I've got a hunch that many readers of this blog have been asking themselves: "When is someone going to write about entropy?"
You're in luck, because the answer is 'now'.
Entropy is a quality of a closed system which can never decrease, according to the third thermodynamic law, which is called-- no kidding-- the Second Law of Thermodynamics. The numerical confusion is due to the inclusion of a "zeroth law" in the 1920's and an inability to modify older works owing to the shortage of copy-monks in the twentieth century.
So the name isn't perfect. In fact, calling it a "law" hardly does it justice, as this invites comparison to entities of lesser stature. For example, the so-called 'law of gravity' (technically a theory) need not apply in all times, places, and theoretical universes. It is even possible (if unlikely) that income tax laws might not exist in some other strange, wonderful version of reality.
But the Second Law is omnipresent and immutable. It cannot be undone even by divine intervention, as such intervention would nullify the 'closed system' caveat. Appropriately, the Second Law itself is like the Godhead of the Holy Trinity, expressible as the Clausius statement, the Kelvin statement, or the Carathéodory statement, each of which is distinct from the others but nonetheless equivalent to the Second Law.
Are you impressed yet? I know I am. So now let's get down to some practical advice. It's summertime in the northern hemisphere of our planet, and not everyone can (or wishes to) afford super-powered air conditioning. As we cannot defy the Second Law, we must understand and embrace it to maximize our comfort.
First of all, any respectable physicist will tell you that "entropy" is not the same thing as "temperature", or even "heat energy". But being a humanitarian, that same physicist will give the layperson permission to pretend like it is (provided that you check back with the physicist every now and then to avoid serious errors, and that you at least consider inviting him to the sort of parties that respectable physicists usually miss).
So now you know: The heat energy of a closed system cannot decrease. And now you know why your air conditioner is mounted in the window or else installed outside your house. In the process of cooling the inside air, it has to dump heat energy outside. If you stuck an air conditioner in the middle of your home, it might feel cooler right in front of the business end, but the net effect on your house would be an increase in temperature. So don't do that.
Some more tips and trivia:
Every joule of energy that crawls up an electrical cord is eventually going to decay into heat, most likely inside your home. A sixty-watt light bulb is a sixty-watt heater, minus a few stray photons that sneak out the window. The same is true for computers, televisions, vacuum cleaners, and robots. I certainly don't recommend turning off your computer at the moment, but appliances and lights which are not in use ought to be shut down; and you should have your robot do the vacuuming at night.
Sound is a form of energy. The louder your stereo, the more energy it produces. But if your neighbor complains about the noise, you can take comfort in knowing that some portion of the stereo's energy production has escaped the premises before decaying into heat, and your neighbor is running his air conditioner to compensate for that energy. Tee hee.
One tricky appliance is the water heater. It produces a lot of heat, but the amount that bleeds into the surroundings depends on how the hot water is used. I recommend showers, which allow the water's heat energy to be carried down the drain, whereas a hot bath will release its energy into the surrounding air.
Fans are appliances. They use electricity. They make the house hotter. So do the arguments I've been having with delusional fan fans for the past few decades.
But a fan can make you cooler, which is worth the cost of making the house slightly warmer. Airflow enhances the skin's evaporation of water (a.k.a. "sweat", "perspiration" or "gleam", depending on whether you're a horse, a man, a woman, or a lady). More importantly, it increases convection. Your body temperature is around 98.6 degrees fahrenheit, which means that when you're walking around on a 90-degree day, you are technically cooling off-- you just aren't cooling off fast enough. Airflow speeds this up.
Ergo: Don't leave the fans running when you aren't around, and don't put up eight different fans thinking it will help. Use a low-powered fan and point it right at your face.
Also, if you have a ventilation fan (like the one in the bathroom), unless you know that it's drawing air from some ultra-cool cellar, try to keep it turned off. Even if it feels like it's creating a breeze, it's almost certainly sucking in hot air from outside. And don't think you can stop this by plugging all the cracks-- if air flows out, air must flow in.
I once knew a girl who hung black blankets over her windows in the summertime.
I thought it was both hilarious and tragic. But once I stepped outside of my "respectable physicist" role (hoping to be invited to a party, which I wasn't), I understood why someone would try this. It's cooler at night, it's cooler in the shade, and barring the existence of some magical technology (like transparent glass) we can expect darkness to be colder.
It certainly was dark, but holy crap it wasn't cool. Light-colored objects reflect light, and darker objects absorb it. That's why they're light and dark. Obvious enough, but perhaps not so obvious is the fact that 100% of the absorbed energy will be released back into the surroundings, mostly by heating the surrounding air.
A light-colored barrier will reflect many of the photons back through the window. That's why window blinds are white. So use those. If you require more blockage, hang a white sheet over the window. Better yet, get an exterior window blind, which will not only reflect light, but vent that small amount of energy which it does absorb on the outside of your home.
And if you're a day-sleeper like me, or just want it to be dark, tape some aluminum foil over your windows. It reflects even better than the blinds.
Heat doesn't rise.
Hot air rises over cooler air. That's not a tip, just a misconception that annoys me.
The exact opposite of summer. More or less. If you use electrical heat, and if you're running the heat almost all day anyway, then wintertime is party time. Your lights, computer, and television essentially run for free, because every joule they consume will decay into heat, thus requiring one less joule of power consumption from your heater. This isn't necessarily true if you have a cheaper form of heat energy, like natural gas, but the electrical appliances will still run at a discount.
Also, take a hot bath instead of a shower. Then don't drain the tub until it's cooled down to air temperature, thus getting double-duty out of all the energy you bought to heat your bathing water. Which brings us to...
Borderline Obsessive Techniques
If you live in an arid environment where the summer nights are cold, stow a dozen garbage cans filled with water inside your house. These will cool down at night, then absorb heat energy during the day. Actually, the addition of any bulk material to the interior will help reduce costly airflow (just like it helps to keep the refrigerator mostly full, so less hot air slips in when you open the door).
Use all the slots on your toaster. If you only want one slice for now, pop an english muffin into the other slot and save it for later. Reheat the muffin by placing it on a sheet of foil and setting it on the porch.
Clean the litter box immediately after use, then haul the waste to the outside trash. It contains heat energy from the cat's body! Conversely, let it sit in the winter until it reaches air temperature.
If you live in an apartment with a downstairs neighbor: During the winter, when you rent a movie, loan it to your neighbor afterwards. This will encourage them to stay inside and burn electricity. In the summer, buy their kids a cheap soap bubble wand that will encourage them to play outside, venting their body heat into the yard instead of (ultimately) your floorboards.
Never kiss a toad
...unless you are a cartoon character. Even if it does turn into a prince, what good does that do you? And did it occur to you that someone might have turned him into a toad for a reason?