Measuring Energy

Everything in the physical world is energy. That's what Einstein's famous equation says, and most people with at least a high school education are familiar with the general idea of the equivalence between mass and energy. Yet energy remains a somewhat abstract concept and we often toss the word around in imprecise ways. Why is that?

I think it has to do with ease of measurement. If you ask, "How tall is Cindy?" Most everyone knows how to answer the question.  You have Cindy stand against a wall, make a mark on the wall with a pencil, and then use a tape measure to see, with pretty good precision, how tall Cindy is.

Other things are easy to measure. How much milk do I add to this pancake batter? Well, the recipe says "one cup", so I get out a measuring cup, fill the measuring cup up to the line that says "one cup" and I pour it in the batter.

How much does Joe weigh? Ask Joe to stand on a scale and read the result.

So in each of these cases, there is a simple instrument – a tape measure, a kitchen utensil, a scale – that we know how to use to answer our question.

It's not that way with energy, however. We don't "experience" energy directly the way we do someone's height and weight or a volume of liquid. Recall from our earlier post that energy is the potential to do work. If we direct some energy to something, say we turn on a light, we see the result of that application of energy (the light bulb glows). We don't, however, directly "see" the energy.

Not only that, but we are also faced with several units of measurement for energy, and these units don't have nice neat relationships between one another.  

If we're measuring length in the metric system, we know that 100 centimeters equals one meter. In the "English" system, there are 36 inches in a yard. There are 16 ounces in a pound. We see the subdivisions, the relationships.

For energy, we are faced with scales of measurement that are more abstract. Again, that's because we don't directly "see" energy. In addition, it is unclear how the various scales relate to each other. What are the relationships between, BTU's, Kilowatt-Hours, therms, and so on?  Not only that, but we often apply volumetric units (cubic feet of natural gas, gallons of propane or gasoline) to sources of energy.

So we need to set some ground rules for how we describe how much energy we're talking about. We'll settle on one primary way to represent the amount of energy that we're using (or saving or delivering) and then we'll show a way to convert other units of measurement – such as those we find on our utility bills – into that one way.

Sort of like comparing apples to apples.