gives the strength of the field. This strength of the field is technically called magnetic flux density (B). So, magnetic flux density is defined as the magnetic flux passing through unit area taken perpendicular to the field. B is also known as magnetic field induction.

Let the flux through the area ‘A’ be Φ.

• What is the flux through unit area perpendicular to the field?

It is equal to Φ/A. The ratio of magnetic flux passing through a plane perpendicular to the field and the area of the plane is called the magnetic flux density.

• Can we generalize the formula of flux for any orientation of the plane taken in the field?

Let ‘θ’ be the angle between magnetic field (B) and normal to the plane with area (A) as shown in figure 3(b). The effective area of the plane perpendicular to the field is A cosθ. Then magnetic flux density is given by,

• What is the flux through the plane taken parallel to the field?


• What is the use of introducing the ideas of magnetic flux and magnetic flux density?

Later in this chapter, you will see how these ideas are used.


• Are there any sources of magnetic field other than magnets?


• Do you know how old electric calling bells work?

In activity 1 we observed that the compass needle is deflected when current flows through the circuit. This observation helps us to conclude that, “Current carrying wire produces magnetic field.”

Let us discuss about the magnetic fields produced by a current carrying wire.