Energy 101

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New Mexico Solar Energy Association 

Energy Concepts Primer

created by Ben Luce 

Overview

This primer is intended to fill in important background information about basic energy physics in a conceptually deep but mathematically simple way.

 If you’re short on time, try reading only highlighted portions of text. 

Table of Contents

 

What is energy?  

What are the different forms of energy?

What are the properties of energy?

How was energy defined and  discovered in the first place? 

How is energy transported and transferred? 

How is energy converted?

How is electrical energy measured, and how well does the Sun provide? (watts, kilo-watt hours, volts, and amps)

How is thermal (heat) energy measured, and how well does the Sun Provide? (BTUs and therms)

What is entropy? (as opposed to energy)

Photosynthesis

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What is energy?

This section covers the concept of energy itself, what it actually is. In the next sections, we’ll discuss its various forms, its properties, how it’s transferred, how we obtain it, and how we use it.

Most of us have an intuitive concept of energy that goes something like this:

Energy is the stuff we need to accomplish physical actions such as walking, lifting a glass, heating some water, or powering a television set.

Although this definition is correct, its a bit indirect because it really only conveys to us what energy is used for, not what energy is, or even how it behaves (for example, what happens to it after you use it?). A curious person might still ask questions like: Is energy a thing? Or is it a property or a condition of a thing? How do we really define it? How was it discovered? What are its properties? These are some of the questions we will try to answer in this and following sections, as completely, briefly, and simply as possible.

With perhaps the exception of energy in the form of light, energy is not a thing per se. Rather, energy refers to a condition or state of a thing.

As we will discuss in more depth later, a book sitting on a table, for example, possesses energy (“potential energy”) because of its condition of being able to fall if nudged off the table. A ball flying through the air has energy (“kinetic energy”) because of its relative velocity with respect to the ground, and it also possesses potential energy because of its height above the ground.

But people speak of energy as if its a thing. Moreover, we all know that energy can be stored, bought and sold, and transported. The reason that energy has all these aspects is, unlike many “conditions” that objects may be subject to,energy is conserved; the condition of having energy is always passed from one object to another, never created anew or destroyed. In this way, energy is pretty unique among conditions.

A good example of how energy is passed along from object to object is a water wave. A water wave gives the impression that there is an object moving across the water because the shape of the water doesn’t change very much. But there really is not an object moving – rather, the movement itself of the water molecules is passed from each collection of water molecules to the next through the forces between the water molecules. 

Similarly, people are familiar with heat flowing from one object to another. For a long time, because molecules are far too small to see, people thought that heat might be a kind of fluid-like substance, which some called “caloric fluid” that flowed from one thing to another. Nowadays, we know that heat energy is the microscopic motion of molecules, and that this state of motion, not the molecules themselves, is what “flows” from hot objects to cold objects. 

The Scientific Concept of Energy

To understand the concept of energy a little more deeply, one needs to first understand the concept of “work” as defined by the branch of science called physics. 

Suppose you push something, say, your couch, across your living room floor. Then the measure of the “work” you do, as defined by the branch of science called physics, is equal to the force you pushed with, multiplied by the distance over which you did the pushing:

Work = Force x Distance.

Suppose you just push on the couch without moving it. Are you doing any work on the couch in this case? No! Although you may feel like you’re doing work (you may get tired), you’re not, because you haven’t exerted the force through a distance(that is, the distance in this example is simply zero). 

Notice that because work is defined as the multiplicative product of force and distance, knowing just the amount of work doesn’t tell you whether you pushed with a little force over a long distance, or a lot of force over a short distance —  you can accomplish the same amount of work either way. 

Now we can give our first scientific definition of energy:

The energy of an object, or of a system, is how much work the object or system can do on some other object or system.

In other words, energy measures the capability of an object or system to do work on anothersystem or object.

Consider a ball flying through the air for example. If the ball collides with another ball, the ball will exert a force on the second ball for a moment, which does work on the second ball and causes it to move. The newly acquired kinetic energy of the second ball after the collision is equal to the amount of work exerted on it by the first ball.

In the example above of pushing a couch, you’re able to do work on the couch because your body has a certain amount ofchemical energy in your body from the food you eat. This chemical energy is released to generate force via your muscles, which you then direct to push the couch across floor. The change in your bodies stored chemical energy is exactly equal to the work you do on the couch, plus any heat energy generated in your body while you do the work.

There are a number of ways in which a system or object can possess energy, i.e. the capability to do work, and each way corresponds to having a different form of energy. The following sections will describe these different forms in more detail. But keep in mind that no matter what the form, energy always means the capability to do work, that is, exert a force through a distance on some object. Sometimes the path to extracting this work from an energy source is difficult and complicated, and compromised by practical considerations involving entropy (discussed in a later section), yet extracting work is always possible in principle. 

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What are the different forms of energy?

 

Energy has a number of different forms, all of which measure the ability of an object or system to do work on another object or system. 

