Machines are so complicated these days it’s difficult to quickly explain how they work. Nonetheless, today’s machines were built using the basic principles of physics that we began harnessing hundreds of years ago.
Washing machine invented by E. Koers. Nationaal Archief, Netherlands 1951.
Work as energy
Steam turbines: more relevant than you might think
Toasters and other electric heaters
Refrigerators and Air Conditioners
WORK IS THE PHYSICS LINGO FOR USING ENERGY
When talking about machines — or anything else — that uses energy, we call the energy used to do an activity work. If you lift up a hat to put it on your head, you do work on the hat. Likewise, if a giant picks you up, and places you on his head, he does work on you.
Because work is a way of thinking about energy, it is measured in the same units (joules, ergs, kilowatt-hours or megawatt-hours), and it is equal to the cross product of the force needed and the total distance traveled; more simply it is the value of the force in the direction of travel multiplied by the value of the distance.
For example, if you push a malfunctioning shopping cart:
Work (you do on the cart to move it) = the force (you push it) x the distance (it goes).
However, the only part of the force that counts is the component of it going in the direction that the cart goes.
Power is work divided by time. If you want to do the same work twice as fast, you need twice the power. Power is measured in watts, kilowatts (1,000 watts), and megawatts (1 million watts).
THE STEAM TURBINE: PERHAPS THE MOST IMPORTANT ENERGY-RELATED DEVICE, EVER [ok, that isn’t the catchiest]
A turbine is a rotating wheel or rotor that converts one source of power to another source of power. A turbine can be rotated using fast-moving fluids or gases, like wind, water, and steam. Windmills use turbines powered by wind.
Steam is like an energy middleman between the many fuels we have and the electricity we want. Using steam may seem like an unnecessary diversion, resulting in some loss of energy, but steam turbines are still generally the best option, and they are used to generate most of the electricity in the nation.
Some version or another of steam turbines have been around for hundreds of years, and today we depend on them for any type of combustion-related power as well as for nuclear power plants and solar panels.
There are a lot of versions and sizes of steam turbine, but they basically function as follows:
1. A source of energy heats water into steam.
Example a: in a nuclear reactor, hot nuclear fission products are several hundreds of degrees Fahrenheit and provide a source of heat to make steam.
Example b : at a coal plant, solid coal is combusted. The coal fire provides heat.
Example c: at a solar thermal power plant, sunlight heats specialized fluid to a high temperature. The hot fluid is sent through pipes to circulate through water, heating the water into steam.
2. Hot, fast-moving water molecules in the steam do work on the turbine, which is made up of a circular set of blades. There are many different possible shapes, such as fins or buckets. The kinetic energy of the steam pushes the turbine blades, cause the whole turbine to rotate around its central shaft.
3. The rotating shaft is connected to a generator, which itself converts theenergy of the rotation into kinetic energy of electrons, electricity.
HOW DOES MY TOASTER WORK?
As discussed above, generating electricity often involves heat. What if you want to do the opposite and make heat with electricity?
That’s just what’s done in electric toasters, ovens, irons, hair dryers, and space heaters. It’s called resistive heating.
Electricity is the flow of electrons in a conducting material. In the case of electric toasters, we send electricity through metal alloys that don’t really conduct electricity that well; the metals have a high resistance (relative to good conductors like copper). Nonetheless, electricity will flow through the metal coils of the toaster, even as it encounters the resistance.
On an atomic level, electrons are losing their kinetic energies as they collide with other subatomic particles inside the metal alloy heating element. That kinetic energy, or work, is transformed to thermal energy of the heating element; the atoms vibrate with the thermal energy, and since they become hotter than their surroundings, heat is transferred, usually to the adjacent air molecules.
Many devices use a fan to direct the hot air molecules at whatever is supposed to get hot, like at your hair, in the case of a hair dryer.
HOW TO MAKE THINGS COLD: REFRIGERATORS AND AIR CONDITIONERS
The fundamentals of the thermodynamics of refrigerators is complex, but simply put, refrigerators exploit some funny characteristics of gases to move thermal energy from inside the refrigerator to the outside.
The funny characteristics are that, under the right conditions, a gas just plain gets hotter when you compress it. The reverse is true too. Let the gas expand and it gets cooler.
Refrigerators use materials that can go from liquid to gas and back easily in a temperature and pressure range that’s convenient. They used to use chloro-fluoro-carbons, and newer refrigerators use tetrafluoroethane.
In the fridge, the tetrafluoroethane gas gets compressed in the aptly-named compressor. It gets hot because it’s been compressed, so hot it’s warmer than the air in your house. Then it passes into the condenser, which allows the tetrafluoroethane to make contact with the air in your house (through conductive metal pipes). Because your house is cooler than the tetrafluorethane, heat passes through the pipe material and to the house. As the tetrafluoroethane cools, some portion of the gas condenses to liquid.
Then the tetrafluorethane is allowed to pass through a valve into a low pressure area, into which it expands suddenly, boiling into a gas and losing temperature rapidly to the point that it is so cold, it’s colder than the inside of the refrigerator. It passes through coils that interact with the inside air of the refrigerator, cooling it by absorbing thermal energy.
Then the tetrafluorethane returns to the compressor.
Air conditioners generally work according to the same compression-expansion cycle, but in the Western United States, where the outside humidity is often low, another type of cooling can be used.
Unlike cooling by compression and expansion, the human body uses evaporative cooling through sweating. If you’ve ever been in a desert climate in hot weather, you might have noticed that your perspiration dries instantaneously, and you don’t feel quite as hot as you’d expect for that temperature. That’s because the air is so dry, it’s easy for water to evaporate into it. Whereas, the same temperature in a more sweltering, humid climate would leave you feeling more overheated because, if the air is humid, with a lot of water vapor already in it, it’s far more difficult for liquid water to evaporate.
As water in perspiration evaporates, a small amount of energy is used just to convert it from a liquid to a gas, called the latent heat of vaporization. When the water turns into a gas and leaves your skin, it turns some of your excess body heat into that latent heat, ultimately cooling you.
The same happens with regular water sprayed on any surface in dry climates. That little heat energy can dramatically reduce the air temperature if the surrounding air is dry enough to allow a lot of water to evaporate.
Evaporative coolers — the most basic of which are also called swamp coolers — exploit precisely that use of latent heat, passing the air over water. The water absorbs heat from the air, using the thermal energy to evaporate. Evaporative cooling uses a lot less energy than air conditioning, but it doesn’t work well in humid climates.
Today’s most efficient coolers make use of both air conditioning technology and evaporative cooling in hybrid systems.
Read more about refrigerators and efficiency here.
- California Energy Commission
- U.S. Energy Information Administration
- National Aeronautics and Space Administration (NASA)
- Halliday, David; Resnick, Robert; and Krane, Kenneth S. Physics 4th Edition(John Wiley & Sons:December 27, 1991)