Questions tagged [energy-conservation]

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.

Filter by
Sorted by
Tagged with
6
votes
1answer
989 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 ...
19
votes
2answers
1k 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 ...
13
votes
10answers
4k views

How to explain independence of momentum and energy conservation in a 2-body collision in elementary terms?

I'm trying to explain to someone learning elementary physics (16 year old) that linear momentum and energy are conserved independently in a 2-body collision. I'm not a professional physicist and haven'...
21
votes
4answers
9k 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
941 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, ...
30
votes
5answers
3k views

If the universe is expanding, what is it expanding into? [closed]

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 be ...
29
votes
5answers
3k 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
3k 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
514 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 ...
18
votes
16answers
8k 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
3k 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 ...
4
votes
3answers
395 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
934 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
778 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?
2
votes
3answers
2k 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, ...
20
votes
3answers
2k 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.
6
votes
2answers
1k 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 receives ...
15
votes
5answers
4k 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 magnetic ...
15
votes
7answers
3k 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 ...
6
votes
2answers
6k 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 ...
34
votes
6answers
23k 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
23k 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 ...
7
votes
3answers
14k 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 ...
63
votes
6answers
16k views

Is the total energy of the universe zero?

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 ...
26
votes
3answers
8k 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 ...
12
votes
5answers
5k 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 ...
46
votes
3answers
13k views

When water climbs up a piece of paper, where is the energy coming from?

Take a glass of water and piece of toilet paper. If you keep the paper vertical, and touch the surface of the water with the tip of the paper, you can see the water being absorbed and climbing up the ...
21
votes
3answers
8k views

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

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, ...
18
votes
5answers
5k views

Hubble's law and conservation of energy

If all distances are constantly increasing, as Hubble's law say, then lots of potential energies of form ~$\frac{1}{r}$ changes, so how is the total energy of the Universe conserved with Hubble's ...
1
vote
1answer
270 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 bead ...
2
votes
1answer
644 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. ...
26
votes
4answers
11k 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 ...
23
votes
6answers
28k 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, ...