I love you, Robert K. Logan
Chapter 9
The Concept of Energy
The concept of energy to be dealt with here is another concept playing a central role in physics, which was developed partially through the efforts of the chemists, further illustrating the point that the division between physics and chemistry is arbitrary.
The concept of energy has always been associated with the idea ofits conservation. The origins of this idea most likely originated with the conservation of mass implicit in Newton’s equations of motion.
This assumption was strictly limited to mechanical reaction for it was thought that for certain chemical reactions, such as burning, mass was not conserved. This misconception was due in part to a lack of an understanding of the process of oxidation, which was thought of as a process whereby the burning object released a substance called phlogiston, a word derived from ancient Greek, which meant “burning up” and in turn was derived from the ancient Greek word phlox, which meant fire. The theory first postulated by Johann Joachim Becher in 1667 postulated that phlogiston had a negative weight to account for the fact that the products of combustion were heavier than the original substance, which burned. Lavoisier on the other hand, correctly believed that burning was due to the oxidation of the burning substance and that the increase in weight was due to the weight of the oxygen that combined with the burning substance. He proved this by carrying out oxidation in a completely closed system and showed that the total amount of mass before and after combustion was the same. He therefore postulated the conservation of matter held for all reactions including both chemical and mechanical ones.
Lavoisier’s conservation of mass did not, however, lead directly to the conservation of energy but provided a model for it. It also provided a model for the erroneous concept that heat is a conserved quantity. Heat was considered to be weightless fluid called caloric. The transfer of heat from a warm body to a cool body involved the flow of the fluid caloric, which was conserved. The generation of heat as a result of friction would appear to contradict the idea that heat was a conserved fluid. The generation of this heat was explained however, as arising fromcaloric being squeezed out of the body by the action of the friction. This would mean that only a fixed amount of heat could be generated from any given body due to friction for once all of the caloric had been squeezed out of the body, it would have no more heat to give. Count Rumford (nee Benjamin Thompson) in 1790 or thereabouts, while working in a cannon factory observed a direct contradiction of this idea.
He noticed that an inordinately large, almost unending, amount of heat was generated in boring the hole necessary to convert a large metal rod into a cannon.
Further study quickly revealed that he could generate as much heat through friction as he wanted, as long as he provided the necessary work to generate the friction. You can perform this experiment yourselves by rubbing your hands together. The only limitation on the amount of heat you create will be the physical exhaustion you experience from rubbing your hands together. Count Rumford discovered that the amount of heat is not conserved and that the creation of heat requires work.
Heat is also generated when a moving object suddenly comes to rest.For example, if a ball falls to Earth, the temperature of the ground andthe ball increase immediately after the ball strikes the ground and comes to rest. In both the example of friction and the falling ball motion is converted into heat. An understanding of these processes involves the realization that the heat of an object is nothing more than the internal motion of the atoms of which it is composed. Motion is converted into heat simply because the external motion of the object is converted into the internal motion of its atoms. Work is converted into heat by first creating motion, which in turn is changed into heat. The connection between work and motion is quite obvious. It takes work to create motion. A horse must work to pull a cart. It takes work to lift an object to a certain height in order to give it motion by dropping it.
The above discussion illustrates the equivalence of heat, motion and work. It is the concept of energy that ties together these three quantities, for it requires energy to perform work, to create motion or to generate heat. The term energy comes from ancient Greek word energos, which meant “active working”.
The conservation of energy is nothing more than the statement that heat, motion and work are equivalent and that for a given amount of work one gets the same amount of motion or the same amount of heat, or that for a given amount of motion one gets the same amount of heat and so on and so forth.
It requires energy in the form of work to give motion to a body, which is initially at rest. The energy acquired by a body in motion is referred to as its kinetic energy. Since all objects are composites of smaller particles called atoms, which are also in motion, the motion of any object can be separated into its external motion and its internal motion. The term, the kinetic energy of a body, usually refers to the energy due to its external motion. The energy of its internal motion, on the other hand, is by definition heat. The amount of heat is exactly equal to the sum of the kinetic energy of each atom’s internal motion. The amount of energy required to move an object depends on its mass and the final velocity of its motion. The greater either the mass or the velocity, the greater the energy. Since energy was defined as a conserved quantity, the kinetic energy of a body was defined equal to one half its mass times its velocity squared (E = 1/2 mv2) to insure that energy remains a conservedquantity. Kinetic energy is not necessarily conserved as was illustrated by bodies that slow down as a result of friction or the ball that came to rest as a result of striking the Earth. In each of these examples the kinetic energy is converted into other forms of energy such as heat in the case of friction or in the case of a ball striking the Earth kinetic energy is converted into heat, sound and a deformation of the ground.
As mentioned in our previous discussion of mechanics, a force is necessary in order to change the velocity a body. In order to exert a force however, energy must be provided in the form of work. The amount of work done as a result of exerting a force is equal to the distance through which the force acts, times the magnitude of the force in the direction through which it acts. If a force acts perpendicular to the motion of a body no work is necessary since this action will only change the direction of the body and hence the kinetic energy will remain fixed. If the body is pushed along its direction of motion, however, the speed of the body will increase and hence the work being expended in pushing it will be converted into the increase in the body’s kinetic energy.
