An electrical circuit requires an energy source to drive the flow of charge, much like a pump moves water through a pipe. This fundamental energy provider is known as a voltage source, which sets up the necessary electrical conditions. It functions by supplying electrical potential energy to the circuit, enabling current to move through the various components.
Defining the Core Function of a Voltage Source
A voltage source is an active two-terminal component that creates a potential difference (voltage) between its connection points. This potential difference is often called the electromotive force (EMF), which represents the energy per unit charge supplied by the source. The primary function is to maintain this steady “electrical pressure” across its terminals, forcing free electrons to move and establish an electric current.
For a direct current (DC) source, such as a battery, the voltage remains constant, pushing charge in a single direction. The water pump analogy helps visualize this: the source is the pump that creates pressure to drive flow. In a circuit diagram, voltage sources are typically represented by a circle containing polarity signs for DC or a sine wave for alternating current (AC) sources.
The voltage source converts non-electrical energy—such as chemical energy in a battery or mechanical energy in a generator—into electrical energy. It continuously moves charge from a region of low potential energy to one of high potential energy. This process ensures a continuous supply of energy is available to overcome circuit resistance and maintain a steady current flow.
Ideal Versus Practical Voltage Sources
The concept of a voltage source is divided into two models: the theoretical ideal source and the real-world practical source. An ideal voltage source is a theoretical construct defined as a device that can maintain a perfectly constant voltage across its terminals, regardless of the amount of current drawn from it. This theoretical behavior implies that the ideal source has zero internal resistance, meaning no energy is lost within the source itself.
In reality, no such device exists because all materials possess some degree of electrical resistance. A practical voltage source, which accurately models real components like batteries or power supplies, is understood to have a small, non-zero internal resistance (\(R_{int}\)). This internal resistance is conceptually modeled as a tiny resistor connected in series with the ideal voltage source.
The presence of internal resistance dictates how a practical source behaves when a load is applied. When a circuit draws current, a small portion of the total voltage is “dropped” across this internal resistance. According to Ohm’s law, as the current increases, this internal voltage drop also increases, causing the terminal voltage available to the circuit to decrease.
This effect explains why the voltage of a battery dips slightly under heavy use, such as when starting a car. The practical source can only supply a finite maximum current before its terminal voltage drops significantly. In contrast, the ideal source, having no internal losses, could theoretically supply infinite current without a voltage change. Engineers use the practical model to predict how a real power supply will perform under load.
Classification of Voltage Sources
Beyond the ideal and practical models, voltage sources are also categorized based on how their output value is determined, falling into either independent or dependent types. An independent voltage source is one whose output voltage is fixed and constant, unaffected by any other voltage or current elsewhere in the circuit. Common examples include standard batteries, generators, and wall sockets, which deliver a predetermined voltage value.
The value of an independent voltage source is set by its own physical design, such as the chemical reaction in a battery cell. These sources are the most common type encountered in everyday applications and are represented in circuit diagrams by a simple circle symbol.
In contrast, a dependent, or controlled, voltage source is one whose output is determined by a voltage or current at another location within the circuit. For instance, a voltage-controlled source might produce an output voltage that is five times the voltage measured across a specific resistor. Dependent sources are primarily theoretical tools used by engineers for analyzing and modeling complex electronic components, such as transistors and operational amplifiers.
These models are essential for understanding amplification and signal processing. They are typically represented by a diamond-shaped symbol in a circuit schematic.