A complete Class 12 Physics & Chemistry guide to Faraday's laws — electromagnetic induction (first & second law, EMF formula, Lenz's law, magnetic flux) and Faraday's laws of electrolysis, with diagrams, right-hand rule, and worked examples.
Last updated: September 23, 2026
Michael Faraday formulated two separate and equally important sets of laws in physics and chemistry, both bearing his name:
This guide covers both in full detail, since they are frequently searched together but are entirely different phenomena.
Electromagnetic induction is the process by which a changing magnetic field induces an electromotive force (EMF), and hence an electric current, in a nearby conductor. Discovered by Michael Faraday in 1831, it is the fundamental principle behind generators, transformers, and induction motors.
Figure 1: Faraday's classic experiment — moving a bar magnet in and out of a coil induces a current, detected by a galvanometer.
Magnetic flux (Φ) is a measure of the total magnetic field passing through a given area. The SI unit of magnetic flux is the weber (Wb).
Φ = B · A · cosθ
Where B = magnetic field strength (tesla), A = area (m²), and θ = angle between the magnetic field and the normal (perpendicular) to the surface.
Flux is maximum when the magnetic field is perpendicular to the surface (θ = 0°) and zero when the field is parallel to the surface (θ = 90°).
"Whenever a conductor is placed in a changing magnetic field (or a conductor moves through a stationary magnetic field), an EMF is induced in the conductor. If the conductor forms a closed circuit, this induced EMF causes a current to flow, called the induced current."
Flux can change due to: (a) relative motion between the magnet and the coil, (b) changing the current in a nearby coil, or (c) rotating the coil within a magnetic field.
"The magnitude of the induced EMF is equal to the rate of change of magnetic flux linkage with the circuit." Flux linkage is the total flux linked with all N turns of a coil (NΦ).
EMF = −N (dΦ/dt)
Where EMF = induced electromotive force (volts), N = number of turns in the coil, dΦ/dt = rate of change of magnetic flux (Wb/s), and the negative sign represents Lenz's law (direction of opposition).
This single equation combines both the first law (that an EMF is induced) and the second law (its magnitude equals the rate of flux change) into one formula — this combined statement is often just called "Faraday's law" in modern textbooks.
Lenz's law states that the direction of an induced current is always such that it opposes the change in magnetic flux that produced it. It was formulated by Heinrich Lenz in 1834 and is a direct consequence of the law of conservation of energy — if the induced current aided the change instead of opposing it, energy would be created from nothing.
Figure 2: As a magnet approaches a coil, the induced current creates a magnetic field that opposes the incoming magnet (like poles repel), consistent with Lenz's law.
The right-hand rule helps determine the direction of induced current or magnetic field. For a current-carrying wire: if you curl the fingers of your right hand in the direction of conventional current flow, your thumb points in the direction of the magnetic field (or vice versa for a solenoid: curl fingers in the direction of current flow around the coil, and your thumb points toward the north pole).
| Term | Definition | SI Unit |
|---|---|---|
| EMF (electromotive force) | The energy provided per unit charge that drives current around a circuit, induced by changing flux | Volt (V) |
| Induced current | The current that flows in a closed circuit as a result of the induced EMF | Ampere (A) |
| Voltage | The potential difference driving current flow (often used interchangeably with EMF in circuits) | Volt (V) |
Self-inductance (L) is the property of a single coil by which it opposes any change in the current flowing through itself, inducing an EMF in the same coil. It is measured in henries (H).
Mutual induction occurs between two separate coils, where a changing current in one coil (the primary) induces an EMF in a neighboring coil (the secondary) due to the changing magnetic flux linking them. This is the working principle behind transformers.
Eddy currents are circulating (loop-like) currents induced within the body of a conductor when it experiences a changing magnetic flux, rather than flowing through a defined wire path. They typically generate heat and oppose the motion causing them — used usefully in induction cooktops and electromagnetic braking, but often minimized (via laminated cores) in transformers and motors to reduce energy loss.
Separate from electromagnetic induction, Faraday's laws of electrolysis (formulated in 1833) describe the quantitative relationship between the amount of electric charge passed through an electrolyte and the amount of substance deposited or liberated at the electrodes.
"The mass of a substance deposited or liberated at an electrode is directly proportional to the quantity of electric charge passed through the electrolyte."
m = ZQ = ZIt
Where m = mass deposited, Z = electrochemical equivalent, Q = charge (coulombs), I = current (amperes), t = time (seconds).
"When the same quantity of charge is passed through different electrolytes, the mass of substances deposited at the electrodes is directly proportional to their chemical equivalent weights (equivalent mass)."
Faraday's constant (F) is the magnitude of electric charge carried by one mole of electrons, equal to approximately 96,485 coulombs per mole. It links the macroscopic quantity of charge to the number of moles of electrons transferred in an electrolysis reaction, and is calculated as F = NA × e (Avogadro's number × elementary charge).
The free PhET Faraday's Law simulation (University of Colorado Boulder) lets you move a bar magnet through a coil and watch the induced current and voltage respond in real time — an excellent way to build intuition for these concepts.
Question: A coil of 100 turns experiences a change in magnetic flux from 0.02 Wb to 0.08 Wb in 0.5 seconds. Find the induced EMF.
Solution:
Given: N = 100, dΦ = 0.08 − 0.02 = 0.06 Wb, dt = 0.5 s
EMF = N(dΦ/dt) = 100 × (0.06/0.5) = 100 × 0.12 = 12 V (magnitude; direction given by Lenz's law)
Question: A current of 2 A is passed through a silver nitrate solution for 1000 seconds. If the electrochemical equivalent of silver is 0.001118 g/C, find the mass of silver deposited.
Solution:
Given: I = 2 A, t = 1000 s, Z = 0.001118 g/C
m = ZIt = 0.001118 × 2 × 1000 = 2.236 g
| Faraday's Law of Induction | EMF = −N(dΦ/dt) |
| Lenz's Law | Induced current opposes the change (negative sign) |
| Magnetic Flux | Φ = B·A·cosθ |
| Faraday's Law of Electrolysis | m = ZIt (1st law); m ∝ equivalent weight (2nd law) |
Faraday's law of electromagnetic induction states that a changing magnetic flux induces an EMF in a circuit, proportional to the rate of change of flux. Faraday also formulated separate laws of electrolysis relating electric charge to chemical change at an electrode.
EMF = -N(dΦ/dt), where N is the number of turns, dΦ/dt is the rate of change of magnetic flux, and the negative sign represents Lenz's law.
Magnetic flux is a measure of the total magnetic field passing through an area, calculated as Φ = B·A·cos(θ), measured in webers (Wb).
Lenz's law states that the direction of an induced current always opposes the change in magnetic flux that produced it, consistent with the conservation of energy.
The first law states that a changing magnetic field induces an EMF in a conductor. The second law states that the magnitude of this EMF equals the rate of change of magnetic flux linkage, EMF = -N(dΦ/dt).
The first law states mass deposited is proportional to charge passed (m = ZIt). The second law states that for the same charge, mass deposited across different electrolytes is proportional to their chemical equivalent weights.
Faraday's constant is the electric charge carried by one mole of electrons, approximately 96,485 coulombs per mole, used in calculations of electrolysis.
Electromagnetic induction is the process by which a changing magnetic field induces an EMF, and hence a current, in a nearby conductor — discovered by Faraday in 1831 and used in generators, transformers, and motors.
Self-inductance is a single coil opposing changes in its own current. Mutual induction occurs between two separate coils, where a changing current in one induces an EMF in the other — the principle behind transformers.
