A few weeks ago, one evening the kids and I had one of those magical dinners. Jo was back at Lariat Ridge hanging out with Carol who is recovering nicely after her second knee replacement surgery. I had taken the kids to Havalah for their haircuts straight after school. We stopped at Central Market on the way home and bought a loaf of freshly baked rosemary bread and two juicy prime ribeyes (32,000 liters of water down the hatch). I thinly sliced the steaks across the grain like Japanese yakiniku. Everyone liked it. My cheap wine tasted great. We sat in the lovely spring evening outside and chatted for a couple of hours.
It took me about 20 minutes to get the grill hot and cook the ribeyes. I spent less than a pound of propane, say 500 grams in the metric world.
When we backpacked in the Ouachita National Forest over spring break, I used 100 g of propane over four days from one of those pocket sized gas canisters that directly screw into an ultralight stove to cook. We boiled water each night for Vivian, Evan and my dinners and for Princess Vivian’s oatmeal in the morning (Evan and I ate cold granola for breakfast). We consumed 25 g of propane per day compared to 500 g to grill the ribeyes in the backyard. The energy in 25 g of propane can be stated in many different ways. Let’s drop into my teacher Daniel Sam’s physics class back at HPS for a quick review, ‘de man.
Energy is defined in physics as the capacity to do work. Work is the force applied on an object times the distance that the object is moved. Force is measured in newtons (N) in Système Internationale (SI) units and distance in meters (m). The unit of work in SI is joules (J) – one J is a force of one N applied to move an object by one m. So a J is also a N-m. As force is mass times acceleration, work and energy are mass x acceleration x distance, or mass x distance2 / time2 which makes a joule the same as a kg m2/s2. If you pick an average sized lemon which weighs exactly 102 g and move it from the floor to a table 1 m high, you have worked against gravity which has a downward acceleration of 9.8 m/s2. You have done 1 m x 102 g x 9.8 m/s2 or 1 J of useful work and if you used a 100% efficient machine it needed 1 J of energy. If you personally did the work you also moved your upper body’s weight against gravity, so you did a lot more work.
Speaking of doing work for no good reason, a calorie is another way to measure energy. We run into the calorie mostly when we are trying to burn more of it or eat less of it. Officially it is the energy needed to raise the temperature of one gram of water through one degree Celsius. A calorie is equal to about 4 J or four lemon pick-ups. A kilo calorie (kcal) is a 1000 calories. When a 12 oz can of Coke says in contains 140 Calories, it really means 140,000 calories. But because no one wants to drink that many calories we drop a few zeros like the peso. Food labels in the US frequently call a kcal a Calorie and spell it with a capital C.
Back to the lemons. When I pick a lemon off the floor to a meter tall table I expend about 0.1 kcal. Don’t quote me on this – I went through some approximate calculations involving toe touches and other stretches because lemon picking isn’t yet an Olympic sport. So 0.1 kcal is 100 calories with a small c or 400 J. Remember we said that the useful work done on the lemon was just 1 J. My body spends 399 J bending down and straightening up while doing 1 J of useful work on the lemon.
I burn about 1860 kcal or 7440 J even when I sit on my ass all day which does not happen infrequently. This number is based on my gender, weight, height, age, and other factors. This is the energy needed to pump blood and maintain my body temperature and think and digest food and do other stuff that keeps my body alive. My liver, spleen, and brain account for almost half of my body’s resting energy needs. If I get off my ass moderately, I need to absorb about 2400 kcal or 20,000 J of energy from food per day. Remember this number. There will be a test later.
One somewhat basic definition of you is that you use energy to create order from chaos. Living things are more organized compared to the world outside them. The chemical energy stored in food along with oxygen from the air we breathe is converted into complex ordered molecules. Some of these molecules do the work associated with living, like reproducing, growing, digesting, healing, and moving. Some molecules store excess energy in my belly fat. And like everything else in our known universe, some of the incoming energy is lost as heat, increasing the entropy of the universe forever. Plants additionally use energy from the sun. Other living things eat them thereby absorbing a portion of the sun’s energy stored in complex ordered plant molecules as chemical energy. The sun and other stars get their energy by smooshing hydrogen atoms into helium atoms. CSNY sang that we are star dust. A less lyrical addendum is that we live on star power.
Recall that energy is the capacity to do work. Power is the rate at which work is done. Returning to our trusty lemon, if a 100% efficient machine moves the lemon very slowly over the course of a year, and another one moves it in a second, the amount of work done by both is 1 J. But the fast machine uses a lot more power. To be exact, it expends 1 J per second, or 1 W (watt) which is the SI unit of power. Conversely, one W-s is one Joule because multiplying power with time gets you back to energy. Think of energy as the capacity of a tank of water and power as the flow rate of water out of the tank. A common measure of energy is the kWh (kilo watt hour), and is 1000 (for the kilo) x 60 (for the minutes) x 60 (for the seconds) J or 3.6 x 106 J. Electrical energy is often measured in kWh. If you loaded up a 100% efficient battery with 1 kWh and powered a 100% efficient lemon picking machine, the machine would be able to pick up 3,600,000 lemons. A fully loaded Tesla car battery stores 100 kWh of energy to move you and itself through 640 km.
There are other units used to measure energy. A British Thermal Unit or BTU which should be renamed to an American Thermal Unit is an archaic unit used in less advanced places and is similar to the calorie except that uses pounds and Fahrenheits instead of grams and Celsius. A TOE or Tonne Equivalent of Oil is the energy released with you burn a tonne of crude oil. Consider it the calorie content of one big ass can of oil. A megaton is the energy released when you explode one million tonnes of TNT. A BTU is equal to about 1000 J or 1 kilo J. A TOE is approximately 42 giga J. A giga is a billion or 9 zeros. A megaton is 4.2 peta Joules (15 zeros). The biggest nuclear bomb, the Tsar Bomba, released 50 megatons of energy during its explosion.
Last year the world used 1.4 x 1010 TOE or 588 exa Joules (18 zeros) of energy. That is approximately 200 x 106 J per man, woman, and child per day. Do you recall how many joules we decided you need to eat to stay alive and engage in moderate activity each day? Of course you do because like me you find all this utterly riveting. So that explains 1/10,000th of my daily energy consumption. Where do I use the remaining 199,980,000 J per day?
I’m looking into it. Feel free to leave a comment with the answer.