January 2001. Its dinnertime on the space station. Cosmonaut Ivanovich makes his way floating gently into the Zvezda module where the galley is stocked with prepackaged foods and drinks, a table, eating trays and utensils, a food warmer, and a water dispenser which doubles as a rehydration station for dehydrated foods. Opening the food storage bin, Ivanovich looks through his personal stash of food clearly labeled with color-coded stickers and selects dehydrated shrimp cocktail and macaroni and cheese, canned, irradiated beef steak, thermostablized fruit cocktail, and strawberry punch. He selects his color-coded eating tray, knife and fork, and a pair of scissors to open the packages. Being careful to not let anything float away, he places the tray on the table using the provided Velcro straps and attaches his metal utensils to the top of the magnetized tray. Then he opens and rehydrates the shrimp and macaroni and places them together with the can of beef steak into the food warmer. He will have to wait for 15 to 20 minutes before the food is sufficiently warmed.

When the space station is fully assembled in a few years, this makeshift, temporary galley will be replaced by one which has an oven, a freezer, and two refrigerators. For now, the astronauts must make do with food stored at room temperature and a single food warmer. Let's examine the food warmer a little more closely. It is a small, portable unit much like a medium-sized ice chest which plugs into a special A/C outlet in the galley. It has room to heat fourteen food packages at once, and uses a warming plate to heat the food. When Ivanovich flips the warmer's switch to "On" he doesn't think twice about the process which brings the necessary power to the warmer, or how the warmer even works.

The warmer, like the microwave or blender in our homes on earth, is made to draw a certain amount of current from an existing power circuit. All it takes is two wires running from the appliance to plug into and connect with two wires in the circuit. If the circuit is "live" then the appliance will be able to draw power.

Although Ivanovich may not be thinking about the power processes on the station while he fixes his dinner, he is fully aware, thanks to his training, about what to do in case of a power system problem.

The goal of the remainder of this article is to help you become aware of what Ivanovich already knows about the power systems on board the station. To have a full understanding, you must learn three concepts:
• The fundamentals of electricity
• Electrical circuits: the power source, conductors, and power loads
• Watts

Fundamentals of Electricity
Electricity, Electrons, and Atoms. Everything, from wood to metal to gas to human beings is made of atoms. Within an atom are three basic components: the neutron, the proton, and the electron. The smallest of these components is the electron, which carries a negative electrical charge. The electron's electrical charge is the smallest quantity of electricity that exists. The hydrogen atom has one electron. The ununbium atom has 112 electrons. The electrons orbit around the nucleus of the atom in layers called energy levels. The ununbium atom has seven energy levels, some with two electrons and some with 32 electrons.

In the nucleus of the atom are neutrons and protons. Neutrons have no electrical charge. They are electrically neutral. Protons have a positive electric charge. In atoms, the positive and negative electrical charges are strongly attracted to each other. The positive protons in the nucleus and the negative electrons are attracted to each other. This attraction is similar to the attraction between the opposite poles in two different magnets. There is also a strong opposing force between two charges of the same kind. Two negatively charged electrons will repel each other and so will two positively charged protons.

In a stable atom the positive charge on the nucleus is exactly balanced by the negative charges on the electrons. It is possible, under certain circumstances, for an atom to lose one or more of its electrons. When this happens, the atom has a positive charge because it has "extra" protons. The result is called an "ionized" atom and, in this case, it is a positive ion. It is also possible for an atom to pick up one or more extra electrons. This atom would then also be called an "ionized" atom. It would be a negative ion.

A positive ion with its positive charge will attract any stray (negatively charged) electron that is nearby. One way it can do this is to attract a single electron or the extra electron from any negative ion in the vicinity.

Some atoms, especially copper, silver, and gold atoms, among others, give up their electrons more easily than others. Imagine a line of copper atoms. When we apply a positive electrical charge to one end and a negative electrical charge to the other end, we initiate a flow of electrons through this line of atoms. One electron from the outer layer of one copper atom will move to the outer layer in the next atom in the direction of the positive charge. It is almost like dominoes falling. This movement of charge from atom to atom is what we call an electric current.

Dominoes falling may be a bad analogy. It is far too slow a process. The flow of electrons along a wire happens very quickly. With dominoes you must wait for the action of falling to propagate along the entire length of the line of dominoes. The longer the line, the longer it would take for the falling action to reach the end of the line. With electron movement, it is more like a tube filled with ping pong balls from end to end. As soon as a ball is inserted in one end, a ball falls out the other end no mater how long the tube. This is why it takes virtually no time at all to light all the lights on a Christmas tree when you plug in one plug-no matter how big the tree.

