Magnetic Flux

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 transformer. The principle of their working is based on magnetic flux. Well, before that, it is important to understand what is magnetic field. Magnetic field is nothing but a force produced by an object exhibiting magnetic properties or by changing the magnitude of an electrical field. A magnetic field is identified by its strength and direction. Electrical currents in transformers also produce a magnetic field. Rather there is a relationship between magnetism and electricity. A varying electric field produces a magnetic field and a changing magnetic field induces an electric field. 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 magnetic field had to pass through a certain medium, it will always travel as "flux" lines (flux lines are imaginary, but continuous lines traveling from north pole of a magnet to its south pole).

Magnetic Flux Measurements

Magnetic Flux Equation: Magnetic flux is the product of the magnetic lines and the sine of the angle formed between the area and the magnetic field. If the angle is 90º (i.e. the area is perpendicular to the magnetic field), then the equation is (magnetic flux lines) x Sin 90º. This application is measured in weber (Wb). The symbol for magnetic flux is Φ (phi).

1 Wb = Sin 90º x 10⁸ magnetic field lines ~ 1 x 10⁸ magnetic field lines.

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

Magnetic Flux Density: The amount of magnetic flux per unit area of the magnetic field, which is perpendicular to the direction of magnetic flux, is termed as magnetic flux density. This equation is measured in Telsa (T).

Magnetic Flux density (T) = Magnetic Flux (Wb) / Area in m²

Example: From the above example, the magnetic flux calculated is 2 x 10⁴ Wb. So, the magnetic 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 = (Magnetic Flux Density) x Area x Cos Ө.

Magnetic Flux Through a Coil

To understand the operation of magnetic 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 time 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 magnetic 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, primary coil and secondary coil. The variation in the electric current in the primary coil winding, creates a magnetic flux in the core of the transformer. This varying flux in turn generates a varying magnetic field through the secondary coil winding. Based on Faraday's Law, changing magnetic flux induces an electromotive force (voltage). So, 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 the 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 our earth which has its own magnetic field, produces its own magnetic flux lines. Earth's magnetic field is quite helpful in navigation purposes, where a magnetic compass is used. A magnetic compass points towards the south pole of the earth's magnetic field when it is left still. Magnetic flux also plays an important role in many industrial applications.