Page 120 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 5 Mechanical and Electrical Power, Work, and Energy 101

2. Alternating current (AC) comes out of our wall sockets in the United States. It
is like a wave of power that fluctuates between 0V and 120V 60 times per
second.
AC power is useful when electricity must travel long distances, like from the power
plant to our homes. Some appliances, like fans and blenders, use AC power directly to
run AC motors. Otherwise, for our purposes, DC power is most useful. Most modern
electronics use AC-to-DC converters (also called AC adaptors or DC power supplies)
that convert the 120V AC from American wall
sockets into around 5V to 12V DC that our
1
electronic components can use. The converter is in FIGURE 5-3 A simple circuit
the bulky black box on the charging cords for your
laptop and cell phone.

A circuit is a closed loop containing a source of
power (like a battery) and a load (like a light bulb or
motor), as shown in Figure 5-3. Current flows from
the positive (high energy) terminal of the battery,
through the light bulb or motor, back to the
negative (low energy, or ground) terminal of the
battery. If you put stuff, like a light bulb or a motor,
in the current’s way in a circuit, it has no choice but
to travel through the light bulb or motor until it
reaches a ground.

The resistance (R) of the load is measured in ohms
(Ω) and represented by a squiggly line in circuit
diagrams, as shown in Figure 5-4. You can think of
resistance as a transition to a skinny pipe put in line
after a fat pipe. All the current, or water, still must
go through it, but it resists the flow, so there is
higher voltage, or water pressure, before the
transition than after it (see Figure 5-4). Electricity
always follows the path of least resistance to
ground.