What is Energy? Measuring and Quantifying Energy

Units:

What is Energy?

Section C. Measuring and Quantifying Energy

When people say the words energy, work, or power in everyday conversations, listeners usually have little trouble understanding what these words mean. For example, one teacher might say to another, "I put a lot of work into my lesson plan last night and it paid off. Today's class went really well." The other teacher would understand immediately what the first is talking about.

But what are the scientific definitions of energy, work, and power? Energy is often defined as the ability to do work. In turn, work has a specific definition in physics, it is equal to the force needed to move an object multiplied by the distance it moved (Work = Force x Distance). Although the teacher planning the next day's lesson during the evening may say that he is doing work, by definition, work is done only when the teacher actually moves something, such as moving his pen to write his lesson plan (Taken from the KEEP Activity Guide " People Power").

ENERGY

Just as there are many forms of energy, there are different ways to measure these forms. In science, work and energy are commonly measured in joules. One joule is the amount of energy it takes to lift an object that weighs one Newton a one meter distance. A Newton is the metric unit for weight and is comparable to the weight of a stick of butter. (1 Newton = 0.445 lbs.)

In our everyday lives, other energy measurements are more familiar to us than joules. Some of these measurements include calories, therms, and kilowatt-hours. Definitions of these and other units are found below.

It might seem confusing to have so many different measurements for energy. The good news is that one unit can be converted to another.

See Conversion and Resource Tables.

Definitions:

British thermal unit - (abbrev. Btu)
1. A unit of energy equal to 1,055 joules or 252 calories. 2. The amount of energy needed to raise the temperature of one pound of water one degree Fahrenheit. 3. The approximate amount of energy in one match tip.

calorie (abbrev. cal; pl. calories; also small calorie): 1. A unit of energy often used when measuring the energy content of food. One calorie equals 4.187 joules or 0.003969 British thermal units (Btu). 2. The amount of energy needed to raise the temperature of one gram of water one degree Celsius.

Food calorie (abbrev. Cal, kcal; also Calorie [written with a capital C], Kilocalorie, Large calorie): A unit of energy equal to one kilocalorie. The food calorie is often used when measuring the energy content of food.

Joule - A unit of energy. One joule equals 0.2388 calories or 0.0009481 Btu.

Kilocalorie (abbrev. Cal, kcal; also Calorie [written with a capital C], Food calorie, Large calorie): 1. A unit of energy equal to 1000 calories, 4,187 joules, or 3.969 Btu. 2. The amount of energy needed to raise the temperature of one kilogram of water one degree Celsius.

Kilowatt-hour (abbrev. kWh; pl. Kilowatt-hours): 1. A unit of energy equal to 3,413 Btu or 3,600,000 joules. 2. An amount of energy that results from the steady production or consumption of one kilowatt of power for a period of one hour.

Therm - (pl. Therms) A unit describing the energy contained in natural gas. One therm equals 100,000 Btu. See British Thermal Unit.

POWER

Power is the rate at which energy is used. Some units of measure for power include horsepower and watts. One way to help distinguish between work and power is to think of two people each eating an apple pie. One person eats the pie faster than the other. They both ate the same amount of pie (they did the same amount of work), one just ate at a faster rate (the faster one was a power eater!). Like with units of energy, power units can be converted to another (see Power Conversions table).

Definitions:

Horsepower (abbrev. hp): A unit of power. One horsepower equals 550 foot-pounds per second or 746 watts.

Kilowatt (abbrev. kW; pl. Kilowatts): A unit of power equal to 1,000 watts.

Power: 1. The rate at which energy is transferred or converted per unit of time. 2. The rate in which work is done. Units of power include Horsepower, Kilowatt, Megawatt, and Watt.

Watt (abbrev. W; pl. Watts): A unit of power. One watt equals the production or use of one joule of energy per second.

Next reading: To help you better understand how to measure and quantify energy, participate in the activity People Power.

Return to What is Energy? - Section B. Two Main Forms of Energy.

To return to What is Energy? index page.

To return to the Course Overview page.