In other words, there are different ways that an object or a system can possess energy.

 

Here are the different basic forms:

 

Kinetic Energy:

Consider a baseball flying through the air. The ball is said to have “kinetic energy” by virtue of the fact that its in motion relative to the ground. You can see that it is has energy because it can do “work” on an object on the ground if it collides with it (either by pushing on it and/or damaging it during the collision).

The formula for Kinetic energy, and for some of the other forms of energy described in this section will, is given in a later section of this primer.

 

Potential Energy:

Consider a book sitting on a table. The book is said to have “potential energy” because if it is nudged off, gravity will accelerate the book, giving the book kinetic energy. Because the Earth’s gravity is necessary to create this kinetic energy, and because this gravity depends on the Earth being present, we say that the “Earth-book system” is what really possesses this potential energy, and that this energy is converted into kinetic energy as the book falls.

 

Thermal, or heat energy:

Consider a hot cup of coffee. The coffee is said to possess “thermal energy”, or “heat energy” which is really the collective, microscopic, kinetic and potential energy of the molecules in the coffee (the molecules have kinetic energy because they are moving and vibrating, and they have potential energy due their mutual attraction for one another – much the same way that the book and the Earth have potential energy because they attract each other). Temperature is really a measure of how much thermal energy something has. The higher the temperature, the faster the molecules are moving around and/or vibrating, i.e. the more kinetic and potential energy the molecules have.

 

Chemical Energy:

Consider the ability of your body to do work. The glucose (blood sugar) in your body is said to have “chemical energy” because the glucose releases energy when chemically reacted (combusted) with oxygen. Your muscles use this energy to generate mechanical force and also heat. Chemical energy is really a form of microscopic potential energy, which exists because of the electric and magnetic forces of attraction exerted between the different parts of each molecule – the same attractive forces involved in thermal vibrations. These parts get rearranged in chemical reactions, releasing or adding to this potential energy.

 

Electrical Energy

All matter is made up of atoms, and atoms are made up of smaller particles, called protons (which have positive charge), neutrons (which have neutral charge), and electrons (which are negatively charged). Electrons orbit around the center, or nucleus, of atoms, just like the moon orbits the earth. The nucleus is made up of neutrons and protons.

Some material, particularly metals, have certain electrons that are only loosely attached to their atoms. They can easily be made to move from one atom to another if an electric field is applied to them. When those electrons move among the atoms of matter, a current of electricity is created.

This is what happens in a piece of wire when an electric field, or voltage, is applied. The electrons pass from atom to atom, pushed by the electric field and by each other (they repel each other because like charges repel), thus creating the electrical current. The measure of how well something conducts electricity is called its conductivity, and the reciprocal of conductivity is called the resistance.  Copper is used for many wires because it has a lower resistance than many other metals and is easy to use and obtain. Most of the wires in your house are made of copper. Some older homes still use aluminum wiring.

The energy is really transferred by the chain of repulsive interactions between the electrons down the wire – not by the transfer of electrons per se. This is just like the way that water molecules can push on each other and transmit pressure (or force) through a pipe carrying water. At points where a strong resistance is encountered, its harder for the electrons to flow – this creates a “back pressure” in a sense back to the source. This back pressure is what really transmits the energy from whatever is pushing the electrons through the wire. Of course, this applied “pressure” is the “voltage”. 

As the electrons move through a “resistor” in the circuit, they interact with the atoms in the resistor very strongly, causing the resistor to heat up – hence delivering energy in the form of heat. Or, if the electrons are moving instead through the wound coils of a motor, they instead create a magnetic field, which interacts with other magnets in the motor, and hence turns the motor. In this case the “back pressure” on the electrons, which is necessary for there to be a transfer of energy from the applied voltage to the motor’s shaft, is created by the magnetic fields of the other magnets (back) acting on the electrons – a perfect push-pull arrangement!

 

Electrochemical Energy:

Consider the energy stored in a battery. Like the example above involving blood sugar, the battery also stores energy in a chemical way. But electricity is also involved, so we say that the battery stores energy “electro-chemically”.  Another electron chemical device is a “fuel-cell“. 

Electromagnetic Energy (light):

Consider the energy transmitted to the Earth from the Sun by light (or by any source of light). Light, which is also called “electro-magnetic radiation”. Why the fancy term? Because light really can be thought of as oscillating, coupled electric and magnetic fields that travel freely through space (without there having to be charged particles of some kind around). 

It turns out that light may also be thought of as little packets of energy called photons (that is, as particles, instead of waves). The word “photon” derives from the word “photo”, which means “light”.  Photons are created when electrons jump to lower energy levels in atoms, and absorbed when electrons jump to higher levels. Photons are also created when a charged particle, such as an electron or proton, is accelerated, as for example happens in a radio transmitter antenna.

