3.1 What is resistance?

We now move on to Module 3 of the Navy Basic Electricity and Electronics series. The first two modules were:

1. Electrical Current
2. Voltage

Today starts the booklet on resistance.

• If current (electron movement) and electromotive force (resulting in voltage, a difference in potential throughout a circuit) are two key elements of a circuit, resistance is a third factor always present.
• "Resistance is the property that opposes current flow" (6).
• Resistance is symbolized by R and the unit is the ohm, symbolized by Ω.
• Conductors have low resistance, insulators or non-conductors have high resistance.
• Some factors that determine resistance include the nature of the material (atomic structure), the area of the cross-section (e.g., of wire), and the length (e.g., of wire). The larger the cross-section, the less the resistance. The longer the wire, the more the resistance. Atomic structure determines how many free electrons are available to move around.

That's the big picture. Here are some additional details.

• One ohm is the amount of resistance that allows 1 amp to flow through a circuit when one volt is applied. 1 amp per volt.
• Resistance is sometimes needed to limit the current flow to a safe value through a circuit. Completely unopposed current can lead to a "short circuit," which is damaging. 
• The resistance of a toaster or iron or bulb is what provides the heat or light.