Everyday Einstein explores how work, energy, and power are related.
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 work 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 work) 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.
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