# Tag Info

8

It depends what you're doing, and indeed most of the quantum optics literature dismisses the term as it does not contribute to the dynamics. However, it is important that beginning students form an intuition for how and where zero-point energies come in, and why they are necessary. Take a look at the eigenfunctions of the harmonic oscillator, in position ...

5

It's valid in the sense that it does tell you the rest energy of a 200-pound person, but it does not tell you how much energy you could get by splitting all those atoms. As a matter of fact, most of the atoms in a human body are carbon, nitrogen, and oxygen; splitting these atoms takes energy, it doesn't produce it. Your character would need to tap into a ...

5

Think about the work-kinetic energy theorem, which states that the net work done on an object is equal to its change in kinetic energy: $$W_{net}=\Delta\mathrm{KE}.$$ You are right that when lifting an object of mass $m$ by a height $h,$ in a uniform gravitational field, the work you do is $W_{you}=mgh$ (assuming, as you said, that you're applying a force ...

4

The poster from that link is saying that the work done by the spring (that's Hooke's law there: $F=-kx$) is equal to the potential energy (PE) at maximum displacement, $A$; this PE comes from the kinetic energy (KE) and is equal to the integral of Hooke's law over the range 0 (minimum displacement) to $A$ (maximum displacement). Anyway, your professor is ...

4

You're missing a somewhat subtle point in your analysis. The block on the left in your diagram, where the spring is at its equilibrium position, is moving, so it has kinetic energy (which you're currently ignoring). I'll leave it to you to sort out what the speed needs to be and check that CoE holds. It needs to be moving because, if it were not, then there ...

3

Yes, and in fact there are several types of electrical chargers that you power by walking. Typically you'd use these for low power applications such as recharging your mobile phone as you walk. There are a lot of frankly daft designs out there, but this one is being funded by the US military, which may or may not be daft depending on your views of the ...

3

No. First of all, I'll take this version of Newton's third law changing force by energy: Third law: When one body exerts a energy on a second body, the second body simultaneously exerts an enrgy equal in magnitude and opposite in direction to that of the first body. You should see that it doesn't make sense to treat energy as a force, because it's not ...

3

The "missing" energy you're referring to actually left the block-spring system when the external force was interacting with the block. One way to think about it is the following. The work-kinetic energy theorem tells us that, since the KE of the block doesn't change during the lowering process, the net work done on the block is 0: $$W_{net}=\Delta ... 3 It depends on what you mean by the energy of light. A light wave has an associated energy flux, that is how much energy passes through a surface at right angles to the light ray. This energy flux is a constant and doesn't change when a light wave passes through materials of different refractive index. However, since the velocity of light is reduced in a ... 3 Energy is an extraordinarily elusive concept, so much so that experienced physicists will use the term only with care and in well defined circumstances. For example it might seems obvious that if I fire a pistol then the bullet has kinetic energy. But Einstein's theory of special relativity tells us that all inertial frames are equally valid, and in the ... 3 There are many problems : 1. As pointed out by Olin, gas cannot exist as a gas at 0 K. 2. In ideal gases, interaction between molecules are absent. Hence, there is no potential energy. Remember that Potential energy always has an additive arbitrary constant. 3. As pointed by Wojciech, you would need (to take}energy to cool that ... 3 These scams typically operate in a very formulaic manner: first they make large press releases for the concept, ask for funding, find funding, make press releases claiming they are a year away from releasing a gigantic version of it, cite technical difficulties when the release date passes and ask for more funding, and so on. They never ask for funding to ... 2 As far as I understand this problem, "GPE seems to exist only when I introduce the ball into the gravitational field" is a correct statement. As You said "You introduce the ball", so first You do the work that is converted to GPE and then GPE does work in accelerating the ball. I don't think there is a way in which a ball can appear somewhere within the ... 2 The Wikpedia article on solar cell efficiency gives a number of reasons that solar cells are less than 100% efficient. One of the large ones is the thermodynamic limit-a photon of less energy (longer wavelength) than the silicon band gap cannot produce an electron and one with higher energy can only produce as much voltage as the band gap. Even if you ... 2 Lenz's rule is related to the principle of conservation of energy. It states that: The induced current produced in a circuit always flows in such a direction that it opposes the change or the cause that produces it. So, induced current is going to stop the fan in your case. I hope the fan is connected to the coil which rotates in between the magnets. ... 2 The problem with that type of schemes is that, because of Newton's third law of motion, when the fan applies a force to some external system to perform work on it, the external system also applies a force on the fan in a direction that will tend to stop it. This performs exactly the same amount of work against the fan's initial energy as it is performing on ... 2 You don't cite a source for the matter ejected by quasars, but I'd guess you're thinking of the jets that are typically expelled from the poles. The answer is that if the jets have enough energy they do indeed disperse into intergalactic space, and space is indeed full of aimlessly drifting intergalactic clouds. For example see this article, or Googling will ... 2 The equation [now corrected] in the question is incorrect by a factor of 2pi, because h-bar should just be h. The equation is not limited to excited states. E is the relativistic energy from E^2 = p^2c^2 + m^2c^4, where m is the rest mass and p is momentum. While the equation was originally for photons, De Broglie extended it to all particles in his ... 1 From a quantum prespective, it is helpful to categorize the energy of molecules as: electronic vibrational (bending and stretching of bonds) rotational and translational. However, these are each inclusive of potential and kinetic energy. 1 Once we have the position 4-vector$$x^\mu= \left( \begin{array}{c} ct\\ \vec{x}\\ \end{array} \right) $$It is natural to define the momentum and energy in a fashion which is analogous to the Newtonian case (and reduces to it in the frame of the particle itself, when \vec{v}=0:$$ p^\mu \equiv m\frac{d}{d\tau} \left( \begin{array}{c} ct\\ \vec{x}\\ ...

