There is a passage from Feynman's Physics, Nobel Prize in physics and one of the most respected scientists of the twentieth century, which is often quoted when it comes to energy: “It is important to realize that, in today's physics, we have no idea what what is energy “. The volumes that collect the lessons were published almost 60 years ago and since then we have discovered many other things in physics, but Feynman's phrase about energy is still true today and for a fundamental reason: among all the physical quantities, the energy continues to be the most difficult and elusive concept to define, as part of the grueling work of identifying and describing the principles and laws that make not just the world but the entire Universe work.
The events of the last few months have made energy a central and much discussed topic, not only among politicians and insiders, but also among all of us, opening up new reflections on things we take for granted, such as seeing a room lit up. dark as soon as we push a switch. The rise in the cost of gas at the end of last summer and the prospect of having less to produce energy following the Russian invasion of Ukraine led us to use terms such as consumption, power and megawatts, focusing mainly on electricity and bills we pay to have it in our homes.
Strict by these needs, we used the word “energy” a lot without having the time to try to grasp what it really is, despite being what allows us to read this article, take a run in the park or send a robot to explore a distant planet. Energy is everywhere, it permeates every aspect of our existence, yet at the same time it is an elusive concept on which philosophers and scientists have confronted and sometimes clashed for centuries, in search of the right definition.
Defining energy
The classic definition of energy, the one studied at school, says that: energy is a physical quantity that measures the ability to something to do work, without necessarily that work being actually done.
Always at school we have learned, or should have learned, that over time we have defined different forms of energy to explain particular physical phenomena and more in a more practical way. For some these distinctions may appear misleading or confusing, precisely because as Feynman said we cannot really understand energy: and the sooner we realize it, the better we will be able to master it.
In everyday experience, we have some at least practical familiarity with energy, and early humans already had it. Recovering firewood requires a certain amount of energy, just as the pieces of wood themselves produce heat and a transfer of energy when they burn. The first watchmakers came up with the idea of ingeniously exploiting the energy that was stored, for example, in the springs used to operate the mechanisms of their watches.
However, energy itself was not taken into account accurately until the end of the seventeenth century, even though previously philosophers and enthusiasts of natural phenomena had begun to make various speculations on the subject. Kepler was among the first to use “energy” to indicate a physical quantity at the beginning of the seventeenth century, but in a systematic way the concept was introduced in the scientific literature only starting from the nineteenth century. It was the scientific diatribes of previous centuries on the concepts of strength and work that gradually brought energy to the center of scientific studies and in particular of physical ones.
In his lectures, Feynman effectively summarized the approach to follow in order not to get too many headaches thinking about energy:
There is a fact, or if you prefer a law, which governs all natural phenomena known to date. There is no exception to this law as far as we know. The law is called energy conservation. It says that there is a certain amount, which we call energy, that does not change compared to the great mutations in which nature goes through. It is the most abstract idea there is, because it is a mathematical principle; it says that there is a numerical quantity that does not change when something happens. It is not the description of a mechanism, or something concrete; it's just a weird fact that we can calculate a number and when we are done observing nature doing its thing and have recalculated the number, this is the same as before.
Simplifying a lot, energy moves between objects and phenomena, but at the end of all the processes we can imagine its quantity remains unchanged. Since it can be conserved, we can imagine that there are reserves of energy and ways by which it is transferred.
Containers and pathways
Containers concern various types of energy such as mechanical (potential and kinetic), thermal, chemical and nuclear, while the pathways are electricity , sound, movement and light. In practice, an energy container can be a cup full of coffee or a barrel of oil, in general: an object.
If I lift a box and put it on a shelf, the energy moves from the chemical energy container of the arms and in general of the body to the box, which being higher than before has a greater potential energy. This is the energy that the box possesses by virtue of its position and orientation with respect to a force field, and on Earth this field is mostly governed by gravity. In the event that it falls off the shelf, the potential energy will become kinetic energy, deriving precisely from its movement (there are other variables and complexities, of course).
In other circumstances, the various steps may be less intuitive. For example, we tend not to imagine a steaming cup of tea as an energy reserve, yet the temperature of the drink derives precisely from the presence of energy. This moves from the energy container (hot tea) to the ceramic of the cup which in turn becomes a container of heat. The energy also moves into the surrounding environment, if this is at a lower temperature than that of the starting container. The effect is that a steaming cup of tea heats a cold room, albeit almost imperceptibly to those inside.
Generally speaking, energy tends to move from places where there is a lot of it to places where there is less. The classic example is precisely that of thermal energy seen a little while ago: an object at a higher temperature transfers its heat towards objects at a lower temperature, until an equilibrium is established. This phenomenon has been happening constantly since the Universe has existed, whose processes will run out just when all the energy is uniformly distributed in the cosmos: but do not worry, it will take a very long time before this happens.
Measuring energy
The unit of measurement of energy (for work and heat) is the joule (J) and derives its name from James Prescott Joule, a British businessman with a passion for science. In the first half of the nineteenth century he was known above all for being a brewer and it was from this that he thought of trying to measure energy in a practical way.
His brewery used some steam engines, powered by coal, and Joule wanted to see if he could save some money by using electric motors, powered by zinc batteries instead. It was therefore necessary to build a system that would allow him to compare the performance of one and the other type of engine. The experiment should therefore have consisted in measuring how much carbon or zinc and acid (for batteries) was needed to lift a certain weight to a certain height.
