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| 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
.
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.
Taken from princeton.edu