1

All are correct. Note that that $I$ is current passing through the resistor and $V$ is potential difference across resistor. Use those equations in which you know the values of variables and not have to calculate them. Why not use all equations in a question and satisfy yourself that all are correct? These are just using ohm's law in $P=VI$. You must ...

1

The field itself carries energy. This is, in fact, a vital point because it can be shown that, if the momentum and energy carried by the fields isn't accounted for, electromagnetism would blatantly violate Newton's third law (and this doesn't have anything to do with special relativity per se).

1

The first equation is the definition of power which is, in words, the rate of energy conversion: $$P \equiv \frac{dE}{dt}$$ Where it is understood that $E$ is the amount of energy converted. For example, in a mechanical system where gravitational potential energy is converted to mechanical kinetic energy and vica versa. In electrical circuits, the power ...

1

Picking up on jinawee's answer, current is charge per unit time: $$I = \frac{Q}{t}$$ So substituting for $I$ in your second equation gives: $$P = V \frac{Q}{t}$$ But $VQ$ is just the work done, i.e. the energy, in moving a charge $Q$ through a voltage difference of $V$. So substituting $E$ for $VQ$ gives us: $$P = \frac{E}{t}$$ which shows that ...

1

Power is defined as: $$P(t)=\frac{dE(t)}{dt}$$ This is valid for any system. If energy is constant, then: $$P(t)=\frac{E(t)}{t}$$ If you're dealing with a resistance in a circuit, the dissipated power is given by Joule's law: $$P=VI$$ So the last one is a particular case of the first one.

1

The result is straight forward. As Landau and Lifshitz explain in p.41, when a body disintegrates into two pieces of masses $m_1$ and $m_2$ respectively, their momenta must be equal in magnitude and oppositely directed by the law of conservation of momentum. So, let each body have momentum $p_0$. Then, $(16.1)$ and $(16.2)$ say that the difference in the ...

1

No energy conversion process completes at 100% efficiency. Using the sun, gasoline or nuclear, none of these will generate power at 100% of total potential power available. That figure of 1KW/m^2 is for total available light energy correct? Due to how solar panels work (I don't think I have enough knowledge to speak on this part directly) solar panels are ...

1

Now my question: Does this make sense? No. Or is it just esoteric hocuspocus? Yes. I didn't watch the video but read the text. Before mentioning the mechanism they tell you about their patents to make themselves seem legitimate. (first "red flag".) They claim that their product work by harnessing Earth Energy. "Earth Energy is electromagnetic ...

1

As of last week, scientists have created a thermoelectric material that increases its efficiency to 15-20%. This is still not good enough to heat or cool your house, but it gives me hope that someday in the future, we have materials so good and cheap that we can build our houses and cars with it. So we may not be able to use the heat inside the house to cool ...

1

I am going to address the question as to why energy and information have time symmetric conservation properties whereas entropy does not. According to the Wikipedia entry on entropy - "The entropy of an isolated system never decreases, because isolated systems spontaneously evolve towards thermodynamic equilibrium, which is the state of maximum entropy." ...

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