Volt
In 1791 Luigi Galvani touched a frog's leg with two different metals, and the leg twitched. Galvani decided that the electricity lived in the frog itself. Alessandro Volta believed it was the metals. To prove it, in 1800 he stacked a pile of copper and zinc discs, separating them with cloth soaked in salt water. The pile gave a current without any frog.
It was the first battery. The unit of voltage was named after Volta in 1881. One volt is a joule per coulomb: the energy each coulomb of charge gains as it passes through the source. An AA battery gives 1.5 V, a wall socket 230 V, and power-line wires hundreds of kilovolts.
| Notation | V |
| Definition | joule per coulomb · J/C · W/A · kg·m²·s⁻³·A⁻¹ |
| Reference values | K_J — 483 597.848 4 GHz/V · Weston cell — 1.018 64 V · 1 eV — 1.602·10⁻¹⁹ J |
| Introduced | 1881, International Congress of Electricians · in the SI since 1960 |
| Standards | ISO 80000-6 · IEC 60050-131 · SI Brochure 9th ed. |
| Typical values | 1.5 V — a battery · 230 V — a wall socket · 400 kV — a power line |
01 · Definition
A joule for every coulomb
Voltage U shows how much energy a charge gains or gives up as it moves from one point of a circuit to another. One volt is one joule per coulomb. A 1.5 V battery gives each coulomb one and a half joules, and a bulb in the circuit takes them and turns them into light and heat.
Voltage is always measured between two points. A voltmeter is connected across both ends of a section, not into a break in the circuit. The current through a section equals the voltage divided by the resistance — this is Ohm's law.
Why power lines carry hundreds of kilovolts
The kettle needs 2 kW of power. It can be delivered with a small current at a high voltage or a large current at a low one. Power is voltage multiplied by current. The wire from the source to the kettle has a resistance of 1 Ω, about 60 m of copper wire with a 1 mm² cross-section. The blue dots are the current: the faster they move, the larger it is. The wire turns red from the heat lost in it.
An electron that passes through a voltage of 1 V gains an energy of 1 eV — an electronvolt. It is a handy measure for atoms and particles. A photon of green light carries about 2.3 eV, an X-ray photon tens of thousands, and protons in the Large Hadron Collider 6.8 TeV.
Pair by pair, volt by volt
On the left is the pile: zinc, copper and cloth soaked in salt water, from bottom to top, pair after pair. Next to it is the potential ladder: each pair raises the potential by one step, and inside the cloth it sags a little while current flows. A wire runs from the top of the pile through a switch to the load and back to the bottom. The blue dots are electrons: they go from the zinc through the load to the copper. On the right is a voltmeter connected to the ends of the pile. While current flows, hydrogen bubbles collect on the copper and the voltage slowly falls. Click the scene to close or open the switch.
Add pairs and watch the voltmeter.
02 · Conversion
| Unit | Value | Where it is met |
|---|---|---|
| {n} | {v} | {note} |
A resting neuron holds about −70 mV across a membrane 5 nm thick. The field in it is about 14 MV/m.
A 12 V car battery gives each coulomb 12 J. The starter draws hundreds of amperes — kilowatts of power.
The statvolt of the CGS system is almost 300 V, more precisely 299.792 458 V.
03 · Orders of magnitude
04 · Measuring instruments
05 · Writing rules
06 · Neighbouring units
07 · Historical section
Since 1990 the volt has been kept with Josephson junctions. A superconducting junction under microwaves of frequency f gives a voltage strictly in steps of f/K_J, where K_J = 2e/h is the Josephson constant. Since 2019 it has been exact: 483 597.848 4 GHz/V. A chain of tens of thousands of junctions gives 10 V with an error of parts per billion.
Before that the standard was the Weston normal cell — a cadmium–mercury jar shaped like the letter H. At 20 °C it gave 1.018 64 V. The international volt of 1908 turned out to be 0.034 % larger than today's.
Catalogue of quantities
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