A faraday is a unit of electrical charge equal to approximately 96,485 coulombs, representing the total charge carried by one mole (about 602 billion trillion) of electrons. The term comes from Michael Faraday, the 19th-century physicist whose experiments with electricity and chemistry laid the groundwork for modern electrochemistry. Depending on context, “faraday” can refer to this unit of charge, the closely related Faraday constant used in science and engineering, or even a Faraday cage, which is a shielding enclosure.
The Faraday as a Unit of Charge
In its simplest form, one faraday (sometimes abbreviated Fd) is the amount of electric charge carried by one mole of electrons. A mole is the standard chemistry counting unit: 6.022 × 10²³ particles. Since each electron carries a tiny charge of 1.602 × 10⁻¹⁹ coulombs, multiplying those two numbers together gives you roughly 96,485 coulombs. That’s one faraday.
To put that in more tangible terms, a coulomb is the charge delivered by one ampere of current flowing for one second. So one faraday is equivalent to about 26.8 ampere-hours, which is a unit you might recognize from battery ratings. If you had a battery rated at 26.8 amp-hours, fully discharging it would move one faraday of charge through a circuit.
The Faraday Constant
Scientists and engineers more commonly use the Faraday constant, denoted by the letter F, which is the same number expressed with precise units: 96,485.33212 coulombs per mole. The 2022 CODATA value from NIST lists this as an exact constant, since both Avogadro’s number and the elementary charge of an electron were fixed by international agreement in 2019.
The formula is straightforward: F = Nₐ × e, where Nₐ is Avogadro’s number and e is the charge on a single electron. The Faraday constant essentially bridges the world of individual electrons (used in physics) and the world of moles (used in chemistry), letting you convert between them.
How the Faraday Constant Is Used
The most classic application comes from Faraday’s own laws of electrolysis, which describe what happens when you pass electric current through a liquid to drive a chemical reaction. The first law says the amount of substance deposited or dissolved at an electrode is directly proportional to the total charge passed through the solution. The second law says that equal amounts of charge will produce different masses of different elements, in proportion to their atomic weights and how many electrons each atom gains or loses.
Together, these laws let you calculate exactly how much material an electrical current will produce. If you know the charge passed and the type of ion involved, you divide by the Faraday constant and the ion’s valence (how many electrons it exchanges) to get the number of moles produced. This is foundational for electroplating, where manufacturers deposit thin layers of metals like chrome or gold onto surfaces by running current through a solution containing metal ions.
The same math applies to batteries and fuel cells. Engineers use the Faraday constant to calculate the theoretical maximum energy a battery chemistry can deliver, based on how many moles of reactants are available and how many electrons each reaction transfers. The gap between that theoretical value and real-world performance tells you how much efficiency is lost to heat and side reactions.
Faraday vs. Farad
These two terms sound nearly identical but measure completely different things. A faraday is a unit of electric charge (how many electrons flow). A farad is the SI unit of capacitance (how much charge a capacitor can store per volt of potential difference). One farad means a device can hold one coulomb of charge at one volt. Both are named after Michael Faraday, which is where the confusion starts and ends. In practice, you’ll see farads on capacitor labels and the Faraday constant in chemistry equations.
The Faraday Cage
A Faraday cage is a mesh or solid enclosure made of conductive material that blocks external electric fields and electromagnetic waves from reaching whatever is inside. Faraday discovered that when electric charge hits a conductive shell, it distributes itself entirely along the outer surface, leaving the interior electrically neutral. Only the very surface of the metal conducts the current; nothing penetrates inward.
This same principle explains why a car protects you during a lightning strike. The charge travels along the outer metal body and flows to the ground without passing through the cabin. Faraday cages also block electromagnetic waves like radio signals. The electric component of the wave induces currents in the cage’s surface that cancel the field inside, while the magnetic component is similarly absorbed. This is why your microwave oven has a metal mesh in its door (to keep microwaves contained), and why sensitive electronic equipment in labs is often housed in shielded rooms.
Michael Faraday’s Legacy
Michael Faraday (1791–1867) was a British physicist and chemist who made foundational contributions to both electromagnetism and electrochemistry, despite having almost no formal education. His electrolysis experiments in the 1830s established the quantitative relationship between electricity and chemical change, which is exactly what the Faraday constant captures. He also discovered electromagnetic induction, the principle behind electric generators and transformers, and demonstrated the shielding effect that now bears his name. Having a unit of charge, a fundamental physical constant, a unit of capacitance, and a type of electromagnetic shield all named after the same person is unusual in science, and it reflects the breadth of his contributions.