Electrical Circuits
Think back to that line of copper atoms again, each giving off one electron and then immediately receiving one from its neighbor. In order for current to flow like this, there has to be a something which attracts the initial electron from the last atom in the line. Think of this as the power consumer, also know as the "load". A load can be fan, a stereo, a light, or a food warmer. But in order for more electrons to flow, and for the leap frog effect to work, there also has to be a "source" of electrons. The source can be a battery, a photovoltaic (PV) cell in sunlight, or a generator. These three items together make up a basic circuit: the power source, the conductors (usually wires), and the load. Each of these three is discussed below.

The Power Source
What makes a good source of electrons? On the space station, one source of electrons is the PV arrays. Energy added to the arrays in the form of sunlight is converted to a flow of electrons which in turn is stored in batteries. Batteries are also a good source of electrons. The electrons added to the batteries from the PV cells are "stored" by the chemicals inside which are specially chosen because of their ability to accept extra electrons. On earth, another source of electrons is called magnetic induction. Electrons may be induced to flow in wires by the spinning action of a nearby magnet. The spinning action may be caused by any number of mechanisms including hydroelectric dams, steam driven turbines inside a coal, oil, or nuclear-powered electrical plant, or a gasoline-powered electric motor.

The electrons flowing in an electric circuit move through the molecular structure of the metal wire, bouncing into each other and into other atoms in their path. The bouncing electrons create heat and may cause energy to be dispersed in the form of light. Too much bouncing causes the wire to glow a bright red. The red, hot glow is electromagnetic energy in the form of red visible light and infrared energy.

In each of the pieces of technology mentioned above, electrons are being freed from the outer energy levels of atoms.

If you have ever installed a battery, you may have noticed the two markings "+" and "-". These represent the two poles of the battery. At which pole would you find electrons? Electrons have a negative electrical charge. A negative charge is represented by the symbol "-". Electrons gather at the negative, or "-" pole of a battery. Atoms with an electron missing are more plentiful at the "+" pole of the battery. (Two batteries in a flashlight have to be lined up correctly or the electrons will not flow and the flashlight won't light.)

Electrical power plants produce, of course, many times more free electrons than simple batteries, but power plants, like batteries, also have a "+" and "-" pole, or terminal. The free electrons produced by a power plant also gather at the power plant's negative terminal. Atoms without electrons, or positively charged, ionized atoms, are more plentiful at the "+" terminal of the power plant.

The greater the difference in the negative and the positive electrical charges between the two terminals, the greater the electromotive force that is being created. The electromotive force determines the electricity's potential power. The term power is another way of describing the amount of work that the electricity in a system can do. You will read more about electrical power and work when you get to the section on watts.

To learn something new, it helps to compare what we know with what is new. This is especially so when learning about electricity. You are probably familiar with flashlight batteries or the batteries in a boom box or hand-held video game. Flashlight batteries create an electromotive force of 1.5 volts. The voltage is printed on the label. You have, also, probably seen an electrical power plant. In the case of water, oil, coal, or nuclear power plants, the electromotive force (difference between + and - terminals) can be measured in tens of thousands of volts. The PV arrays of the space station create an electromotive force of 168 volts. The space station's batteries create 124 volts of electrical energy. Either a 120 or 240-volt electrical service, services the home in which you live.

You probably haven't seen the "power plant" in the Earth's magnetosphere. Nobody has. The electromotive force that can be generated by an electric field in the magnetosphere can be greater than 100,000 volts. This electromotive force is created when electrically charged particles from a coronal mass ejection cross the magnetic field lines of the Earth. The magnetic field lines are highly concentrated at the earth's poles, like the poles on a magnet. The crossing could create a huge flow of electricity if there was good enough conductor to bring it to the surface of the earth. On occasion, under certain atmospheric conditions, the flow of charged particles will travel down the magnetic field lines causing disturbances in the atmosphere (the aurora borealis, or Northern lights) or on the ground in power stations.

Conductors
For electricity to flow from the power source to the power load, there has to be some way of conducting the electricity. The most common way is to use wires- which have been designed to make use of the special conductive properties of certain atoms. Remember that some atoms give up their electrons more readily than others. Materials that give up their electrons easily, and which make excellent electric wires, are called "conductors." The following materials have atomic structures that make them good conductors.

• Metals: gold, silver, copper, zinc, steel, etc.
• Carbons
• Acids
• Water

Gold is one of the best conductors. Gold is also very expensive, which is why electrical wires are commonly made of copper. It is far less expensive than gold, and it has excellent conductive properties. A house full of gold wires would be quite a temptation.

