# What is Energy?

what does this term mean? because We hear about energy everywhere.

why we hear about it everyday? and how important it is?

We pay bills for energy every month, Athletes eat high-energy foods, and put as much energy as they can into the swing or the kick or the sprint.

we hear this word many times everyday. Like most words, “energy” has multiple meanings. This article is about the scientific meaning of energy, which is fairly consistent with most, but not all, of the meanings of the word in the preceding paragraph.

What is energy, in the scientific sense? I’m afraid I don’t really know. I sometimes visualize it as a substance, perhaps a fluid, that permeates all objects, endowing baseballs with their speed, corn flakes with their calories, and nuclear bombs with their megatons. But you can’t actually see the energy itself, or smell it or sense it in any direct way—all you can perceive are its effects. So perhaps energy is a fiction, a concept that we invent, because it turns out to be so useful.

Energy can take on many different forms. A pitched baseball has kinetic energy, or energy of motion. When the ball is high above the ground, we say it has gravitational energy. Stretching a rubber band stores elastic energy in it.

Corn flakes and gasoline store chemical energy, while uranium and plutonium store nuclear energy. A hot potato contains more thermal energy than it did when it was cold. Electrical energy is transmitted along wires from power plants to appliances, and radiant energy is given off by lightbulbs, lasers, stovetops, and stars. Figure below lists the most important types of energy and their physical manifestations.

These multiple forms of energy can be converted into each other. As the baseball flies upward, its kinetic energy is converted into gravitational energy; as it falls, the gravitational energy is converted back into kinetic energy. Before the energy entered the ball, it was stored as chemical energy in the batter’s breakfast. When the ball hits the ground and rolls to a stop, its kinetic energy is converted into thermal energy, warming the ball and the ground very slightly. A slingshot converts elastic energy into kinetic energy; a burning matchstick converts chemical energy into

thermal and radiant energy. Our sun converts nuclear energy into thermal and radiant energy. A hydroelectric dam converts gravitational energy into electrical energy, while an electric motor converts electrical energy into kinetic energy.

Energy conversions have been familiar to humans for thousands of years. Only over the last few centuries, however, did scientists gradually realize that during every energy conversion, they could account for the energy, unit by unit, in a quantitative way. If you carefully measure the amount of energy before and after some process, taking all forms of energy into account, you find that the total amount of energy never changes. Energy can be converted from one form to another, but it cannot be created, nor can it be destroyed. This principle is one of the deepest in all of science, and also one of the most useful. In scientific jargon, this principle is known as the law of conservation of energy, and also as the first law of thermodynamics.

In everyday life, “conservation of energy” has a completely different meaning, namely, using less of it. Please don’t confuse this use of the word “conservation” with the more technical meaning in the previous paragraph. But if energy cannot be destroyed, what does it mean to “use” energy? The answer lies in the fact that some forms of energy are more useful than others. Our modern industrial society converts energy from more useful forms (mostly chemical energy) into less useful forms (dispersed thermal energy) at a tremendous rate. This is some cause for

concern, because supplies of energy in useful forms may be limited, and because energy conversions often have unwanted side effects. “Conserving” energy, in the everyday sense of the word, simply means carrying out these conversions to a lesser extent.

Thermal energy is the least useful form of energy. Although it can be partially converted into other forms (as in an automobile engine where it is partially converted to kinetic energy), this conversion is never complete. Furthermore, thermal energy tends naturally to disperse over time, and once it is widely dispersed, it is effectively useless. These annoying properties of thermal energy are summarized in the second law of thermodynamics, a principle that is just as important as the principle of conservation of energy (the first law).

**Units of Energy**

Just as distances can be measured in inches or meters or miles or microns or light-years, so also energy can be measured in many different units: joules, calories, British thermal units, kilowatt-hours, electron-volts, and quads, to name a few.

Although life might be simpler if there weren’t so many different energy units, each of these units came into use for excellent reasons, and every educated person needs to learn at least the most common energy units, and how to convert a quantity of energy from one unit of measure to another.

The official, internationally accepted energy unit for scientific work is the joule, abbreviated J. A joule is a rather small amount of energy, roughly equal to the kinetic energy of a very gently tossed baseball, or to the gravitational energy that

you give to a baseball when you lift it by two feet (70 centimeters). A more familiar energy unit is the calorie (cal). The original definition of the calorie was the amount of thermal energy required to raise the temperature of a

gram of water by one degree Celsius. This amount of energy turns out to equal about 4.2 joules, and the calorie is now defined to equal precisely 4.186 joules. This is still a rather small amount of energy. The familiar “food calorie,” used to measure chemical energy that our bodies can extract from food, is actually a kilocalorie (kcal), or 1000 calories—enough energy to raise the temperature of a kilogram of water by one degree Celsius. In this course I’ll always refer to food calories as “kilocalories,” to avoid ambiguity.

