Lab 5
DC Circuits

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 conservation of energy in simple DC circuits. A common household problem will also be explored. See Figure 1.

Figure 1. A Breadboard

Review Theory

In order to run any electrical device, no matter how simple, it is necessary to make a complete circuit. In other words, there must be a flow of charge from a power supply, through one or more circuit elements, and back again to the power supply. If an open circuit exists, the flow of charge is interrupted and the circuit elements will not function. A switch can be used to control the flow of charge in a circuit. The flow of charge is called the current and is given the symbol, I. The current is measured in a unit called an Ampere, A, where:

A power supply provides a potential difference (also called a voltage, or a voltage drop) which causes charge to flow. Potential difference is measured in a unit called a Volt, V, where:

It may be helpful to think of the power supply as a pump, and the voltage as the pressure it produces. If a pump is connected to a pipe that contains water, the water will flow (analogous to the current in an electric circuit). If the pipe becomes obstructed, the water will experience a resistance and will not flow freely. The resistance in an electric circuit is caused by an obstruction to the flow of charge. Resistance, R, is measured in a unit called an Ohm, Ω, where:

All circuit elements (except superconductors) have some resistance at temperatures above absolute zero. The wires that connect the elements of a circuit together are called conductors because they are made of a metal (usually copper) which has a very low resistance compared to other materials. Some materials like silicon and germanium have a much higher resistance than metals and are called semiconductors. Other materials like glass, sapphire, plastics, and ceramics are called insulators because they have an extremely high resistance.

The voltage, the current, and the resistance in a circuit are interrelated. For a given voltage, a higher resistance means less current will flow. An equation called Ohm's law relates the voltage, the current, and the resistance. Ohm's law applies to most materials of interest and states that:

In all electric circuits, the power supplied by all sources must equal the power dissipated by all components. This is a statement of the conservation of energy, one of the most powerful and universal laws of nature.

Power is measured in a unit called a Watt, W, where:

From the definition of the volt and the ampere, electric power, P, becomes:

Ohm’s law can be used to substitute for either, I,  or, V, giving two equivalent equations for the power generated or dissipated by the components in a circuit: