eNotes: Electric and Electronic Circuits

## 3.1 CIRCUIT COMPONENTS AND QUANTITIES

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Although in reality circuits involve complex interactions of potential and magnetic fields, we tend to simplify components into discrete and independant parts.

Typically each simple circuit component will act as a "black box" with an applied current creating a voltage, or an applied voltage creating a current.

Current and voltage are very important terms that are not well understood by the beginner. Consider an electron/proton pair. If both are together they are stable and steady. If we separate them they exert a force of attraction, much like gravity. This potential of attraction is called voltage. If we create a channel for these electrons to flow back to the protons (electrons are much lighter and more mobile than protons), the flow of electrons is called a current. The electrons do not flow freely, the restiction on flow is called resistance.

Resistance is the simplest of all circuit elements, and is found in all circuit elements, but there are a variety of other simple elements found in circuits,

- capacitors
- inductors
- voltage sources
- current sources

All of us have seen an electromagnet at least once in our lives. This is effectively a large inductor. The device is best described as resisting current flow changes (almost as if preserving the momentum of the current). The resulting relationship is,

When you get a static shock you are touching a basic form of capacitor. An electrical capacitor typically allows current to flow freely when a voltage is applied, but the current will quickly reach a steady stae. The relationship is,

Voltage sources are also very common. Disposable batteries (e.g, 1.5V, 9V) are one good example. When we use these normally we assume that the batteries will supply any amount of current, at the rated voltage. The schematic symbol is shown below,

A similar method is used when considering current sources

Some theoretical treatments of circuit elements make use of dependant (variable) voltage and current sources. The schematic symbols are often as shown below,

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