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Watts For Lunch? (Or Why Humans Are Like Light Bulbs)

Monday, June 10, 2013 - 01:06 PM

 

There's a new lunch place down the block, so like you do when the menu looks interesting, I walked in and ordered something mysterious, which for me was the "Red Lentil and Edamame Salad," mostly because I can never remember what edamame is, and because that word suggests doing something slightly frightening, like munching accidentally on one's mother.

How Much Energy Am I Eating? Enough To Power A Flashlight?

What arrived was a bowl of lentils, roasted carrots, raisins, mint and (I'm guessing) edamame beans. I took the bowl to a window seat, and that's when my mind began to wander. My mind doesn't need much to go free. It slips off whenever I let it, when I'm by myself and alone with my thoughts, which, at this moment, were: "So I'm chewing these beans and I'm breaking them into little bits, which will become littler bits in my stomach, (bond-breaking, as the chemists would say) so I'm turning food into energy. But how much energy am I getting? Does a salad produce enough calories or watts or whatever, to light a flashlight? Or run an electric toothbrush for 10 minutes?

(Do you ever do this? I do this all the time.)

Robert Krulwich/NPR

After lunch I looked up the answer. I found it in a fine little book by Wayne State Professor Peter Hoffman, called Life's Ratchet: How Molecular Machines Extract Order from Chaos. In it, he says humans will typically eat roughly 2,500 calories a day.

1.5 Million Joules Is ... ?

Professor Hoffman is a physicist. He goes on to make some quick calculations. If one food calorie equals 4,184 joules of energy, at 2,500 calories a day, that means our bodies break down or release 1.5 million joules. Sounds like a lot, no? But if you divide those joules by the number of seconds in a day (86,400), that works out to a rate (where 1 watt = 1 joule per second) of about — 120 watts a day. In other words, that's all I need to dream, wake, dress, shower, work, walk to a restaurant, order a salad, ask myself how much energy am I using, and then look it up, think about it, and write this essay. I can power all trillion cells (of me) for a day at the same rate that it takes to light one 120-watt light bulb.

That's it?

That's it. Peter Hoffman writes, "Humans talk, write, walk and love using the same amount of energy per second as a light bulb."

I'm humbled. I will now confess that when I got back from the salad place, I diddled, I called friends, yakked with office mates and used up lots of time to avoid writing this, and yet — down deep, at a cellular level, it turns out I'm a mind-boggling display of energy efficiency. You too, of course.

On certain afternoons, this is a nice thing to know.

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Comments [2]

John from Indiana

Sometimes they screw it up the other way around: "the plant uses as much power as it takes to light 100,000 homes for a month". It's impossible to parse that sentence in any sensible way.

I think engineers may have caused this confusion by using the unit "kilowatt-hour" for energy. Here a rate is multiplied by a unit of time to get a total amount of energy. Technically correct, but confusing in much the same way as "light-year" is, where a rate (speed of light) is multiplied by a unit of time to get a unit of distance.

On a side note, for anyone trying to do any unit conversions between food and energy, be aware that while physicists define a calorie as the energy needed to raise 1 gram of water by 1 degree Celsius, what we commonly call a "food calorie" is actually a kilocalorie, or 1000 calories. Robert got this right, but the difference is usually noted by spelling (or abbreviating) a food calorie with an uppercase "C" (or "Cal" for the abbrev.- see your nearest nutritional label).

Jun. 13 2013 05:31 AM
John from Indiana

This article runs heads-first into a major pet-peeve of mine about science journalism- since energy is one of the most important issues in life, shouldn't journalists, or at least science journalists, have a basic understanding of the difference between energy, and power, which is a _rate_ of energy (whether production, transfer, or consumption)? This is taught as basic info in just about any high school science class, and should be considered part of "basic grammar" for science journalists.

Take this sentence in the story: "If one food calorie equals 4,184 joules of energy, at 2,500 calories a day, that means our bodies break down or release 1.5 million joules. ... if you divide those joules by the number of seconds in a day (86,400), that works out to a rate (where 1 watt = 1 joule per second) of about — 120 watts a day." Robert correctly distinguishes between energy and power all the way through that, until the last two words. He divides joules (energy) consumed in one day by seconds in a day to get joules per second (a rate known as a watt). All correct until he adds "a day" after "watts". The "per day" in the two initial parts of the calculation cancel in the division. "Watts per day" makes no sense, unless you were talking about a change in the rate (change in watts) per day (much like acceleration is a rate of change of a rate).

In the final sentence of that paragraph, "I can power all trillion cells (of me) for a day at the same rate that it takes to light one 120-watt light bulb", the "for a day" is not quite incorrect, but it's irrelevant. The rate- 120 watts, or 120 joules per second, whether used by your body or a light bulb, is the same regardless of how long you continue to use energy at that rate.

I've seen endless documentaries about large facilities- power plants, steel mills, etc,- where the narration says something like "this plant will produce (or consume) as much energy every day as it takes to power 100,000 homes. Again, the "per day" makes no sense in that sentence. The plant might use/consume as much energy in one day as it takes to power 100,000 homes _for that same day_, but it's not stated that way- rather they talk in terms of energy in the first half of the sentence and power in the second (and often make it more confusing by using the word "power" rather than energy in the first half, so again we'd have a completely meaningless "rate per day" statement). The sentence can be simplified (and corrected/clarified) by eliminating the "per day" altogether, since if on one day the plant uses as much energy as it takes to run 100,000 homes for a day, that will presumably be the same on any random date in the future. In this case the word "power" should be used in both parts of the sentence- "The plant uses as much power as it takes to power (or run) 100,000 homes."

(continued...)

Jun. 13 2013 05:29 AM

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