Lab 4
Resistance

A breadboard is an extremely useful device because it can be used to easily interconnect electronic components in an variety of ways. A breadboard is a set of formed, metal sockets inserted into a plastic housing which holds them firmly in place. Sockets called tie points are attached to connector strips built into the breadboard. The connector strips are visible on the back side of the breadboard.

The goal, today, is to use a breadboard to investigate the property of a material called its resistance. A series of resistors called a voltage divider will be explored. See Figure 1.

Figure 1. A Breadboard

Review Theory

The movement of charge, q, defines a current, I, where:

Current flows in response to the presence of an electric field. The field must be applied continuously or the current will stop. The direction of the current is defined by the movement of a positive charge in the field.

In many materials, the current density, J, is proportional to the strength of the electric field, E, that produced it. That is:

where the current density is defined as the current per unit area, and the proportionality constant, σ, is called the conductivity of the material. If the conductivity is independent of the applied electric field the material (e.g. copper or carbon) is called an ohmic conductor.

Suppose the electric field that drives a current, I, is created by applying a constant voltage, V, across an ohmic conductor of length, L. From the previous equation:

The constant of proportionality between the voltage and the current is called the resistance, R, where:

The resistance is usually expressed in terms of the reciprocal of the conductivity which is called the resistivity of the material, ρ,:

The unit of resistance is called the ohm (Ω).