The weight was one pound (454 grams) and the height one foot (about 30 centimeters), which effectively led to a first unit of measurement for comparing energy. Joule had found a way to calculate the amount of energy needed to lift a pound of a foot vertically, eventually discovering that it was better for him to continue using coal-fired steam engines.
In addition to producing beer, Joule had other important merits in the study of energy and in particular for having found confirmation of the conservation principle for thermodynamic systems, demonstrating how heat can be considered a form of mechanical energy. He invented a device, the “Joule whirlwind”, with which to measure the mechanical equivalent of the heat produced as a result of the transfer of a known quantity of mechanical energy.
The system consisted of a water container, inside which was inserted a reel that could rotate on itself along its vertical axis. The reel was attached to a pair of weights on the outside of the container, locked in a certain position. When these were unlocked and began to descend, they made the reel rotate through a system of lines and discs (pulleys). The rotations were slowed down by the friction exerted by the water, which was heated precisely by virtue of this phenomenon. Joule had shown that mechanical energy did not “disappear”, but simply transformed into another form.
The joule, intended as a unit of measurement, is defined as the work done by applying a force of one newton to the distance of one meter (the newton is the unit of measurement of force). An example that is often used to try to visualize the joule, more difficult than units of measurement close to us such as those for mass or length, is to think that it is more or less equivalent to the work required to lift an apple for one meter. , opposing the force of earth's gravity.
The joule can be defined as the basic unit of energy and, as we have seen, it is the most important in physics, but it is not used much in everyday activities. In the last months of the energy crisis, we have mostly heard about watts and kilowatt hours.
Power and consumption
The watt (W) is the unit of measurement of power and is equivalent to 1 joule per second (or in electrical units to one volt multiplied for one ampere). Equivalence therefore tells us that power is the energy transferred in the unit of time. A machine with “many watts” is therefore able to transfer a large amount of energy in a very short time. Power can be thermal (heat transfer), mechanical (labor transfer) or electrical (electrical energy transfer).
The watt represents the energy produced or consumed instantaneously, for this reason the watt hour (Wh) is often used, which as the name suggests is used to measure the amount of energy supplied in an hour of time (for domestic consumption the kilowatt hour, i.e. the consumption of one thousand watts in one hour).
A 500 MW power plant is a 500 million watt plant, but this does not tell us how much the actual energy produced is because the plant could be used at varying rates over the course of the day, depending on demand. We know that in one hour the plant generates up to 500 MWh (megawatt hour), so to know its annual production it is sufficient to multiply that figure referred to one hour by all the hours of operation of the plant at full speed. Estimating 6 thousand hours of activity in a year, the energy produced will be 3 million MWh.
In short: the power (W) indicates the capacity of the power plant, but to understand the amount of energy produced (Wh) we need to know how long the plant is in operation. The two data are very important to understand which systems are more efficient in producing electricity and which measures to adopt to improve their yield, especially when deciding to draw on more sustainable sources.
A hypothetical 100 MW coal-fired power plant and a 100 MW wind farm do not produce the same amount of energy in a year. The coal-fired power plant can in fact operate for almost the entire year, because the raw material it uses to produce electricity is always available and in stable quantities, while the wind farm works best only when there is wind. In terms of megawatt hour, the power plant will certainly be more productive coal-fired, but it will also produce a huge amount of carbon dioxide and other pollutants, which wind does not produce. Similar speeches can be applied to other energy production plants that rely on sources that are not permanently available, such as photovoltaic parks.
W and Wh are also important on smaller scales to deal with one's household consumption. By keeping an electric heater with a power of 1,000 watts (one kilowatt) on for one hour we will consume 1000 watt hours, while if we used it for 12 minutes we would consume one fifth of the 1000 watt hours, therefore 200 watt hours equal to 0.2 kWh (kilowatt hour). If the cost of energy is 20 euro cents per kilowatt hour, we would spend 4 euro cents (20 • 0.2) to keep the heater on for 12 minutes.
For simplicity in everyday life, reference is almost always made to energy and its costs, meaning electricity and the work (in physical terms) necessary to produce and distribute it. This mainly derives from the fact that the kilowatt hour is used as a sales unit for electricity by those who produce it and then supply it to end users. The presence of the word “watt” in the two units of measurement often leads to further confusions.
And beyond
Returning to the concept in general, the definition of energy has undergone numerous evolutions especially in the twentieth century, thanks to numerous discoveries in the field of physics, from infinity large to infinitely small. Some theories have found confirmation thanks to experimentation, while others are still waiting to be demonstrated in practice.
One of the best known and most elegant equations to define energy is undoubtedly that of Albert Einstein:
E = mc2
Derived from the special theory of relativity, it shows the substantial equivalence of matter and energy. The decay of heavy metals, such as uranium, leads for example to lighter metals, with the missing mass corresponding to the release of energy in the form of radiation.
Many aspects of energy have yet to be investigated, and there are fascinating and mysterious frontiers. Dark energy, for example, is a hypothetical form of energy that cannot be directly detected and that would be scattered evenly everywhere; it is assumed that it alone accounts for 68 percent of the mass-energy of the Universe. Its existence would help explain the accelerated expansion of the Universe, but to date its characteristics and nature are unknown.