Every material that conducts electricity also offers some degree of resistance to the flow of charges. Remember that as energy is added to a system, it may be transformed in a variety of ways. Some of the energy may take the form of electrons moving from one atom to another atom. But energy may also create greater vibration among the atoms of the conductor and generate heat. Every conductor will give off some level of heat as electrons move through it. Superconductors are made of materials which at low temperatures have less resistance to the flow of electrons. Some materials which conduct electricity may also have enough resistance that not only do they get hot, but some of the energy is given off in the form of light. Think of a lightbulb. The filament of the lightbulb conducts electricity, but offers enough resistance to generate light (and heat). Think of the red-hot top of an electric stove. The redness is energy being emitted by excited atoms!

Some materials are made of atoms that resist giving up their electrons under all but the most extreme conditions. Here is a list of common materials, most of which consist of atoms clustered in very complex molecular structures. These materials make up a list of what scientists call insulators.

• Dry Air
• Wood
• Porcelain
• Glass
• Rubber
• Plastic

Insulating materials play a very important role in the electrical world. Plastic, for instance, is used to wrap the copper strands in wires. The plastic keeps us from getting shocked every time we plug in a hair dryer or an electric razor. A car's rubber tires also act as an insulator for passengers during a lightning storm. Glass materials are woven into gloves used by electricians who work with heavy power lines. Porcelain is used on telephone poles and power lines to minimize the risk of electric shock.

Semiconductors, on the other hand, are materials that can conduct electricity under some conditions but not others, which makes them effective for the control of electrical current. Using semiconductors allows for the precise design and control of current within a power circuit. Computer chips, silicon wafers used in PV cells, and the layered carbon disks used in transistors are all examples of semiconductors. In the last 50 years, semiconductors in computers have literally changed the course of mankind.

Power Loads
Electricity flows around a circuit just like track athletes run around a quarter mile track. A running race has a beginning and an end. An electrical circuit has a beginning, the negative pole or terminal of a battery or power plant, and an end, the positive pole or terminal. The circuit's track is usually a wire, sometimes a heavy cable. A circuit also has to have a load, something for the electricity to do during its journey, like light a light or turn a fan or heat an oven. Officially, a circuit doesn't need a switch. But as in every well-run race, electrons follow commands and are well-behaved if handled with respect. They like to be told when to start and when to stop. A switch is the starter and race judge, all in one.

There are a number of different kinds of circuits, usually to handle different types of power loads. A flashlight is a simple circuit of a power source (1-2 batteries), connectors (usually copper strips), and a load (the lightbulb). The circuit is designed to provide a steady amount of current to the filament in the bulb at a level which will keep the bulb bright. What happens as the batteries run down? The bulb begins to dim. Flashlights which use less current (like a keychain light) have been designed in accordance with the load, and can produce a similar brightness of light to the bigger flashlight, although for shorter periods of time.

The mark of a good electrical engineer is the design of circuits to handle the appropriate power loads. In your home, you have a number of circuits of electricity, all attached to a main power line outside. You have a circuit breaker box which has several kinds of circuits designed for the expected loads. In newly constructed homes, high power loads such as washing machines, electric dryers, or air conditioners are often given their own circuits. In places like kitchens, bathrooms, and garages where appliances, hair dryers, and power tools may need a lot of power, there are often multiple circuits for the one room.

When you plug something in, you are attaching it to a circuit. When you have too many power loads on a circuit operating at once, you may cause the fuse to blow or the circuit breaker to trip, stopping the flow of electricity. This is a safety measure put in place by the designers of the home's circuits. This safety measure may prevent fires by limiting the flow of electricity through the wires. Too much electricity too fast may cause overheating of the wires (remember resistance!), eventually leading to cracking the plastic sheathing and creating a source for sparks.

The space station also has a circuit breaker system run by computers. One of the critical circuits on the space station is dedicated to the life support equipment. The appliances in your house and the equipment on the space station all represent the load on the different circuits.

Regardless of which circuit we are considering, the main power grid, the circuit to your stove, even the circuits within your dishwasher or clothes washer, the electrons flow from negative to positive, all stop when a switch is thrown, and all can only "finish the race" if the circuit is switched on.

Watts
Power is measured in watts (W) or kilowatts (kW). Look at the end of a light bulb. It tells you how many watts of power are needed to make it shine.

Every circuit has a load. If all of the appliances and lights attached to a circuit are turned on then a lot of electrons are needed to get everything to work. The circuit needs a lot of watts of electrical power.

On Space Station Alpha, each circuit can be defined by the number of kilowatts required to get everything on that circuit to work. The PV arrays of the space station can produce 24,490 watts per hour, or 24.49 kilowatts every hour. This has to be divided amongst all of the circuits and loads on the station. Computers regulate this division of power. During an emergency, the division of power is done manually, upon recommendation of Mission Control and the Mission Specialists.