A student fixes a sheet of white paper on a drawing board. He places a bar magnet in the centre of it. He sprinkles some iron filings uniformly around the bar magnet. Then he taps the drawing board gently and observes that the iron filings arrange themselves in a particular pattern. (a) Why do iron filings arrange in a particular pattern? (b) What does the crowding of iron filings at the ends of the magnet indicate? (c) What do the lines, along which the iron filings align, represent? (d) If the student places a cardboard horizontally in a current carrying solenoid and repeats the above activity, in what pattern would the iron filings arrange ? State the conclusion drawn about the magnetic field based on the observed pattern of the lines.

(a) The bar magnet exerts its magnetic influence in the region surrounding it, creating a magnetic field. The iron filings experience a magnetic force due to this field, which causes them to align themselves along the magnetic field lines, forming a distinct pattern.

(b) The crowding of iron filings at the ends (poles) of the magnet indicates that the magnetic field is strongest at the poles. The degree of closeness of the field lines represents the relative strength of the magnetic field.

(c) The lines along which the iron filings align themselves represent the magnetic field lines.

(d) Pattern: Inside the solenoid, the iron filings will arrange themselves in parallel straight lines along the axis of the solenoid. Outside the solenoid, they will arrange in a curved loop pattern similar to that of a bar magnet.

Conclusion: The parallel straight lines of the magnetic field inside the solenoid indicate that the magnetic field is uniform (has the same magnitude and direction) at all points inside the solenoid.