Energy

What It Is and Why It Matters

Energy is a magical quantity. We talk about the energy consumed by a man carrying a stick of lumber, the energy of a moving vehicle, the energy in electricity, and the energy of a gallon of gasoline. Why do we lump these disparate phenomena into one concept, and what do they have in common? And why does energy have such an impact on how we live and what we can do? Ultimately, where does it come from? I hope this essay will give you some clarity on these questions.

Notice that I began by asserting that energy is a quantity. As such, it must be measurable, and its amount expressed as a number. Work is the form of energy resulting from accelerating a mass over a distance. We say the man is doing work by accelerating the mass of the stick of lumber from rest to move it a distance. The units of work are mass x acceleration x distance.

You may recognize one measure of energy, the kilowatt-hours, abbreviated kWh, on your electricity bill. A typical American home consumes about 500 kWh of electricity per month. Other measures of energy are British thermal unit (Btu), joule (J), and calorie (cal). All these measures are defined as variants on the units of work. For example, 1 joule is the energy used to accelerate a 1-kilogram mass by 1 meter per second squared over a distance of 1 meter. One kWh is defined as 3,600,000 joules. It would be ridiculous for the April electricity bill to report your usage as 1,800,000,000 joules. A calorie of energy stored in food is defined as 4,184 joules, a convenient unit for reporting food energy.

Since every form of energy is measured in some variant of the joule, we can add amounts to find the total energy of a system. For example, if I consume a gallon of gasoline to drive to the market, the energy stored in its chemical bonds is converted to the energies of motion, heat, and electricity (to charge the battery). The sum of the quantities of these energies will equal that stored in the gallon of gasoline. How do I know? Because many scientists have carefully measured energies before and after conversion and found that the before and after totals are always equal. This fact is so useful for predicting how a system’s behavior that it has a name, Conservation of Energy. A significant consequence of this law is that one cannot create or destroy energy. The only possible action is to convert one form to another.

Energy has no physical existence. You cannot point to any physical object and say that is energy. Humans invent and believe ideas that don’t exist in the physical world. Examples are the United States, General Motors, love, justice, and a photon. For none of these examples can I exhibit a physical object. Indeed, Yuval Harari has argued in his book Sapiens that this capability distinguishes humans from all other living beings. I say energy is a magical quantity. However, unlike some magical things, we know how to measure it, the rules for working with it, and the mathematics of predicting its behavior.

So why does this magical thing matter so much to our lives? You may have noticed that in the examples above, converting one form of energy to another played a significant role. The man carrying a stick of lumber is converting the chemical energy stored in his food to motion and body heat. The energy stored in a gallon of gasoline is useless until I pour it into my gas tank and start the engine, converting it to heat, motion, and electricity. In all these examples, what matters is converting one form of energy to another suiting the purpose at hand.

There are many ways to tell the story of humans’ rise to mastery of the earth, but this essay is about energy. So I will describe its role in human history. The prime reference for this subject is Vaclav Smil’s book Energy and Civilization. Smil describes in detail how humans changed from just another animal to masters of the earth by increasing their exploitation of energy sources and conversions. Unfortunately, the book is an example of academic condescension, making it hard to read despite its illuminating ideas.

In the beginning, maybe two million years ago, the only energy available to humans came from converting the chemical energy stored in food to heat, muscle, and brain. Muscle power directed by human brains did all the work of living. By the third decade of the 21st century, humans had mastered converting energy of primary sources – such as coal, oil, and uranium – to forms required to run a complex civilization and modify the planet to suit themselves.

Significant events in the history of energy conversion include controlling fire (300,000 years ago), controlling photosynthesis (agriculture 12,000 years ago), mining coal (400 years ago), mining oil and gas (150 years ago), and developing nuclear fission (80 years ago). Each of these events increased the net energy available for human activities. For example, wood burns at too low a temperature to melt iron, but coal burns at a much higher temperature, making it possible to produce iron and steel girders on a large scale. The net energy from burning coal is typically 2 to 3 times that from burning wood. The net energy from commercial nuclear fission is 1,000 times that of wood. There exist technologies, tested but not yet exploited, for increasing the net energy from fission by a factor of 10.

Another way to understand why energy matters is to ask what are the sources of essential materials and activities. Some illustrations will clarify. The homes we live in are the most prominent and essential items of modern civilization.. They are constructed of concrete, steel, wood, and plastic produced by energy-intensive processes unavailable to muscle- driven pre-1700 society. Yes, previous societies used wood and concrete for construction, but they could not produce them in the quantity and quality now common.

The water supplied to these homes is collected from surface water or pumped from wells, usually far away. The delivery system is built with machines powered by oil, and the water purification system is operated by more machines powered by electricity and using chemicals produced by energy-intensive processes. Before these machine and chemicals were available, water was supplied either directly by muscle power or by aqueducts built with muscle power. Women carried water daily from the well or stream to the house.

Electricity became available to modern homes less than 150 years ago. This energy source is so embedded in our lives that I need not describe how essential it is. An example of where electricity comes from is a coal-fired generating station. A simplified chain of conversions from coal to electricity is:

1. digging the coal out of the ground and breaking it into chunks of convenient size,

2. transporting the chunks to the generating station,

3. burning coal converts its stored chemical energy into heat,

4. the heat converts water into steam to carry the energy,

5. the steam drives a turbine, converting its energy to rotating a shaft,

6. the rotating shaft spins a coil of wire in a magnetic field to produce electricity.

The primary energy source for a coal-fired generating station is the coal laid down millions of years ago as dead plants. Photosynthesis converted sunlight into the plant matter that geologic processes transformed and stored deep in the earth as coal. I reserve the term "primary source" for currently available sources such as wood, coal, oil, natural gas, and fissionable elements. These sources have been produced by processes over which humans have little or no control.

I hope you now know what energy is and what it does for us. But where does it come from? The physicists have discovered that one cannot create energy, only convert it from one form to another. If you think about the planet earth, how it was formed and its subsequent history, you realize that all our available energy comes either from energy stored at its formation or from the daily radiant energy from the sun. That is to say, most of our energy comes from photosynthesis of the sun’s radiant energy to make complex organic molecules and oxygen. We get from plants both the food we eat and the oxygen that supports burning. The energy stored in wood, coal, oil, and natural gas comes from plants. The energy stored in fissionable elements comes from the material collected at the earth’s formation.

Some will complain that I have not even mentioned energy harvested directly from the sun: wind and water power and photovoltaic electricity. They are omitted from this essay because my goal is to show why energy matters to human flourishing. These direct solar sources contribute trivial amounts of net energy for human activities. There are good reasons to believe they never will provide significant energy, but these properly belong to an essay on the role of energy density in human life.