Volt: Definition, SI Units, and the Josephson Effect
In the world of physics and electrical engineering, the volt serves as the fundamental unit of electric potential. Whether you are analyzing a simple battery or complex industrial circuitry, the volt provides the standard measure for the "pressure" that pushes electrical charges through a conductor.
Named in honor of the physicist Alessandro Volta, the volt is a derived unit within the International System of Units (SI). While its symbol is always capitalized as V, the word itself is treated as a common noun and is only capitalized at the start of sentences or in titles.
Defining the Volt
A volt is formally defined as the electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. In simpler terms, it represents the potential difference that imparts one joule of energy per coulomb of charge passing through it.
Because it is a derived unit, the volt can be expressed through several different mathematical relationships depending on the context of the measurement:
- Power and Current: Watts per ampere (W/A).
- Energy and Charge: Joules per coulomb (J/C).
- Resistance and Current: Amperes times ohms (A · Ω), following Ohm's law.
- Magnetic Flux: Webers per second (Wb/s).
- Elementary Charge: Electronvolts per elementary charge (eV/e).
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SI Base Unit Representation
To reach the most fundamental level of physics, the volt can be broken down into SI base units: meters (m), kilograms (kg), seconds (s), and amperes (A). The resulting expression is:
V = kg · m2 · s−3 · A−1
The Josephson Junction and Voltage Standards
Maintaining a precise global standard for the volt requires advanced quantum physics. Historically, from 1990 to 2019, the "conventional" volt (V90) was implemented using the Josephson effect—a phenomenon that allows for the exact conversion of frequency to voltage—combined with a caesium frequency standard.
The core of this measurement is the Josephson constant (KJ), defined as 2e/h, where e is the elementary charge and h is the Planck constant. For years, the conventional value KJ-90 = 0.483 5979 GHz/μV was used. However, following the 2019 revision of the SI, the Josephson constant was assigned an exact value of KJ = 483 597.848 416 98... GHz/V.
In practical application, this standard is realized through an array of several thousand or tens of thousands of series-connected junctions. These are excited by microwave signals ranging between 10 and 80 GHz. Empirical evidence confirms that this method remains accurate regardless of the material used, the device design, or the specific measurement setup.
Key Facts
- Named after: Alessandro Volta.
- SI Symbol: V (uppercase).
- Fundamental Ratio: 1 Volt = 1 Joule per Coulomb.
- Ohm's Law Relation: Voltage = Current × Resistance.
- Modern Standard: Based on the Josephson effect and the Planck constant.
- 2019 Update: Transitioned from a conventional value (KJ-90) to an exact Josephson constant.
Summary of Volt Equivalencies
| Relationship | Formula | Units |
|---|---|---|
| Power / Current | W / A | Watts per Ampere |
| Energy / Charge | J / C | Joules per Coulomb |
| Current × Resistance | A · Ω | Amperes times Ohms |
| Magnetic Flux / Time | Wb / s | Webers per second |
Frequently Asked Questions
What is the simplest way to define a volt?
The simplest definition is that one volt is the potential difference between two points that imparts one joule of energy for every one coulomb of charge that moves between them.
Who is the volt named after?
The unit is named after Alessandro Volta, the physicist who invented the first chemical battery.
How did the definition of the volt change in 2019?
In 2019, the SI system moved away from the "conventional" Josephson constant (KJ-90) and replaced it with an exact value for the Josephson constant (KJ) based on fundamental constants.
What is the Josephson effect used for in voltage measurement?
The Josephson effect is used for exact frequency-to-voltage conversion, allowing scientists to realize a highly precise and stable voltage standard using microwave signals.
What are the SI base units of a volt?
The SI base units for a volt are kilograms, meters, seconds, and amperes, expressed as kg · m2 · s−3 · A−1.