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What Is Magnetic Flux and How to Calculate It

Narayani Karthik May 13, 2019
Magnetic Flux is a measurement of magnetism exhibited by an object in a two dimensional area. This is also referred to as electromagnetism and is used to calculate the density of the magnetic field.
Most of the electronic appliances that we come across in our day-to-day life, work on the principle of magnetic flux. The best example is a transformer. The principle of its working is based on this phenomenon. Well, before that, it is important to understand the term, magnetic field.
It is nothing but a force produced by an object exhibiting magnetic properties or by changing the magnitude of an electrical field. It is identified by its strength and direction. Electrical currents in transformers also produce it.
Rather, there is a relationship between magnetism and electricity. A varying electric field produces a magnetic field, and vice versa. This principle is often known as the electromagnetic induction.
Magnetic flux can be defined as a measure of magnetic field in a certain medium. In simple terms, if the field had to pass through a certain medium, it will always travel as "flux" lines (they are imaginary, but continuous lines, traveling from north pole of a magnet to its south pole).



Magnetic flux is the product of the field lines and the sine of the angle, formed between the area and magnetic field. If the angle is 90º (i.e. the area is perpendicular to the field), then the equation is (flux lines) x Sin 90º. The unit of measurement is weber (Wb). The symbol used is Φ (phi).
1 Wb = Sin 90º x 108 field lines ~ 1 x 108 field lines.

Example: If a flux has 5000 field lines then how much is the flux? 108 field lines/5000 = 2 x 104 Wb


The amount of flux per unit area of the field, which is perpendicular to the direction of flux, is termed as flux density. It is measured in Telsa (T).

Density (T) = Flux (Wb) / Area in m2
Example: From this example, the flux calculated is 2 x 104 Wb. So, the flux density for that area is 2 x 104Wb/m2 ~ 2 x 104 Telsa. Similarly, one can calculate the flux if the density is provided. Then the equation would be Φ = (Flux Density) x Area x Cos Ө.

Magnetic Flux Through a Coil

To understand the operation of flux through a coil, Faraday's Law has to be understood first. Faraday's law states - "The electromotive force induced in a circuit is directly proportional to the rate of change of magnetic flux through the circuit". Hence, any change in the magnetic field of coil will cause an electromotive force to be induced in the coil.
Electromotive force (EMF) is nothing but the voltage generated in the coil. This voltage is produced by changing the orientation of the field, which could be by moving the magnet towards/away from the coil or by rotating the coil relative to the magnet.
Most of the appliances like thermoelectric devices, solar cells, electrical generators, transformers, and voltaic batteries, work on the principle of Faraday's Law. A transformer is a device which transmits the electricity from one circuit to another through inductively coupled coils.
A transformer has two coils, a primary coil and a secondary coil. The variation in the electric current in the primary coil winding, creates a flux in the core of the transformer. This varying flux in turn generates a varying field through the secondary coil winding.
Based on Faraday's Law, changing flux induces an electromotive force (voltage). Hence, because of the changing magnetic field in the secondary coil winding, an EMF or voltage is produced in the secondary coil. This effect is often known as mutual induction.
Now when the load is connected to the secondary coil, there is a direct flow of charge through the secondary winding from the electric energy produced in the primary coil.
Precisely, for an ideal transformer, the voltage produced in the secondary winding of the transformer due to mutual induction is directly proportional to the primary voltage of load, depending on the number of turns in the coils of both primary and secondary windings.
Even the Earth, which has its own magnetic field, produces its own flux lines. This can be quite helpful for navigation purposes. A compass points towards the south pole of the Earth's field when it is left still.