Tagged Questions

Energy is a quantity which gives an overview of the amount of work doable by the system.

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6
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2answers
1k views

How are the calories in food calculated?

This is intended to be a fun question. Calorimetry used for calculating the heat generated from chemical changes has been around for centuries, however, I suspect the process for calculating food ...
1
vote
1answer
299 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 = ...
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?
7
votes
3answers
3k views

Why does frozen water burst a pipe?

When water freezes in a pipe it can crack the pipe open. I assume this takes quite a lot of energy as when I try to crack a pipe it can be hard work! I think water freezing is a result of energy ...
42
votes
7answers
13k views

What Is Energy? Where did it come from?

The simplistic undergrad explanation aside. I've never really understood what energy really is. I've been told that it's something when converted from one kind of something to another kind does some ...
12
votes
6answers
2k views

What is the symmetry which is responsible for conservation of mass?

According to Noether's theorem, all conservation laws originate from invariance of a system to shifts in a certain space. For example conservation of energy stems from invariance to time translation. ...
7
votes
2answers
190 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
552 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 ...
43
votes
8answers
8k views

Why does holding something up cost energy while no work is being done?

I read the definition of work as $$W ~=~ \vec{F} \cdot \vec{d}$$ $$\text{ Work = (Force) $\cdot$ (Distance)}.$$ If a book is there on the table, no work is done as no distance is covered. If I ...
22
votes
5answers
3k views

Do photons gain mass when they travel through glass?

Please correct me if I'm wrong, but I believe that photons slow down when travelling through glass. Does this mean they gain mass? Otherwise, what happens to extra kinetic energy? I understand now ...
2
votes
3answers
852 views

Does the energy of a magnetic field decrease when it moves a conductor carrying a current?

When a charged particle moves in an electric field, the field performs work on the particle. Thus, the energy of the field decreases, turning into kinetic energy of the particle. Does the magnetic ...
11
votes
6answers
11k views

How efficient is an electric heater?

How efficient is an electric heater? My guess: greater than 95%. Possibly even 99%. I say this because most energy is converted into heat; some is converted into light and kinetic energy, and ...
1
vote
1answer
260 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 ...
7
votes
3answers
1k views

Maxwell's Demon Constant (Information-Energy equivalence)

New Scientist article: Summon a 'demon' to turn information into energy The speed of light c converts between space and time and also appears in e=mc^2. Maxwell's Demon can turn information supplied ...
3
votes
2answers
625 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
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. ...
68
votes
16answers
16k views

Why does kinetic energy increase quadratically, not linearly, with speed?

As Wikipedia says: [...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $mv^2/2$. Why does this not increase linearly with speed? Why does it take so much ...