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Theory:

In the previous sections, we have learnt about magnetic fields in detail. In this session, through an activity, we will practice drawing the field lines of a bar magnet by ourselves by ourselves through an activity.
 
Let us perform the following activity.

Steps:
  • To do this activity, take a small compass and a bar magnet.
  • Using an adhesive material, locate the magnet on a sheet of white paper fixed on a drawing board.
  • Mark the boundaries of the magnet.
  • Locate the compass near the north pole of the bar magnet. How does it act?
The south pole of the needle moves towards the north pole of the bar magnet. The north pole of the compass is pointed away from the north pole of the bar magnet.
  • Observe and mark the position of two ends of the needle.
  • Now relocate the needle to a new position such that its south pole obtains the position previously occupied by its north pole.
  • In this way, progress step by step until you reach the magnet's south pole, as shown in the below figure.
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Position of the needles from north to the south pole
  • Connect the points recorded on the paper by a smooth and uniform curve. This curve represents a field line.
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Magnetic field lines of a bar magnet
  • Repeat the above steps and draw as many lines as you can. You will get a pattern shown in the above figure. These lines represent the magnetic field around the magnet. These are known as magnetic field lines.
  • Note the deflection in the compass needle as you move it along a field line. The deflection increases as the needle are moved towards the poles.
We know that the magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is known to be the direction in which the compass needle's north pole moves inside it. Therefore, it is the practice that the field lines arise from the north pole and join at the south pole (note the arrows indicated on the field lines in the figure).

Inside the bar magnet, the direction of magnetic field lines is from its south pole to its north pole. Thus the magnetic field lines are looked at as the closed curves.
 
The relative strength of the magnetic field is related to the degree of closeness of the magnetic field lines. The field is stronger; the force acting on the pole of another magnet placed is greater where the field lines are crowded (see the figure).
 
No two field lines are found to cross each other. If they cross each other, it would mean that the compass needle would point towards two directions at the point of junction (intersection), which is practically not possible.