for the neutrino presumably does not change when a neutrino changes lepton flavor, though the mass is different, what compensates for the gain or loss of mass? Does the ...
6
votes
3answers
3k views

Do mirrors increase the amount of light in a room?

So if you have a light bulb in a room, and you had a tool to measure the amount of light that's in the room, then let's assume the amount of light only caused by the bulb is "1" If you place a mirror ...
20
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5answers
4k views

Total energy of the Universe

In popular science books and articles, I keep running into the claim that the total energy of the Universe is zero, "because the positive energy of matter is cancelled out by the negative energy of ...
13
votes
3answers
2k views

Energy conservation in General Relativity

I understand that energy conservation is not a rule in general relativity, but I'd like to know under what circumstances it can still be possible. In other words, when is it possible to associate a ...
9
votes
5answers
1k views

What happens to light after it enters an eye

What happens to the light [energy] after it enters an eye and hits the rods and cones? I presume the energy becomes electrical, and it must be near 100% perfect, else our eyes would heat up? Or am I ...
13
votes
2answers
2k views

Is kinetic energy a relative quantity? Will it make inconsistent equations when applying it to the conservation of energy equations?

If the velocity is a relative quantity, will it make inconsistent equations when applying it to the conservation of energy equations? For example: In the train moving at $V$ relative to ground, ...
1
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1answer
185 views

conversion of information to energy [duplicate]

Possible Duplicate: Maxwell's Demon Constant (information-energy equivalence) I was reading: Demonic device converts information to energy : Experiment inspired by a paradox tempts a ...
1
vote
1answer
217 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. ...
12
votes
4answers
4k views

Is energy really conserved?

In high school I was taught energy was conserved. Then I learned that nuclear reactions allow energy to be converted into mass. Then I also heard that apparently energy can spontaneously appear in ...
12
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4answers
8k views

Where do magnets get the energy to repel?

If I separate two magnets whose opposite poles are facing, I am adding energy. If I let go of the magnets, then presumably the energy that I added is used to move the magnets together again. However, ...