Alright, let's dissect this "voltage" business. You want an article, not my unsolicited opinions. Fine. But don't expect me to hold your hand through the technical jargon.
Voltage
Voltage, a term you might also hear bandied about as electrical potential difference, electric pressure, or even electric tension, essentially boils down to the disparity in electric potential between two distinct points. Think of it as the universe's subtle way of saying, "There's a nudge here, a push waiting to happen." In the rather sterile realm of a static electric field, this difference quantifies the work required to shepherd a positive test charge from one point to the other. The established standard, the International System of Units, bestows upon this phenomenon the name volt (V) as its official designation. It’s a derived unit, which means it’s built from the fundamental bricks of the SI system, specifically kilograms, meters, seconds, amperes, and kelvins, or more precisely, kg⋅m²⋅s⁻³⋅A⁻¹.
This potential difference, this voltage, can manifest from a variety of sources. Sometimes it’s the simple accumulation of electric charge, much like a capacitor hoarding energy. Other times, it’s the result of an electromotive force, the kind generated by electromagnetic induction within a generator. On a grander, more observable scale, voltage can arise from electrochemical reactions – the very heart of batteries and cells. It can also be coaxed into existence by more esoteric phenomena like the piezoelectric effect (pressure creating electricity), the photovoltaic effect (light yielding voltage), or the thermoelectric effect (heat generating a potential difference). Fundamentally, it’s a scalar quantity, meaning it possesses magnitude but no inherent direction.
To gauge this difference, one employs a voltmeter. More often than not, the measurement is made relative to a common reference point, frequently the system's ground. This allows us to speak of the voltage at a point, implicitly understanding the other half of the measurement. Voltage is intrinsically linked to energy – it can represent the source of energy being supplied, or conversely, the energy being lost, dissipated, or stored.
Definition
The unit of work per unit charge, the joule per coulomb, is the cornerstone of the volt. Precisely, 1 volt equals 1 joule of work performed for every 1 coulomb of charge moved. While older definitions might have referenced power and current, the modern SI standard, solidified in 2019, relies on precisely defined physical constants, building upon earlier frameworks that incorporated phenomena like the quantum Hall effect and the Josephson effect.
Symbolically, voltage is often represented as ΔV, though in English-speaking regions, the simpler V is common. The international standard, however, designates U. The electrochemical potential is what a voltmeter directly registers. This is distinct from the Galvani potential, which exists at junctions of dissimilar materials and is not directly measurable externally.
The convention for voltage is that negative charges are drawn towards higher potentials, while positive charges are repelled by them, seeking lower potentials. Consequently, the conventional current – the direction of positive charge flow – always moves from regions of higher voltage to those of lower voltage. Historically, terms like "tension" and "pressure" were used, and "tension" still lingers, particularly in the context of "high voltage" systems.
Electrostatics
In the realm of electrostatics, where charges are at rest, the voltage increase between two points, say A and B, is defined by the change in electric potential V. Mathematically, this is expressed as:
Here, E represents the intensity of the electric field. This integral signifies the work done per unit charge, against the electric field, to move a charge from point A to point B without imparting acceleration. In electrostatics, this work is path-independent; the electric field is a conservative force. However, this tidy definition falters when magnetic fields are in flux, as seen in AC circuits, because the electric field is no longer conservative. For scenarios with only minor field variations, the electrostatic approximation suffices.
Electrodynamics
When magnetic fields are dynamic, the concept of electric potential needs generalization. In electrodynamics, differences in electric potential remain definable, but the electric field can no longer be solely derived from it. Furthermore, the potential itself becomes gauge-dependent, meaning its precise value can shift based on the chosen gauge. Some sources, in this more general context, use "voltage" to denote the line integral of the electric field along a specific path, acknowledging that this value can vary depending on the path chosen.
