Open and Closed Circuits

A closed circuit is one in which a series of device(s) complete a connection between the terminals, and charge is allowed to flow freely.

An open circuit is a section of a circuit for which there is no connection. Current does not flow between the terminals of an open circuit, although a voltage may be applied between the terminals, and a capacitance may exist between them. At steady state, there is no current flow in an open circuit, and most examples will assume that there is no capacitance between nodes of an open circuit, for simplicity.

"Shorting" an element

We will often hear the term "shorting an element" in later chapters of circuit analysis. Shorting a circuit is equivalent to placing an ideal wire across the terminals of the element. Because current will take the path of least resistance, shorting an element redirects all current around the element. Because the potential difference between the terminals of the device is zero, no current can flow through it . This practice must be done with care, because reducing the resistance of a certain portion of a circuit to zero can theoretically raise the current to infinity, and the circuit will become damaged.

Measuring instruments

Voltmeters and Ammeters are devices that are used to measure the voltage across an element, and the current flowing through a wire, respectively.

Ideal Voltmeters

An ideal voltmeter has an infinite resistance (in reality, several megaohms), and acts like an open circuit. A voltmeter is placed across the terminals of a circuit element, to determine the voltage across that element.

Ideal Ammeters

An ideal ammeter has zero resistance (practically, a few ohms or less), and acts like a short circuit. Ammeters are placed in-line in a circuit, so that all the current from one terminal flows through to the other terminal. By convention, current into the + terminal is displayed as positive.

Sources

Sources come in 2 basic flavors: Current sources, and Voltage sources. These sources may be further broken down into independent sources, and dependent sources.

Current Sources

Current sources are sources that output a specified amount of current. The voltage produced by the current source will be dependent on the current output, and the resistance of the load (ohm's law).

Voltage Sources

Voltage sources produce a specified amount of voltage. The amount of current that flows out of the source is dependent on the voltage and the resistance of the load (again, ohm's law). This can be dangerous because if a voltage source is shorted (a resistance-less wire is placed across its terminals), the resulting current output approaches infinity! No voltage source in existance can output infinity current, so the source will usually melt or explode long before it reaches that value. This is an important point to keep in mind, however.

An example of a voltage source is a battery, which is specified as being "9V" or "6V" or something similar. The amount of current that the circuit draws from the battery determines how long the "battery life" is.