THE LAW OF CONSERVATION OF ENERGY

Energy is conserved. What does this really mean, and why is it true?

In physics, the law of conservation of energy states that the total energy of an isolated system cannot
changeit is said to beconserved over time. Energy can be neither created nor destroyed, but can
change form, for instance chemical energy can beconverted to kinetic energy in the explosion of a stick
of dynamite.
A consequence of the law of conservation of energy is that a perpetual motion machine of the first kind cannot
exist. That is to say, no system without an external energy supply can deliver an unlimited amount of energy to
its surroundings.
Water in a reservoir is more or less conserved. So the amount of water can always be calculated from the
amount that was there some time ago, plus the amount that has come in, minus the amount that has gone out
(you may have to take account of evaporation as well as water drawn off).

Another way of saying the same thing is that water cant be made or destroyed. For there to be more, it has to
come in; for there to be less it has to go out.

Energy is similar. If you take any volume of space, then the total energy inside that volume at a given time is
always the amount that was there earlier, plus the total amount that has come in through the surface, minus the
total amount that has gone out through the surface.

Another way of saying the same thing is that energy cant be made or destroyed. For there to be more, it must
have come from somewhere; for there to be less it must have gone somewhere else. This also means that
energy is a calculable quantity. The practical teaching implication here is that it is important to do sums about
energy changes how much in, how much out and not just to talk generally about it.

The conservation laws, such as the conservation of energy, give physics its backbone. They are not really
statements of knowledge but they contain implicit assumptions and definitions. They are however tied to the
natural world, and they contain experimental knowledge.

The emergence of energy physics
By the early 19th century, steam engines were widely used. Both physicist and engineers sought to understand
them by developing a theory of steam engines. Through the 1840s, as part of this process, several key people
developed the concept of energy and its conservation : Mayer, Joule, Helmholtz and Thomson.

Julius Mayer, a German physicist, was the first person to state the law of the conservation of energy, in an
1842 scientific paper. Mayer experimentally determined the mechanical equivalent of heat from the heat
evolved in the compression of a gas (without appreciating that heat could be explained in terms of kinetic
theory).

In 1847 another German physicist, Hermann von Helmholtz, formulated the same principle in a book titled On
the conservation of force. By contrast with Mayer, Helmholtz did view heat as matter in motion. The idea of
conservation arose from his interest in animal (body) heat. He may not have known about Mayers prior work.

Between 1839 and 1850 the English brewer James Joule conducted a remarkable series of experiments,
seeking to unify electrical, chemical and thermal phenomena by demonstrating their inter-convertibility and their
quantitative equivalence. His numerical results and conclusion were published in the Philosophical
Transactions of the Royal Society with the title 'On the mechanical equivalent of heat'.

William Thomson (later Lord Kelvin) took the next step, considering the problem of irreversible thermal
processes, until that time simply a contradiction between Carnot and Joule. Carnot, in his 1824 theory of heat
engines, had argued that heat could be lost; more recently Joule argued that energy was convertible from one
form to another but could be destroyed. In Thomsons 1851 scientific paper 'The Dynamical Equivalent of Heat',
he contended that energy was "lost to man irrecoverably; but not lost in the material world". Thomson was thus
the first person to understand that all energy changes involve energy dissipation.

From energy to thermodynamics
In the second half of the 19th century Thomson and other scientists (including Clausius, Rankine, Maxwell and,
later, Boltzmann) continued to develop these ideas, . Kinetic theory and the science of thermodynamics
gradually became established, with energy conservation as its First law and energy dissipation as its Second
law.

Everyday Examples: Law of Conservation of
Energy
The law of conservation of energy can be seen in these everyday examples of energy
transference:
Water can produce electricity. Water falls from the sky, converting potential energy to
kinetic energy. This energy is then used to rotate the turbine of a generator to produce
electricity. In this process, the potential energy of water in a dam can be turned into kinetic
energy which can then become electric energy.
When playing pool, the cue ball is shot at a stationary 8 ball. The cue ball has energy.
When the cue ball hits the 8 ball, the energy transfers from the cue ball to the 8 ball,
sending the 8 ball into motion. The cue ball loses energy because the energy it had has
been transferred to the 8 ball, so the cue ball slows down.
Kelly ran across the room and bumped into her brother, pushing him to the floor. The kinetic
energy she possessed because of her movement was transferred to her brother, causing
him to move.
When a moving car hits a parked car and causes the parked car to move, energy is
transferred from the moving car to the parked car.
When playing the lawn game bocce ball, a small ball is thrown with the intention of hitting
larger balls and causing them to move. When a larger ball moves because it was hit by the
small ball, energy is transferred from the small ball to the larger one.
When you push a book across the table, the energy from your moving arm is transferred
from your body to the book, causing the book to move.
A cat sitting on the highest branch of a tree has what is known as potential energy. If he
falls off the branch and falls to the ground, his potential energy is now being converted into
kinetic energy.
When kicking a football that is sitting on the ground, energy is transferred from the kickers
body to the ball, setting it in motion.
Sam was rearranging furniture, and needed help to push the heavy sofa. His brother came
over, and together they were able to lift the sofa onto sliders. This made it easy to push the
sofa across the room. When Sam and his brother pushed the sofa and it slid across the
wood floor, energy was transferred from the men to the piece of furniture.
A fly ball hits a window in a house, shattering the glass. The energy from the ball was
transferred to the glass, making it shatter into pieces and fly in various directions.
Two football players collided on the field, and both went flying backwards. Energy was
transferred from each player to the other, sending them in the opposite direction from which
they had been running.
Claire threw the ball and it hit her mothers vase, knocking it over. Energy was transferred
from the moving ball to the stationary vase, causing the vase to move.
Fingers hitting piano keys transfer energy from the players hand to the keys.
Billy hit the punching bag, transferring energy from his arm to the stationary bag.
The dog ran in into the Christmas tree and knocked it over. Energy was transferred from the
moving dog to the stationary tree, causing the tree to move.
When the car hit the road sign, the sign fell over. Energy was transferred from the moving
car to the stationary sign, causing the sign to move. No energy was lost in the transfer.
These law of conservation of energy examples show how commonplace this physics concept is
in everyday life.