In previous lessons, we concentrated on how moving electric charges behave when subjected to magnetic fields. However, there is a fundamental symmetry in nature: when an electric charge experiences a changing magnetic field—or more precisely, a changing magnetic flux—an electromotive force (EMF) is induced, causing the charge to move. This phenomenon is known as electromagnetic induction.
In this upcoming series of lessons, we will explore the core principles that govern induction—including Faraday's Law and Lenz's Law—as well as practical applications such as electrical transformers, generators, and electromagnetic braking.
What is Magnetic Flux?
To understand induction, we must first define magnetic flux (Φ). While the concept of "flux" appears in several areas of physics to describe flow through a surface, in electromagnetism it serves as a measure of the total magnetic field passing through a given area.
Quantitatively, magnetic flux depends on the strength of the magnetic field (B), the area of the surface (A), and the angle (θ) between the magnetic field vector and the normal (perpendicular) to the surface:
Φ = BA cos(θ)
Where:
- Φ is the magnetic flux in Webers (Wb, or T·m2)
- B is the magnetic field strength (magnetic flux density) in Teslas (T)
- A is the surface area in square meters (m2)
- θ is the angle between the field lines and the line perpendicular to the surface area
Maximum flux occurs when the surface is perpendicular to the field lines (θ = 0°, cos(0°) = 1), whereas zero flux passes through a surface aligned parallel to the field lines (θ = 90°, cos(90°) = 0).
Watch the 3-minute video below for a visual breakdown of magnetic flux and field line density.