As an example that is both vivid and useful (if not particularly nutritious), consider a typical jelly donut, which provides about 250 kilocalories. Since a kilocalorie is about 4000 joules, one jelly donut provides approximately one million (250×4000) joules, or one megajoule, of chemical energy. Some physicists go so far as to define a unit of energy called the jelly donut (JD), equal to exactly one megajoule (MJ).

A typical American adult consumes the equivalent of about ten jelly donuts each day, or roughly 2500 kilocalories.

In the British system of units, the analogue of the kilocalorie is the British thermal unit (Btu), defined as the amount of thermal energy required to raise the temperature of one pound of water by one degree Fahrenheit. Since a pound is smaller than a kilogram and a degree Fahrenheit is smaller than a degree Celsius, a Btu is smaller than a kilocalorie—about one fourth the size, it turns out. This means that a Btu is approximately 1000 joules. This is still a rather small amount of energy compared to what’s involved in heating or cooling a building or a large tank of water, so it’s common (in the U.S.) to see thermal energies measured in millions of Btu’s (MBtu). The natural gas that I use to heat my home is billed in these units; I currently pay about $6 per MBtu.

Electrical energy, meanwhile, is most often measured by the kilowatt-hour (kWh), a unit whose origin I’ll explain in the next section. One kilowatt-hour equals exactly 3.6 million joules, which is approximately 860 kilocalories or 3400 Btu. For this amount of electrical energy I currently pay about seven cents. Table below summarizes the conversions among these various energy units. I suggest that you memorize the approximate values of these conversion factors, to help develop your intuition for various amounts of energy. Although these energy units are the ones we’ll use most frequently in this course, I’ll occasionally introduce other units when they are convenient for specific applications.

**1 kcal = 4186 J = 3.97 Btu = 0.00116 kWh**

**1 Btu = 1054 J = 0.252 kcal = 0.000293 kWh**

**1 kWh = 3, 600, 000 J = 860 kcal = 3413 Btu**

**Power: The Rate of Energy Conversion**

Often we’re interested not just in the total amount of energy converted in some process, but also in the rate at which that energy is converted. A person consumes 2500 kilocalories per day; a furnace puts out 100,000 Btu per hour; a power plant generates one billion joules per second. To calculate the rate of energy conversion, all you have to do is divide the amount of energy converted by the amount of time it took. Scientists use the term power for this ratio:

For instance, if in climbing a flight of stairs you gain 2000 joules of gravitational energy over a period of 5 seconds, we would say that the power output of your legs is :

(Since you are not 100% efficient at converting chemical energy into gravitational energy, the rate at which your muscles consume chemical energy is actually greater; the rest of the chemical energy gets converted to thermal energy, as you’ve probably noticed while exercising.)

see also our Energy Books section

The joule per second (J/s) is the official scientific unit of power, and has its own name: the watt (W). Thus, we could just as well say that the power output of your legs in climbing the stairs is 400 watts. Since a watt is a rather small amount of power (very roughly the rate at which a flashlight bulb converts electrical energy into thermal and radiant energy), we often attach to it the prefixes kilo (for a thousand), mega (for a million), or giga (for a billion). A typical American home consumes electrical energy at an average rate of about a kilowatt (1 kW = 1000 J/s = 103 J/s); a large truck consumes chemical energy (diesel fuel) at a rate of about a megawatt (1 MW = 1,000,000 J/s = 106 J/s); and a large power plant generates electricity at a rate of about a gigawatt (1 GW = 1,000,000,000 J/s = 109 J/s).

Another unit of power that you’ve probably heard of is the horsepower, approximately the power output of a draft horse working steadily. Since all horses are not created equal, today the horsepower is defined as exactly 746 watts; whether your horse can actually produce one horsepower is your own problem. Other units of power can be created by combining any unit of energy with any unit of time, as in Btu/hr (used for heating and cooling appliances) or kcal/day (convenient for talking about the human diet).

If you know the power involved in some process and want to calculate the total energy, you have to multiply by the time elapsed:

**energy = (power) × (time)**

This is just an algebraic rearrangement of equation 1.6. For instance, if you leave a 1000-watt electric heater running all day long, its total energy consumption is:

more than 86 megajoules. If you convert this number to kilowatt-hours, you’ll find that it’s exactly 24 kWh. This is because the kilowatt-hour is defined to equal a kilowatt multiplied by an hour:

Thus, if we want the answer in kilowatt-hours (which is more useful in calculating the cost of the electricity), we can simplify the calculation in equation above as follows:

Just remember that the kilowatt-hour is a unit of energy, not power, while the kilowatt (or megawatt, etc.) is a unit of power, not energy. (Newspaper writers seem to get this wrong about as often as they get it right, indicating that they’re just guessing randomly. Now you know better!)