Topic Cluster

Electromagnetism

Electromagnetism: electric charge, fields, current and magnetism. From Coulomb’s and Ohm’s laws to electromagnetic induction and circuits — how charges and fields actually work.

Electromagnetism — the core equations of electromagnetism, and every article on the topic
The equations this topic keeps coming back to.

What Is Electromagnetism?

Electromagnetism is the physics of electric charge and magnetism, and of the fields that carry the force between them. It runs from the current in a wire to the light leaving a star, because visible light is itself an electromagnetic wave.

Core equations in electromagnetism
RelationshipEquationWhat it tells you
Coulomb's lawF = k·q1·q2 / r²Force between two point charges
Ohm's lawV = I·RVoltage, current and resistance in a conductor
Electric fieldE = k·Q / r²Field strength around a point charge
Faraday's lawε = −N · ΔΦ/ΔtEMF induced by a changing magnetic flux
Magnetic forceF = q·v·B·sinθForce on a charge moving through a field
Electrical powerP = V·IPower delivered by a circuit
Electric Potential (V = kQ/r)
Electromagnetism

Electric Potential (V = kQ/r)

Electric potential is the energy per coulomb at a point in space, measured in volts against a zero set infinitely far away, and around a point charge it is V = kQ/r. This page covers where the formula comes from, why potential is a scalar that adds as plain signed numbers, the two kinds of zero that catch people out, equipotential surfaces and what a negative potential really means. Seven worked problems, two original diagrams, an interactive lab and a calculator.

22 min read
Magnetic Flux: Formula, Units and Worked Examples
Electromagnetism

Magnetic Flux: Formula, Units and Worked Examples

Magnetic flux is the total magnetic field passing through a surface, Phi = B A cos theta, measured in webers. This guide fixes the one thing that goes wrong most often - the angle is measured from the normal to the surface, not from the surface - then works through the weber, flux against flux density, what happens past 90 degrees, and why the net flux out of a closed surface is zero, with an angle table, two original diagrams, seven worked problems and an interactive simulator.

18 min read
The Right-Hand Rule in Physics: Three Clear Rules, One Hand
Electromagnetism

The Right-Hand Rule in Physics: Three Clear Rules, One Hand

The right-hand rule turns a cross product into a direction you can point at: fingers along the first vector, curl them towards the second, thumb along the third. This page fixes the axis convention first, then works through all three rules - the force on a moving charge, the field circling a wire, and which end of a coil is north - with a 36-cell direction table, eight worked problems, two original diagrams and an interactive trainer.

24 min read
Superconductors Explained: How Cold, How Much Field, How Much Current
Electromagnetism

Superconductors Explained: How Cold, How Much Field, How Much Current

A superconductor carries current with no measurable resistance below a critical temperature, and expels magnetic field while it does so. This page puts numbers on all three limits that end it: lead at 4.2 K holds 52.70 mT, a 1 mm-diameter lead wire carries 131.8 A for nothing where copper would burn 371.3 W per metre, and twenty-eight years of persistent current bound the resistance below 1.1 times ten to the minus eighteen ohms. Eight worked problems, three original diagrams, an interactive lab and a calculator.

22 min read
Eddy Currents: What They Are, What They Waste and Where They Help
Electromagnetism

Eddy Currents: What They Are, What They Waste and Where They Help

An eddy current is a loop of current a changing magnetic field drives inside solid metal, with no wire to guide it. In a thin sheet the power it wastes is P/V = pi^2 B^2 f^2 d^2 / (6 rho), which is why a metal core is cut into slices: the same 100 cubic centimetres of copper goes from 1300.08 W as one 20.0 mm block to 1.53 W as forty sheets. That is a cut of 849.79 times where the squared rule promises 1600, and this page measures exactly where the formula stops telling the truth. Eight worked problems, three original diagrams, an interactive lab and a calculator.

21 min read