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Q & A: Energy neither created nor destroyed

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Most recent answer: 09/04/2016
Q:
energy neither be created nor distroyed explain
- naveeen (age 18)
st.antonys high school, hubli,karnataka,india
A:
Hi Naveeen,

This is one of the most important rules that scientists have found which describes natural phenomena. Unfortunately there is no non-circular proof of energy conservation -- in the end, all laws of physics that we know of are the result of observation, formation of hypotheses, making predictions, and testing them. Conservation of energy is one such law. If energy could be created or destroyed, all of our ideas of how the world works would have to be modified in some way (and weíd learn something very perplexing). But so far, energy seems not to be created or destroyed.

Energy can be converted from one form to another, though. Mechanical energy, such as the kinetic energy of motion, can be converted to heat energy, for example in the heating of a carís brakes when it slows down. Chemical energy in the gasoline of the car can be converted into both heat energy in the exhaust and heating the engine, and into mechanical energy to move the car. Potential energy, such as the gravitational potential energy stored in an object which is on a high shelf, can be converted into kinetic energy as the object falls down. Electrical energy can be converted to heat or mechanical energy or sound energy in a variety of useful ways around the house using common appliances.

It is often the conversion of one form of energy to another which is the most important application of this rule. Often predictions of the behavior of physical systems are very much more easily made when using the idea that the total amount of energy remains constant. And careful measurements of different kinds of energy before and after a transformation always show that the total always adds up to the same amount.

Historically, of course not all the forms of energy were known to begin with. Scientists had to keep inventing more forms to keep the law of energy conservation true. If that process had gotten too messy or complicated to make sense, we would have had to give up the law.

One very interesting feature of energy is that other forms can be converted into rest mass and back again (particle physicists do this every day in their accelerators). Einsteinís E=mc^2 gives the relationship between the rest mass of a particle (measured in standard mass units) and the amount of energy that corresponds to (measured in standard energy units). It even applies to other systems where particles are neither created nor destroyed. If a box contains some air at a temperature, and then is warmed up, it will become ever so slightly more massive because of the extra energy given to it. You can call that rest mass of the whole box or the mass equivalent of the kinetic energy of the particles in it- nature doesnít care what names you give it.

Tom J. (w Mike W.)

(published on 10/22/2006)

Follow-Up #1: mass-energy

Q:
Wouldnít antimatter and natural matter annihilation be creating energy from mass?
- Jay Berkovitz (age 22)
Gushikawa, Okinawa, Japan
A:
That depends on how you use the words. The thing which is conserved is the total energy including all the rest-energies of the particles. Itís that total "E" which is related to the "m" which is the source of gravity by E=mc2. So one way of viewing that annihilation is that it is converting the rest-mass form of energy into other forms. Thatís the way we think when we say that the law of conservation of energy works.

Mike W.

(published on 10/22/2007)

Follow-Up #2: Loss of our energy while working

Q:
i wanna ask that when we work we losse our energy how?where does it go?
- bhawna (age 17)
U.P
A:
When we do physical work, for example lifting up a bucket of water,  we utilize the chemical energy in our body. This energy is supplied by the food we eat and is used up in a variety of ways: transfer of potential energy to the bucket of water,  kinetic energy of a thrown ball, etc.
A large fraction of the food energy is used up in heating of our bodies.   Nevertheless, adding it all up....    Energy in = Energy out.     Total energy is conserved.

LeeH

(published on 10/22/2007)

Follow-Up #3: in the beginning

Q:
I would like to ask what your theory is on the beginning of energy. Where did it come from? There is the theory of God, but are there any other legitimate theories?
- Emily (age 13)
Southern Georgia, United States
A:
Perhaps your question is really about the beginning of there being anything at all. The beginning of " energy" itself may not be quite the issue, because at least in some pictures of the universe (simple closed geometry) the total energy is and always was zero.

Our standard pictures can trace things back from now to a tiny fraction of a second after some dramatic 'origin' event. That may be the sort of inflationary Big Bang most often discussed or a collision between two 3-D 'branes' in a higher dimensional space. The consequences of those pictures for our current world would be only a bit different, so sorting out which picture is better isn't done yet.
But you're really asking about what got things started 'before' that. We don't know if the idea of 'before' makes sense here, because in the mathematics describing this phase, there may be nothing like time which can be smoothly traced all the way through. Still, we can take the question as being "how does the universe fit into some bigger mathematical picture that all hangs together?" In other words, is there some big picture of spacetime in which the 'beginning' of our universe obeys the most basic laws, instead of just coming from nowhere?
We don't know the answer to that yet, because we don't yet know the most basic laws. String theorists and others are trying to work on those laws, but until they make more progress we won't be able to say much about the very beginning of how our universe got started. Youíre young though, so you may well live to see real progress on these questions.

Mike W.

(published on 12/18/2007)

Follow-Up #4: light energy

Q:
If energy cannot be created nor destroyed then where does the energy "physically" go after it is used? Example: I turn on a lamp. The light bulb in the lamp uses electricity (energy) to light a room. In order to keep the light shining I must ensure the on/off switch is "on" in order for the energy to be drawn from the electric socket into the light bulb. So - where does the energy "physically" go after it has been drawn from the socket and after it has lighted the bulb? Does it go into the air? How is the energy physically "recovered" after it has lighted the lamp if energy cannot be destroyed?
- Janice
Los Angeles CA, USA
A:
The light energy gets converted to thermal forms- mainly jiggling of molecules- when it is absorbed by walls, rugs, etc. Of course you can sense that directly by how your hand warms up when light shines on it. Also, the bulb converts a lot of the electrical energy directly to thermal forms, so the bulb and the surrounding air heat up. The big advantage of compact fluorescent bulbs is that they send a much higher fraction of the input energy out as light than do incandescent bulbs.

