Is Energy Conserved?

Found in a School of Physics. Apparently somebody didn't do his homework...

      More often than not, we hear this phrase, 'please conserve energy' Is this true? Can energy be conserved? Let's read an article from princeton about this.

Conserve Energy!

If ever there was a vacuous admonition, this is it. As anyone who has taken a physics course can tell you, energy is always conserved. There are no exceptions. No one has ever conserved or, for that matter, wasted a single ounce of energy! But, we all have a sense that some activities are wasteful while others are not. And, it seems that what is being wasted is energy. Right? Well, not exactly.

To understand what is going on, let's consider the big picture. Where does Earth get its energy from and, if it doesn't vanish, where does it go? 

The Sun shines on us. 

Most of the energy we get, comes as light from the Sun. Most of this light is visible light (the stuff we can see). Some is higher energy (i.e., more energy per photon) ultraviolet light and some of it is lower energy infrared radiation. But, the lion's share of it is middle-of-the-road visible light. What happens to this light? Most of it bounces off us right back out into the blackness of space. But, some of it gets absorbed. What happens to that light? Does "absorption'' violate conservation of energy? No. It gets absorbed in a number of ways. Some of the light's energy goes into increasing the kinetic energy of individual atoms that get hit. The overall kinetic energy of atoms, is what we call heat. Much of this heat involves the warming of the atmosphere and eventually gets converted to wind energy that can be used to turn windmills, which convert the wind energy into electrical energy. This electrical energy is fed through wires into our electrical grid and comes to our homes and offices where we convert it back to mechanical energy (say to run a vacuum cleaner), heat (if it is winter and we are cold), light (for when it is night and we want to read), etc. 

Some of the ``absorbed'' light goes into helping make chemical reactions take place. Photosynthesis is the most important example of this. So, the light energy gets converted to chemical energy. The chemical energy allows many things to happen. A simple one is that trees grow tall---the chemical energy gets converted to gravitational potential energy, which will at some future date get converted back to mechanical energy when the tree falls or is cut down. More importantly, however, is that the plants provide chemical energy that we animals ``use''. We eat plants and our biochemistry extracts the chemical potential energy so that we can do the activities that we do. 

Some of the plants that lived millions of years ago died with much of their chemical energy intact. These plants got buried and over the millenia turned into buried oil fields containing lots of chemical potential energy. This process is very inefficient. It takes millions of years. For this reason, exploitation of oil fields for energy conservation today is considered a non-renewable exploitation. It's not that it's really non-renewable. Rather, it's that renewal would take millions of years

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Here's another source of energy. The Earth rotates.

It's difficult to put the brakes on but if we could, there is a lot of energy to play with. Actually, because of the gravitational effects of the Moon, the Earth's rotation is slowing slightly. Where does that energy go? Some of it goes into sloshing of the oceans---an effect we call tides. The rest of it goes into frictional heating of the interior of the Earth---making so-called geothermal energy. There's quite of bit of energy due to this rotation. But, it is the ultimate in non-renewable resources.

What happens after we ``use'' the energy? 

Let's consider lighting our homes. A light-bulb produces light. A tiny fraction of that light enters our eyes and becomes part of the biochemistry of seeing, thinking, living. But, most of that light simply gets absorbed by other things in the room. Some, of it could be used for photosynthesis, if you have plants in the room, but most of it just heats up the room ever so slightly. The walls, the tables, the chairs, the floor, even the air, gets slightly warmer. It takes a lot of light to produce even a small amount of heat. 

Eventually, warm things cool either by emitting an infrared photon or by coming in contact with something else that is cooler and transferring some of the heat energy to the cooler thing. At some point, the warmth gets from inside the room to outdoors and eventually the heat is lost to outer space by giving off an infrared photon that happens to escape back into space. 

So, what came in as a photon, eventually goes out as a photon. The Earth is roughly in equilibrium.

So, what makes one type of light-bulb ``efficient'' while another type is called wasteful, if in all cases energy is conserved? An incandescent light-bulb does not only produce light. It also produces heat---lots of it. If you wanted heat, then an incandescent light-bulb is good. If you wanted only light, then it is wasteful. This is the entire difference. So-called wasteful bulbs simply convert much of the electrical energy directly into heat energy by-passing the desired intermediate step of conversion to light energy. Hence, more electrical energy is needed in order to produce a given amount of light. 

It's important always to keep the big picture in mind. There is still the feeling that something is being ``used'' as we go about our daily living. If it's not energy, what is it? It is a measure of the orderliness of things. As time marches forward, the universe as a whole is becoming less ordered, sloppier if you like. In our Solar System, the Sun represents a great deal of order. It consists of an enormous quantity of hydrogen gas compressed into a rather tight ball. That is order. But, it is ``burning''. It is becoming more disordered---it's entropy is increasing. We, here on Earth, capture those photons coming at us from the Sun and we harness them to create a sense of order here on Earth---locally our entropy is decreasing but it is at the expense of an overall increase. We are taking the Sun's order and creating order here on Earth. Energy and entropy are closely related. When scientist and engineers tell us to conserve energy, they really mean conserve entropy. Roughly speaking, energy can be classified into different types. Mechanical and chemical energy are high on the energy food chain whereas infrared radiation and heat are low on the food chain. Entropy measures the slow degradation from energy of the high sort to energy of the low sort. 

So: Conserve Entropy!

Taken from princeton.edu