lines. If you place a compass at any point on the line, the needle comes to rest along the tangent to the line. So we conclude that the tangent drawn to the field line at a point gives the direction of the field.
The field lines appear to be closed loops, but you can’t conclude that lines are closed or open loops by looking at the picture of field lines because we do not know about the alignment of lines that are passing through the bar magnet. We will come to know about this point later in this chapter.
Observe the spacing between lines. In some places the field lines are crowded (near the poles of a bar magnet) and in some places the field lines are spread apart (at long distances from the bar magnet). From this picture we can conclude that the field is strong when lines are crowded and field is weak when lines are spaced apart.
Thus, the field drawn is non uniform because the strength and direction both change from point to point.
We may define the nature of the field with its characteristics such as its strength and direction. The field is said to be non uniform when any one of the characterstics of field i.e., strength or direction changes from point to point. Similarly the field is said to be uniform if both strength and direction are constant throughout the field. Let us define the strength of a uniform magnetic field.
Consider a uniform magnetic field in space. Imagine a plane of certain area ‘A’ placed perpendicular to the field at a certain point in the magnetic field as shown in figure 3(a). You notice that a few field lines pass through this plane. This number gives an estimation of strength of the field at that point.
The number of lines passing through the plane of area ‘A’ perpendicular to the field is called magnetic flux. It is denoted by ‘Φ’.
Magnetic flux represents the number of lines passing through the imagined plane in the field. Of course, flux depends on the orientation of the plane in the field. But here we are concerned only with the perpendicular case. The S.I unit of magnetic flux is Weber. Now strength of the field is easily defined using the idea of flux. If the imagined plane is perpendicular to the field and has unit area, then the flux through this plane of unit area