Mike W.

(published on 09/17/2008)

Follow-Up #5: how does a room cool?

Q:
Ok, but then where does the heat energy go after the lamp is turned off and the room cools?
- Neil (age 38)
China
A:
There are lots of ways that heat can leak out of a room. Maybe you have the windows open and the warmer air just blows out. Maybe it's sealed up, and the heat leaks out by conduction, the gradual transmission of energy via thermal jiggles from one part of a material to the next.

Mike W.

(published on 09/11/2012)

Follow-Up #6: where does energy go?

Q:
It seems as if kinetic, sound, and potential energy all terminate. When pottery falls off of a shelf, there is no more potential energy, and after time and space sound is no longer detectable (right)? With kinetic energy, you throw a ball, and the energy of that system may be enough to change the shape of soil (causing an indent), but when the ball lands and physical changes are complete, the energy is destroyed, right? Where does it go when the "work" is complete in these examples?
- Carl (age 26)
Seward, AK, US
A:
In all those cases, it goes into the thermal jiggling of atoms, molecules and electrons. Then it gets radiated as mainly infrared light out into space.

Mike W.



(published on 02/21/2013)

Follow-Up #7: source of car energy

Q:
I understand that energy can't be created or destroyed but with a car, how does it just start? Where is the energy needed to power a car when it is stationary? Also what do you mean by the jiggling of atoms? Thanks
- William Duncan (age 17)
Adelaide, South Australia
A:

In a typical car, the initial starting energy is stored in a battery. The chemicals in the charged-up battery are not in their lowest free-energy state. Then once the motor starts, the energy comes from combining the fuel with oxygen, which also lowers the chemical free-energy.

By atoms jiggling we just mean that they move around, with some kinetic energy. They also squash into each other as they jiggle, and that raises their potential energy. Just picture a bunch of little balls connected by springs, rattling around.

Mike W.


(published on 11/19/2013)

Follow-Up #8: energy conservation in the universe

Q:
Does the conservation of energy apply to the entire universe? That would mean that if the universe is expanding its energy density must be decreasing?
- Andy Findlay (age 52)
Switzerland
A:

We've addressed the question of cosmological energy conservation in the expanding universe in other threads, e.g.: The quick summary is that it's over our heads.

At a more local level, however, it is quite true that as the universe expands the energy density goes down. It's very similar to the adiabatic expansion of a gas. The densities of massive particles and of photons go down. The wavelengths of the photons stretch out, so there's less energy per photon. The stretching of the photons means that there's not only a lower energy density, but also lower absolute energy within the region that some collection of photons occupy. That all looks quite a bit like the classical expansion. One difference is that classically the lost energy goes into pushing on the walls, doing work on things outside. Cosmologically, there aren't any walls, so this is trickier.  Another issue is that there is some dark energy, or an effect that acts like dark energy, for which the energy density doesn't change as the space expands. This dark energy is not currently well understood.

Mike W.


(published on 01/05/2016)

Follow-Up #9: relativistic energy

Q:
Hi Mike,In relativistic energy, an object's energy increases if it starts moving. Because of the conservation of energy, the question is, where does the extra energy come from? Is the total energy conserved? That seems not to be right, because relativistic energy alters the total energy, doesn't it? Thank you.
- David Martin (age 44)
Arizona
A:

The relativistic case really isn't much different from the non-relativistic description here. The total energy is conserved, in any particular frame. The kinetic energy may come from reduction of some potential energy, e.g. a weight falling, just as in the classical case. It gets trickier for cosmological general relativistic problems, but special relativistic treatments aren't tricky.

Mike W..


(published on 08/28/2016)

Follow-Up #10: gravitational potential energy

Q:
Thanks Mike. So out in space without gravity, in a given frame the energy that moved an object is the same as the energy it gains from moving.But an object in free fall in a gravity field steadily gains speed, so it gains energy. Where does that extra energy come from? What loses energy, to allow the object to gain energy? Thank you.
- David Martin (age 44)
Arizona
A:

The gravitational field loses energy. As two masses approach, their fields get closer to adding coherently, making the integral of the square of the field get bigger. There's a negative energy associated with that field, and thus it gets more negative. It's very similar to the famouus E2+B2 term in the electromagnetic energy density, except that it has the opposite sign. (This whole answer is in the flat-spacetime approximation, which is all I can handle for this purpose.)

Mike W.


(published on 09/02/2016)

Follow-Up #11: loss of field energy

Q:
Thanks. So does the field lose energy because the freefalling object's position in the field changes, which would be like a relative loss of energy, or does it lose energy from its total, overall energy somehow?
- David Martin (age 44)
Arizona
A:

The field consists of a part from other objects, plus the part from the one we're saying "falls". These just add together to make the total field. For simplicity, let's say the other objects are very massive and at rest with respect to one another, so we can ignore their motions. Falling causes the total field pattern to change, since it involves motion of one object with respect to the others. That's why the total field energy changes.

Mike W.


(published on 09/04/2016)

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