Ways Physics Teachers Save Energy

Submitted by Shawn Reeves

2006-01-16 12:59:40

Turning down the thermostat and driving hybrid cars work fine for the average citizen, but what do physics teachers do when they want to save a penny's worth of energy?
1. Heat things up with cold water. When I'm hungry for something small, I look in the freezer to see if we have any frozen peas; they're great with steak sauce. But, I don't heat them up from frozen, that would waste energy. I put them in a cup with cold water.
Five degrees celsius seems cold to us, but warm to something frozen. To raise 100 grams of water (equivalent to a decent serving of peas) from -10 to 55 degrees celsius takes 34000 Joules (4.18 Joules per gram-degree for warming plus 335 Joules per gram for melting). To raise 100g of water from 5 to 55 degrees celsius takes only 20900 Joules.
But what about pumping that tap water to warm up the frozen peas? To raise 300 grams of tap water 50 meters takes 150 Joules, not significant in this situation.
So, for this lunch, I saved about 13 kJ, 0.0036 kWh. At thirteen cents a kilowatt-hour, I can save a penny every 22 lunches.
I don't think that I don't save enough energy for this to be important, but that our energy is too cheap.
2. Paint the attic white.
3. Consider those who say it takes so much energy to start up the car/tv/lights that they just leave it on if they need it in 5/10/15 minutes.
A TV might take 200W times 600 seconds to run it for 10 minutes, 120kJ. To use up that much energy to start a TV, to charge up the capacitors in the TV, which takes about 20 seconds, would suck 120kJ divided by 20s, or 60kW, which would blow my circuit breaker 40 times over! Turn your TV off whenever you're not watching it; you can even save energy by turning it off for three minutes of commercials. The same goes for lights.
A car takes less than 60VAh, which is 220kJ, to start. To regenerate the battery takes somewhat less than double that from the alternator, let's say 400kJ. Let's also say the alternator puts out 1kW. Thus, it would take 400s to put that 220kJ back into the battery. Lesson: Turn of the ignition if you're leaving the car for more than 5 minutes.
By the way, did you know that modern alternators are three-phase generators? I didn't until I read (don't trust everything you read) the following page:
This kind of physics-thinking, using skills involving algebraic formulas and unit conversions, gets easier with practice and gives me something to think about while I'm driving or adjusting my thermostat. Should the 'basics' be taught first so students can learn to think this way, or should the teaching go 'backwards' so that students feel like they have a reason to learn the basics? I leave that to each teacher to decide. I would usually prefer the latter, resting on resarch into inquiry in the classroom.
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