Magnetic lines of force

Magnetic lines of force

1.4         Magnetic field : Experiments have shownthat a magnet or a current carrying conductor can exert force on the nearby magnetic substances even when they are not in touch with each other. It implies that there is a magnetic force around a magnet or a current carrying conductor. The area in which magnetic force is present and magnetic effect is felt is called magnetic field.

Detection of the presence of a magnetic field.

Keep a magnetic needle in the area to be tested. If the magnetic needle deflects, then it means that there is a magnetic field.

Usually magnetic field is represented by a set of lines. These lines are called magnetic lines of force. Why is magnetic field represented by a set of lines ? Magnetic field or magnetic lines of force are invisible. The fact that the iron filings arrange themselves along a few curved lines (fig 1.12(b)) in a magnetic field might have resulted in the concept of lines of force. The magnetic lines of force can also be traced by a magnetic needle. The pattern of magnetic lines of force depends on the shape of the magnet and its strength. Therefore the pattern of magnetic lines will be different for different magnets. The study of magnetic lines of force around a powerful bar magnet reveals the following properties.

1.      Magnetic lines of force are relatively compact wherever the field strength is strong (near poles) and less in number wherever the field strength is weak.

2.      Magnetic lines of force are spread from pole to pole. The convention is to show them as starting from north pole and ending at south pole.

3.      Magnetic lines of force keep pubshing each other away and never corss each other.

4.      Magnetic lines of force appear to possess tnsion as in a bent cane held pressed in the from of an are.

5.      Magnetic lines of force can penetrate substances like paper, glass etc.

6.      The field strength decreases as one mover away from the source causing it.

Do it yourself

Keep a bar magnet on a table. Keep a rectangular card board horizontally on the magnet. Sprinkle iron fillings uniformly on the card board. Gently tap the cardboard. Gently tap the cardboard. Observe the arrangement of iron fillings on the cardboard. Draw a figure of what you observe.

Magnetic substances

The substances that are attracted by a magnet are called magnetic substances. Example : Iron, nickel, cobalt, steel etc. 

Nonmagnetic substances : The substances that are not attracted by a magnet are called nonmagnetic substances. Example : Paper, glass, water, common salt, plasti etc.

In a sense, all substances have magnetic properties. Howerer the degree of magnetism exhibited by them differs. Based on their properties when brought under the influence of powerful magnetic field, They are classfied into three groups – Diamagnetic, Paramagnetic and Ferromagnetic substances.

Do You Know ?

Experiments have revealed that the ability of paramagnetic substance to get magnetised goes on increasing as we go on cooling it. This abillity goes on decreasing as its temperature increases and at one specific temperature it altogether vanishes, Can you guess why ?

Magnetic field and magnetic lines of force : Space around a magnetic pole or magnet or current carrying wire within which it's effect can be experienced is defined magnetic field. Magnetic field can be represented with the help of a set of lines or curves called magnetic lines of force.

General Properties of Magnetic Lines of Force
Magnetic lines of force have a number of important properties, which include:
They seek the path of least resistance between opposite magnetic poles. In a single bar magnet as shown to the right, they attempt to form closed loops from pole to pole.
They never cross one another.
They all have the same strength.
Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability.
Their density decreases with increasing distance from the poles.
They are considered to have direction as if flowing, though no actual movement occurs.
They flow from the south pole to the north pole within a material and north pole to south pole in air.