# what is energy?

by ngjingyi
Tags: energy
 P: 19 hello i know that energy can be transferred but cannot be created or destroyed... but how does energy transfer from one to another(eg. kinetic to sound?) does it passes through an unknown medium? 2nd question, since energy cannot be created why does it exist? Thanks
 PF Patron Sci Advisor P: 1,767 To answer your first question the transfer of energy is most often effected by the action of a force. Recall the classic definition of energy as "the ability to do work". Well work is the action of a force across a distance. So for example if you hit a steel plate with a baseball the ball's kinetic energy is transferred to the steel plate by the action of a force at the "point" of contact. The work is done by the large force over the small distance the plate is deforming as it stops the ball. The plate then vibrates which pushes (applies a force) to the air around it. The molecules of air then vibrate as pressure waves (pressure=force/unit area) carrying the energy away as sound. As to your second question, this is a deep one. There is a theorem in mathematical physics called Noether's theorem (discovered by Emma Noether) which relates conserved quantities such as energy and momentum to symmetries in the laws of physics. Energy is conserved (cannote be created or destroyed) only so long as the laws of physics do not change over time. So one way of answering your question is to speculate that sometime past the laws of physics did change allowing energy to be created. This would be the "big bang" event. Another way to answer your question is to delve deeply into general relativity. First recall that Einstein's special theory of relativity (SR) equates mass and energy. We can view the mass of an object as stored up energy which can later be released (as in a nuclear reactor, or more completely in the mutual annihilation of matter and anti-matter.) Well also recall that masses produce gravitational fields. When you try to calculate the energy stored in the gravitational field itself i.e. in the curvature of space-time then you run into an interesting phenomenon. The energy in the gravitational field of a given mass is exactly equal in magnitude to the mass itself but with negative value. So the energy in a mass plus its gravity totals to zero. Now this is a bit slippery as defining energy in general relativity is ... complicated... but the crux of it is you can speculate that the total energy of the universe adds up to zero but as long as you allow negative and positive energy you can work with it. Ultimately though this is mostly philosophical speculation. When we do physics we are working within a finite domain and all the energy is merely assumed to have come from outside. What's more it is best to view energy (as well as all the other quantities in physics) as our way of quantifying how the universe behaves. The universe just does what it does and the "why" questions are best restricted "why do we describe it this way" formats. Asking "why" beyond this context risks stepping outside physics and into theology. That's ok but one should do so consciously and not think you're still talking science when you've crossed the border into metaphysics.
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P: 15,596
 Quote by jambaugh The universe just does what it does and the "why" questions are best restricted "why do we describe it this way" formats. Asking "why" beyond this context risks stepping outside physics and into theology. That's ok but one should do so consciously and not think you're still talking science when you've crossed the border into metaphysics.
Well said.

P: 19

## what is energy?

 Quote by jambaugh To answer your first question the transfer of energy is most often effected by the action of a force. Recall the classic definition of energy as "the ability to do work". Well work is the action of a force across a distance. So for example if you hit a steel plate with a baseball the ball's kinetic energy is transferred to the steel plate by the action of a force at the "point" of contact. The work is done by the large force over the small distance the plate is deforming as it stops the ball. The plate then vibrates which pushes (applies a force) to the air around it. The molecules of air then vibrate as pressure waves (pressure=force/unit area) carrying the energy away as sound.
but how do lets say kinetic energy be converted to light?
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P: 1,767
 Quote by ngjingyi but how do lets say kinetic energy be converted to light?
Well we stretch the meaning of "force" but only a little bit. Recall an electromagnetic field exerts a force on a charged particle. Likewise in a sense the charged particle exerts a "force" on the surrounding electromagnetic field. Since electromagnetic fields carry momentum and energy when a charged particle moves or more importantly accelerates it will induce an additional component to the surrounding electromagnetic field which carries away energy and momentum. Newton's 2nd law continues to hold when you take into account the interaction between charged particles and electromagnetic fields. Note this also applies to gravitational fields as well.

Light is just like radio waves it is an electromagnetic wave.

You can picture the analogy of a boat's kinetic energy being transferred to waves in the water. This analogy is not perfect but pretty good.
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P: 15,596
 Quote by jambaugh Well we stretch the meaning of "force" but only a little bit.
I don't think you even have to stretch the meaning a little bit as long as you use the original defnintion f = dp/dt instead of the "high school" definition f = ma.
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