Energy is the ability to create change
Energy is converted from one form to another, but never created or destroyed
Two main categories of energy are kinetic and potential energy
Energy systems of conversion and use are at the heart of our political, economic and social structures
What is energy?
Energy is a quantity that describes the ability to create useful or desirable change. This ability includes generating motion (moving an object, for example) or causing a physical change (heating an object, for example).
In the universe, energy can not be created or destroyed, it simply transforms from one form to another. However, as energy is converted to different forms, some energy can be lost as non-useful energy. This is also called entropy, and this is why it is important to think about the efficiency of our energy conversions.
Types of energy
Kinetic and potential are the two major categories of energy. Kinetic energy is often what first comes to mind when you think about energy, and includes moving and heating things. Potential energy is harder to conceptualize, but is equally important for our energy systems.
Kinetic energy is the energy of motion. Some examples of kinetic energy include wind energy, the motion of a vehicle, the motion of a turbine, and thermal energy (heat).
Potential energy is stored energy, to be released when we need it. One example of potential energy is gravitational energy, like water sitting at the top of a mountain that has the potential to flow downhill and be converted to kinetic energy. Other examples include chemical energy stored in the bonds between atoms (firewood, coal), nuclear energy stored in the bonds within an atom’s nucleus (uranium), and electrical potential energy between charged particles such as electrons.
Energy, power, and units
When discussing different forms of energy, it can be confusing to keep track of the various units used in different applications. Below we give a few examples to help give some context for terminology you may encounter when learning about energy.
Energy describes an inherent quantity, and can be expressed in the units of calories (cal), watt-hours (Wh), British thermal units (Btu), and more. Calories are used to communicate the energy content of food, watt-hours describe electricity, and British thermal units convey the amount of energy in fuels.
When using energy it is also valuable to think about how fast it can be used. Power is the rate at which energy is expended, or energy consumed per an amount of time. Units for power include watts (W), horsepower (hp), and more. Watts describe the rate of electrical energy use of household appliances, and horsepower is more commonly used for vehicle engines.
Different scales are represented with different prefixes, such as mega- (M), a million times; or kilo- (k), a thousand times.
An example to help conceptualize energy, power, and units
On the electrical bill at the end of the month, the units reported are watt-hours (Wh), which is the total amount of energy consumed during the month.
Lightbulbs are described in terms of watts (W), which is the power (energy per a unit of time) consumed to light the bulb. For incandescent lightbulbs, a higher power correlates with a higher brightness.
In 2021, the average household in the US used 880 kWh per month, which is the equivalent of leaving over twenty 60 W incandescent lightbulbs on 24/7 for a month!
880 kWh / 672 hours in a month (24 hours x 7 days x 4 weeks) = 1309 W total
1309 W total / 60 W per lightbulb = 21.8 lightbulbs
Other household appliances use more power than a single lightbulb. A space heater is about 1,800 W, a refrigerator/freezer is 700 W, a laptop is 50 W, for some examples. To learn about the power of other household appliances, see a list here.
Energy in society
All the types of fundamental “energies” mentioned above exist, and humans have developed methods and technologies to use them to do work and generate power.
We use kinetic energy for transportation, electrical energy to power appliances, thermal energy to heat buildings and more!
Energy systems are at the heart of our economic, political, and social systems. Energy literacy requires understanding where energy comes from, how we convert it into useful forms, how we transport energy, and how to store it to be easily accessible when it is needed.
In these modules, we will explain some of the examples, concepts, and considerations of energy sources, conversion, distribution, and storage.