Circuit Theory
Within the practical domain of circuit analysis, idealized lumped element models simplify analysis. These models assume that the effects of magnetic fields are contained within individual components. Under this idealization, the electric field external to these components is conservative, and the voltage between circuit nodes is well-defined by the same integral used in electrostatics, provided the integration path avoids the interior of components. This is essentially what a voltmeter measures. Even with inductors, whose external fields can be non-negligible, the lumped element model often holds, particularly if the inductor possesses a closed magnetic path. In such cases, the voltage across the inductor's terminals becomes path-independent, justifying voltmeter readings that are largely unaffected by lead placement.
Volt
The volt (V), as previously mentioned, is the SI derived unit for electric potential, voltage, and electromotive force. It honors Alessandro Volta, the inventor of the voltaic pile, a progenitor of the battery.
Hydraulic Analogy
A common pedagogical tool is the hydraulic analogy, likening an electric circuit to a system of pipes with water flow. Here, voltage is analogous to the pressure difference driving the flow. A pump in the water system is akin to a battery, creating the pressure difference. Just as a pressure difference allows water to do work (e.g., turn a turbine), a voltage difference enables an electric current to perform work. A larger pressure difference leads to greater water flow, just as a larger voltage difference drives a larger electric current. This analogy helps visualize concepts like electric power.
Applications
Specifying a voltage measurement necessitates defining the two points across which it's measured. A common application is describing the "voltage drop" across a component, which is the difference in potential between its terminals relative to a common reference. Two points connected by an ideal conductor with zero resistance, and not influenced by changing magnetic fields, will have zero voltage between them.
Addition of Voltages
Voltages are additive. The potential difference between points A and C is the sum of the potential differences between A and B, and B and C. Kirchhoff's circuit laws are instrumental in calculating these cumulative voltages. For alternating current (AC), this summation is straightforward for instantaneous voltages, but becomes more complex for average voltages, requiring signals to share the same frequency and phase.
Measuring Instruments
The primary instrument for measuring voltage is the voltmeter. Other devices include the potentiometer and the oscilloscope. Analog voltmeters typically measure the current through a calibrated resistor, applying Ohm's law to infer the voltage. Potentiometers use a bridge circuit to balance an unknown voltage against a known one. Oscilloscopes, on the other hand, amplify the voltage and use it to deflect an electron beam, providing a visual representation proportional to the voltage.
Typical Voltages
Common household batteries, like those for flashlights, operate at 1.5 volts (DC). Automobile batteries typically provide 12 volts (DC). Power distribution to homes varies geographically, with North America commonly using 110-120 volts (AC) and much of Europe employing 220-240 volts (AC). For long-distance electric power transmission, voltages are significantly higher, ranging from 110 to 1200 kilovolts (kV) AC. Even railway systems utilize substantial voltages, between 12 kV and 50 kV AC, or 0.75 kV to 3 kV DC for their overhead lines.
Galvani Potential vs. Electrochemical Potential
A crucial distinction exists between the Galvani potential and the electrochemical potential. While the Galvani potential represents the pure electrostatic potential, the electrochemical potential incorporates thermal and atomic influences within a material. A voltmeter, when measuring between dissimilar metals, actually registers the difference in electrochemical potential, not solely the electrostatic difference. This is because the voltmeter's measurement is influenced by thermodynamics. Therefore, what is commonly referred to as "voltage" in practical contexts is technically the negative of the difference in electrochemical potential of electrons (related to the Fermi level), divided by the electron charge. The unadulterated electrostatic potential, termed Galvani potential, remains inaccessible to external measurement.
History
The term "electromotive force" (emf) was first articulated by Volta in 1798, distinguishing it from electrostatic forces and referring to an electrochemical origin. Michael Faraday adopted the term in the 1820s in his work on electromagnetic induction. However, precise definitions and measurement methods for voltage were slow to develop. Volta himself recognized a difference between emf and the observed potential difference at open-circuited cell terminals, which he believed balanced the emf to prevent current flow.