But because light can also be described as waves, in addition to being a packet of energy, each photon also has a specific frequency and wavelength associated with it, which depends on how much energy the photon has (because of this weird duality – waves and particles at the same time – people sometimes call particles like photons “wavicles”). The lower the energy, the longer the wavelength and lower the frequency, and vice versa. The reason that sunlight can hurt your skin or your eyes is because it contains “ultraviolet light”, which consists of high energy photons. These photons have short wavelength and high frequency, and pack enough  energy in each photon to cause physical damage to your skin if they get past the outer layer of skin or the lens in your eye. Radio waves, and the radiant heat you feel at a distance from a campfire, for example, are also forms of electro-magnetic radiation, or light, except that they consist of low energy photons (long wavelength and high frequencies – in the infrared band and lower) that your eyes can’t perceive. This was a great discovery of the nineteenth century – that radio waves, x-rays, and gamma-rays, are just forms of light, and that light is electro-magnetic waves

Sound Energy:

 Sound waves are compression waves associated with the potential and kinetic energy of air molecules. When an object moves quickly, for example the head of drum, it compresses the air nearby, giving that air potential energy. That air then expands, transforming the potential energy into kinetic energy (moving air). The moving air then pushes on and compresses other air, and so on down the chain. A nice way to think of sound waves is as “shimmering air”.

Nuclear Energy:

The Sun, nuclear reactors, and the interior of the Earth, all  have “nuclear reactions” as the source of their energy, that is, reactions that involve changes in the structure of the nuclei of atoms. In the Sun, hydrogen nuclei fuse (combine) together to make helium nuclei, in a process called fusion, which releases energy. In a nuclear reactor, or in the interior of the Earth, Uranium nuclei (and certain other heavy elements in the Earth’s interior) split apart, in a process called fission. If this didn’t happen, the Earth’s interior would have long gone cold! The energy released by fission and fusion is not just a product of the potential energy released by rearranging the nuclei. In fact, in both cases, fusion or fission, some of the matter making up the nuclei is actually converted into energy. How can this be? The answer is that matter itself is a form of energy! This concept involves one of the most famous formula’s in physics, the formula,

E=mc².

This formula was discovered by Einstein as part of his “Theory of Special Relativity”. In simple words, this formula means:

The energy intrinsically stored in a piece of matter at rest equals its mass times the speed of light squared.

When we plug numbers in this equation, we find that there is actually an incredibly huge amount of energy stored in even little pieces of matter (the speed of light squared is a very very large number!). For example, it would cost more than a million dollars to buy the energy stored intrinsically stored in a single penny at our current (relatively cheap!) electricity rates. To get some feeling for how much energy is really there, consider that nuclear weapons only release a small fraction of the “intrinsic” energy of their components.

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What are the properties of energy?

So far, we have learned that energy is a measure of the capability of an object or system to do work, and we have also learned about the basic different forms of energy. 

But these concepts still don’t quite do justice to the full concept of energy, for energy has a number of very special additional properties we have not fully discussed yet. If you think about these carefully, and don’t take them for granted, you’ll realize that they don’t follow from simple intuition. Rather, these properties had to be discovered or proven somehow. We’ll explore briefly how these properties were proven in the next section. In this section, we’ll first review them:

These properties are;

1. Energy can be transferred from one object or system to another through the interaction of forces between the objects (unlike the condition of, say,  being the color red, which is intrinsic to the object in question).
2. Energy comes in multiple forms: kinetic, potential, thermal (heat), chemical, electromagnetic, and nuclear energy. (as discussed in the previous section).
3. In principle, energy can be converted from any one of these forms into any other, and vice versa, limited in practice only by the Second Law of Thermodynamics (we discuss the Second Law, that is “entropy”, in a later section).
4. Energy is always conserved, that is, it is never created anew or destroyed – this is called the First Law of Thermodynamics.     Thus, when an object does work on another object, the energy can only be converted and/or transferred, but never lost or generated anew. In a sense, energy is like perfect money – transferred but always preserved, assuming no inflation or deflation!

Although most people are aware of these facts nowadays and take them for granted, these are really amazing properties if you stop and think about them.  How was anyone ever able to prove such properties? These properties go far beyond the intuitive concept of energy given at the beginning of this primer. You may find this hard to see now, because we generally take these ideas for granted. But for thousands of years, people didn’t have a clearly defined concept of energy, and didn’t know, for example, that there is a definition of “energy” which refers to a quantity that is always conserved. 

Moreover, even after kinetic energy and potential energy became understood, it still took people centuries to figure out that heat is just another form of energy.  

Before our present understanding of physics evolved, it was still a logical possibility that the Universe might have been constructed quite differently, such that energy, in the sense of power to modify the world, would not have been conserved and/or things in even everyday life might have been controlled by some kind of supernatural beings. We can now see easily that such a world would likely look very different from our own, because the basic properties of energy are actually responsible for “constraining” many aspects of our world: Everything from the branching structures of trees to the way that our bodies and the planets move are all strongly constrained by the properties of energy.