#### Basics of Voltage and Current

Two basic electrical measurements are voltage and current.

We can compare electrical voltage with water pressure. The 12 volts (V) in your automobile is a relatively low electrical "pressure." By analogy, low water pressure could be used in, say, a drinking fountain. The 120-volt service in United States homes provides a higher electrical "pressure." If you were to increase the water pressure in your drinking fountain tenfold, and the pipes didn't burst, the water might squirt across the room when you went to take a sip.

Current is analogous to the water's flow rate. The small pipes in a drinking fountain carry a small current of water -- only a liter or so per minute. Large pipes supplying a fire hydrant can carry a substantially higher "current," perhaps thousands of liters per minute.

You can vary the flow rate in your drinking fountain by changing the pressure, and this is true in electrical circuits as well: increase the voltage supplied to a resistor such as a lightbulb, and more current will flow through it. You could continue to increase the voltage until the increasing current overloads and burns out the light bulb (bursts its "pipes").

Current, I, equals electromotive force (voltage) E, divided by resistance, R. Current is measured in Amperes (also called Amps), A, electromotive force in Volts, V, and resistance in Ohms, Ω.

 I   = E --- R

If the voltage across a resistor increases, the current will also increase. If the resistance increases, the current will decrease.

You wouldn't use drinking-fountain-sized pipes to fight a fire. Likewise, it takes larger-diameter electrical wires to safely carry a larger electrical current.

Electrical current is measured in amperes (A). The 12-volt fan in your automobile might consume 1 A at its low setting, and 4 A on high.

Volts multiplied by amperes gives a convenient single measure of power: watts, abbreviated W.

W = VA

Alternatively, watts are called volt-amps, VA.

If a current flows steadily in one direction, as it does right out of a battery, or RTG, or solar panel, it's called direct current, DC. If it flows back and forth as it does in most residential service, it's called alternating current, AC. Some components on a spacecraft may require AC, so they use an inverter to change DC to AC. If you need to change AC to DC, you'd use a rectifier. It's easy to step voltage either up or down if it's AC, using a simple wire-wound device called a transformer. DC can be stepped down in voltage, and that voltage held constant, using a voltage regulator.