While the water is flowing through a pipe or hose, it experiences some loss of pres-
sure through friction. The same thing happens to electricity as it flows through a circuit:
there is friction that causes a loss in pressure. This friction is calledresistanceand is mea-
sured in units calledohms. The amount of water that flows through a pipe for a specified
length of time, such as the gallons-per-minute, is called thevolumeof the water flow. For
an electrical current, volume, or the rate of flow, is measured inamps.
Another electrical term you should know iswatt. The amps (rate of electrical flow) in
combination with the volts (pressure in a circuit) form the watts, or the electrical power
in a circuit. When an electrical circuit is open, there is no current or flow in the electricity.
However, there can still be pressure (volts) in the circuit. A standard household electrical
outlet has voltage (electrical pressure) just waiting for you to plug in a household appli-
ance, which completes the circuit and starts the flow. This is why you don’t want to stick
your finger into an electrical outlet. You would be supplying the extra circuitry to close
the circuit and cause the electricity to flow—through you, in this case. And because you
are not well insulated and you don’t make a particularly good conductor of electricity,
you feel the electricity flowing through you as a shock. The degree of shock you feel
when you close the circuit depends on the watts, or electrical power in the circuit. Please
do not try this at home (or work or on vacation, for that matter); this is one of those facts
of nature you should just accept as fact without question. Chances are you have already
tested it anyway.AC Power and DC Power
An electricalcurrentis a movement of electrons through a copper wire or some other
conducive property (more on this topic later). There are two types of electrical currents:
alternating current (AC) and direct current (DC).Alternating current(AC) changes the
direction of the electrical flow at a rate of about 60 times per second. The voltage changes
from a positive charge to a negative charge, causing the electrical flow to change direc-
tions. The electrical flow in adirect current(DC) keeps a constant pace and flows in the
same direction all the time. Direct current flows from a negative charge to a positive
charge and does not fluctuate. AC power is what you very likely have at your standard
householdelectricaloutlets(atleastintheUnitedStatesandCanada),butDCiswhatthe
computer must have to operate properly. Let me repeat this for you: the computer needs
DC power, but your wall outlet provides AC power. What’s wrong with this picture?
Inside your PC is a module called thepower supply,which converts the electricity in
your household circuit (HC) from AC power into the DC power that the computer needs.
Yes, this means that the HC provides AC, but the PC needs DC. All of the circuitry, the
electronics on the motherboard, including the microprocessor, the disk drive motors, and
all other electrical parts, require DC power to operate. It is the function of the power
supply to convert the AC power to DC power. Chapter 14 covers the power supply in
shocking detail.Chapter 2: Basic PC Concepts and Terminology^33