The Bizarre Love Triangle of Work, Energy, and Power
Ask Science explores how work, energy, and power are related.
A long time ago when I was much younger, I worked with my father in his landscaping business, which consisted primarily of cutting the grass at gas stations. Now, you might not think that gas stations have much grass, but I can tell you that some of them would surprise you. But no matter how much grass a gas station had, I earned a steady stream of $5 per gas station.
Unfortunately for me, while it took quite a long time to earn that money, I spent it at a much faster rate. This was mainly because I was an avid comic-book reader and all of the gas stations that needed their grass cut also sold comic books. (It was diabolical!)
In my mind, cutting grass, earning money, and buying comic books will forever be connected. Such is the relationship between work, energy, and power.
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Energy, the Currency of Work
If you remember our discussion about , you know something about energy already, such as the fact that energy can’t be created or destroyed, only converted from one form to another. One of my favorite physics websites is https://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html. On that site they call energy the “currency” for performing work.
When I was cutting grass for my father, I was storing up currency, which in my mind was the capacity to buy comic books. The value of comic books I could purchase was equal to the amount of currency that I had stored up through cutting grass.
Similarly, the /education/science/all-work-and-no-play you can do is equal to the amount of energy you have available. Just like I was paid in the same currency that I needed to buy comic books (U.S. dollars), energy and work are measured using the same quantity, joules. If I wanted to buy $5 worth of comics, I had to do $5 worth of grass cutting. If you want to perform 100 joules of work, you need 100 joules of energy.
I Have the Power!
When my father received his payment for our work, it typically lasted us an entire month. He’d spend it on things like a month’s worth of mortgage, a month’s worth of food, or a month’s worth of electricity. I typically spent my money much, much, faster that my father. He would get paid by the store manager, hand me my money, and I would immediately grab my chosen comic books and hand my money back to the store manager. While it took us about the same amount of time to earn the money, the rate at which I used my money was much higher.
Power is the word used to describe how fast energy is used up, or equivalently, how fast /education/science/all-work-and-no-play is performed. The two most commonly used units of power, horsepower and the watt, share an interesting history.
Back in the 1700’s, an engineer named James Watt was working on making a better steam engine. When he sold someone a steam engine, the price he charged was equivalent to a third of the cost of the coal the buyer saved by using his more efficient engine. The trouble was, some buyers weren’t replacing old engines; they were replacing horses. So Watt figured out how much work per minute the average horse could do and called that “1 horsepower.” He could then sell his engines to people based on how much horsepower they had.
Unfortunately, horsepower is defined in foot-pounds of force per minute, which isn’t very convenient for scientific calculations. So in the 1800’s scientists came up with the watt (named after James Watt) as a unit of power. One watt is equal to one joule of energy (or /education/science/all-work-and-no-play) being done per second. One horsepower is equal to about 746 watts.
The average power used in doing work can be calculated by taking the amount of work done and dividing it by how long it took to do the work. So if one person does 100 joules of work in 2 seconds, they have used 50 watts of power. If a second person can do the same amount of work twice as fast, (100 joules of work in 1 second), they have used 100 watts of power.
Conclusion
So now you know that if you spend all summer cutting grass, you probably shouldn’t spend all of your hard-earned money on comic books in a single day. You also know how work, energy, and power are all related. Energy is the currency of work, 1 joule of energy buys you 1 joule of work, and power is the rate at which work is carried out, or the rate at which energy is used.
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James Watt image from Shutterstock
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About the Author
Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech.
