The Arduino Inventor's Guide Read Free Page B

positive charge. Though technically the electron is the part of the atom that can be moved, it is still common to refer to current as the movement from positive to negative.
    Except for lightning, arc welding, and the odd static shock, we don’t normally see electricity directly. Even the bright light we see in lightning is merely the air molecules changing form as charges move through them.
    Charges move when there is an electrical force that acts on them and a path for the charges to move along. That electrical force is created by an
electrical potential difference
, or what we commonly call
voltage
. Voltage is what ultimately causes charges to move, and it’s measured in volts (V). For a reference, typical batteries range involtage from about 1.5 V to 12 V. A 12 V battery will cause charges to move faster than a 1.5 V battery.
Types of Electricity
    In general, electricity can be broken down into two basic types:
direct current
(DC)
and
alternating current (AC)
. AC is the kind of electricity in the power lines outside your house and in your wall outlets. AC electricity is great for power generation (for example, power plants), transmitting power over long distances (like from the power plant to your home), and driving large devices (like motors and heaters). We don’t use AC electricity for most of our household electronics, however. Most small appliances and household electronics that plug into the wall outlet require DC electricity and use a
transformer
to convert from AC to DC. Further details on AC and DC are beyond the scope of this book, but the projects you build here will focus on DC electricity.
What Is a Circuit?
    Even with the electrical forces pushing them, charges need a path to follow from a point of higher potential to a point of lower potential. The path by which charges move from the positive (+) side of a battery (high potential) to the negative (–) side of the battery (low potential) is called a
circuit
. A circuit consists of a closed path from the positive terminal to the negative terminal through a device such as a light-emitting diode (LED), resistor, light, or motor. Figure 1 shows a simple circuit containing an LED, a battery, and a resistor. Notice that the shape of the circuit loosely resembles a loop or a circle, hence the name
circuit
.
    FIGURE 1: A basic DC circuit

    In order for charges to move, the path must be made out of a material that is
conductive
. Conductivity is not an absolute measure but more of a continuum. While some materials are generally considered conductors and nonconductors, most materials occupy a range of values for conductivity. In other words, some materials allow charges to move more freely than others. Think of driving a car on different surfaces. On the smoothly paved interstate, you can go much faster than if you were off-roading or driving down dirt roads. Different roads allow for different speeds the same way that different materials allow for more or less conductivity. We use the term
resistance
to describe how much a material slows down the movement of charges.
Ohm’s Law
    As you may already have guessed, there is a relationship between current, voltage, and resistance. This relationship is commonly called
Ohm’s Law
, and it is represented mathematically as follows:
    V
=
I
×
R
    In this equation,
V
represents the voltage,
I
represents the current, and
R
is the resistance. (Don’t let this bit of math scare you: this is one of only about three equations you’ll see in this book.)
Visualizing Electricity as Water in a Pipe
    To understand what’s going on in a circuit, it’s useful to think of electricity like water moving through pipes. Imagine water flowing through a garden hose. When you turn on the valve, water starts to flow through the hose to the other end, as shown in Figure 2 .
    FIGURE 2: Water and electricity model

    The water molecules moving in the hose represent the flow of charges (current). If we turn the water valve up or down, we can