Imagine how dark the nights were a few hundred years ago. I remember reading about a party in the 17th century at a wealthy person's home where the writer commented on the extravagance of the affairs - something like a dozen beeswax candles flickered away on the big dinner table even though the moon was nearly full.
It takes nearly fifty candles to equal the amount of visible light that is produced by a single sixty watt incandescent lamp - even the little fifteen watt lamp in your refrigerator produces as much light as a dozen high quality candles. Of course low light levels present problems as to what people can do.
A la chandelle, la chèvre ressemble à une demoiselle
- an old French saying dating from before the 17th century1
The cost of artificial lighting has dropped dramatically over the years. Candles were fairly expensive coming in two basic flavors - beeswax and animal tallow. Tallow candles required constant work, smelled awful and became rancid, but you could make them from the fat of any slaughtered animal. Beeswax candles were superior but were so expensive only the church and wealthy individuals could afford them.
Oil lamps were extensive and a subject of serious technological progress, but by the time petroleum was discovered in Pennsylvania, a gallon of lamp oil cost about half the wage of the average worker. Gas lamps were common and expensive in cities at the time and the advent of electric lights, even though electricity cost about one hundred times as much as it does now, adjusting for inflation, was cost effective. Breaking the darkness was an enormous problem that consumed an enormous amount of money.
We had about 20 hours without electricity yesterday and had to make do with some beeswax candles. To conserve them we only kept three going at a time - and only one in a room at any time. Story telling is a good diversion for a few hours, but the fact that our gas furnace has an electric fan caused the temperature to drop and electricity at 15 cents a kWh seemed like a fantastic deal.
Apart from wishing for warmth it made me think of a few things -
Without electricity you begin to worry about how much energy is left in your mobile phone. Landlines are powered by batteries in the central office and conventional telephony is fairly bullet-proof as long as the lines aren't broken, but many of us now rely on our own batteries.
My iPhone 4 has a 1420 milliwatt 3.7 volt battery - about 5.25 watt-hours of energy.2 I have an external battery pack that holds about 20 watt-hours but, of course, it wasn't charged.
Five and a quarter watt-hours is not a lot in human terms and tiny compared with the 25 or so kilowatt-hours of electricity the average home uses in a day. To move that energy into the phone requires a few hours of charging, but it is interesting to compare it to the energy we use in the form of food. Converting to units we all are familiar with gives about 4.5 nutritional Calories. A single M&M contains about 3.5 Calories of energy. Too bad we don't have a little fuel cell from Mars that burns candy. Then we could pop an M&M in every few hours and forget about the wall plug.
It is amusing to compare the power transfer rates we see eating meals and compare that to what our home wiring can provide. Food can be a remarkably dense form of chemical energy. (I leave that as a fun little exercise).
The back of the envelope was a quick approximation an athlete friend offered. She isn't technical, but we had been talking about energy and power as we were writing a few pieces and the question of leaving a light on or turning it off came up and my back of the envelope technique has rubbed off on her.
A compact fluorescent lamp needs more power to start up. How much energy is required and where is the break-even point for leaving the lamp on rather than turning it off (this doesn't consider the cost of a possible failure from turning a lamp on and off - that turns out to be very low for high quality CF bulbs -- there are other factors that contribute to less than optimal lifetimes).
her reasoning...
The highest power a circuit in my apartment can provide is about 1800 watts as all of the lights are on 15 amp circuit breakers (power is current times voltage). Since turning on a lamp doesn't trigger a circuit breaker, 1800 watts is an upper limit for the startup power of a CF bulb. The bulb needs about a second to warm up, so the startup energy for a CF bulb has to be less than 1800 watt-seconds. My apartment has four CF bulbs on a circuit and you can turn them on with the circuit breaker switch and not overload it, so the start up power per bulb upper limit is 450 watt-seconds. The bulbs are 25 watts each, so it would take them 18 seconds of normal use to equal the 450 watt-seconds of startup energy. The worst case is the break-even point is 18 seconds and is probably much less. It makes sense to turn off the lights when you leave the room.
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1 By candlelight a goat looks like a lady
- Thanks to Juliette for the right words. I'm a poser when it comes to French.
2 As rechargeable non-chemical energy storage devices the lithium-ion polymer battery does an excellent job compared to most other technologies. Moving a 175 pound person about 80 feet vertically stores about 5.25 watt-hours of potential energy in the person.
Two people asked for detail on the power transfer involved in eating -- so here is a calculation:
Since it is Halloween day one observes a lot of people can eat a candy bar in about a minute. A Snickers bar has about 280 Calories of energy - that's about 325 watt-hours or 19.5 kilowatt-minutes. Power is just the rate of energy transfer so if you eat it in a minute the power is simply 19.5 kilowatts. Note that the electric element on your range is about 1.8 kilowatts. Another way of looking at it is to note that is about 26 horsepower.
People are very efficient when it comes to transferring energy into our bodies.
Posted by: steve | 10/31/2011 at 12:14 PM