Work can also be used to change the position of a body in a force field by overcoming the force such as lifting a body from the ground to some given height. The energy or the work done on the body is stored in the form of potential energy by virtue of its position. Once the body begins to fall this potential energy is converted into kinetic energy.
Summing up our discussion of work, we see that work can generate the different forms of energy we have so far encountered namely heat, motion (or kinetic energy) and potential energy, and that these different forms of energy are interchangeable.
In addition to the forms of energy that we have so far discussed, there are other forms, which also deserve mention. For example, there is chemical energy, the energy that is released by chemical reactions. It is the source of energy that generates the heat within our bodies and the
work and motion we are able to generate with our bodies. There is also light energy, the energy of electromagnetic radiation, which is the form of energy by which the Sun transmits its energy to Earth. Sound is another form of energy, which in fact is the kinetic energy of the air molecules, the oscillations of which create sound.
There is also nuclear energy, the energy generated by the nuclear processes of fission, the splitting of a heavy nucleus of an atom into smaller nuclei and neutrons, and nuclear fusion, where smaller nuclei combine to form a larger nucleus. In both fission and fusion mass is destroyed and converted in energy according to Einstein’s famous formula E = mc2. This energy is popularly referred to as atomic energy, when in fact it is nuclear energy. It refers to the energy released both peacefully by atomic reactors and violently by A-bombs and H-bombs. It is also the source of the tremendous amount of energy generated by the Sun. Energy takes many forms. The processes of nature may be considered as the conversion of energy from one form to another. The Sun is the original source for the different forms of energy we find on Earth with the exception of geothermal energy due to radioactivity. The energy we derive from our food whether animal or vegetable originates with the vegetation of the Earth. The energy stored in plants is a result of the process of photosynthesis whereby plants transform the energy of sunlight into chemical energy and store that energy in the form of hydrocarbons. The energy generated in the Sun by nuclear fusion is converted into heat or internal motion of atoms of the Sun. The internal motion of the atoms causes them to radiate light, which propagates to the Earth and through photosynthesis is converted into chemical energy or food. This chemical energy or food is then converted through oxidation into body heat and motion. The energy of our bodies, therefore, had its genesis in the Sun and has assumed many different forms in order to be transferred to us. It began as nuclear energy and then through successive processes became heat energy, light energy, chemical energy and finally the heat energy and kinetic energy of our bodies.
Global Warming and Greenhouse Gases
The sunlight from Sun warms the Earth constantly heating the atmosphere and the surface of the Earth. As the atmosphere and the surface of the Earth heat up a certain amount of that heat is radiated back into outer space otherwise our planet would overheat to the point that life on our planet would not be sustainable. There has been a delicate balance in nature in which the amount of incoming energy and outgoing energy are approximately equal which keeps the temperature on Earth in the moderate range. Over the course of time there have been moderate fluctuations in the amount of energy coming from the Sun, due to volcanic activity, fluctuations in the Sun’s output and fluctuations in the Earth’s orbit about the Sun known as Milankovich cycles. These fluctuations have resulted in Ice Ages, the last one of which ended 10,000 years ago. Another factor affecting the warming and cooling of the planet is fluctuations in albedo, the reflection of sunlight back into outer space due to ice and snow cover. One of the factors affecting the amount of heat radiated out from the Earth into outer space is the amount of carbon dioxide, CO2, in the atmosphere, which is approximately 0.0383% of all the gases in the atmosphere. CO2, however, absorbs a significant percentage of the heat radiated off the surface of the Earth and therefore contributes to the warming of the planet.
The balance of CO2 in the atmosphere before the massive intervention of industrial age human activity was maintained by plants absorbing or breathing in CO2 and exhaling oxygen. The replenishing of CO2 into the atmosphere comes form the respiration of animals breathing in oxygen and exhaling CO2. There is also a certain amount of CO2 that enters the atmosphere due to forest and brush fires caused by lightning and certain amounts of emissions due to volcanic activity. That balance has been seriously eroded by our burning of fossil fuels, which is dumping large quantities of CO2 into the atmosphere. As the amount of CO2 has increased a greenhouse effect has taken place where more energy in the form of sunlight has entered the Earth’s atmosphere than is now exiting.
There is an analogy with a glass greenhouse because glass is transparent to visible light but is opaque or blocks the lower frequency infrared 88 The Poetry of Physics and The Physics of Poetry radiation. This is why a greenhouse is many degrees warmer than the outside air temperature even in the winter.
Great work condensing all energy into two forms everyone understands (potential and kinetic) and then using relativity to show that they're one and the same. I know people who take college-level physics struggling to make sense of the concept of energy. I find it funny that it took me so long to figure out what you've explained in a 4 minute video. Hope this gets more views!
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https://www.britannica.com/science/mechanical-energy
Key
forms of energy
types of energy
forms of energy
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