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Answers
Magnetic field lines are lines representing magnetic field .
They are closed curves.
Outside the bar magnet the emerge at the North Pole and merge at the South Pole.
The strength of magnetic field is shown by degree of closeness of field lines, they are crowded at the poles ,this shows that the magnetic field is the strongest at the poles.
Activity
Take a circular coil having many turns , and take a rectangular cardboard.
Punch two large holes into it and insert the coil in it.
Connect both the ends of the coil to a circuit comprising a battery, a rheostat and a key.
Spread some iron fillings uniformly onto the cardboard.
Close the key.
Observations, when current is passed through the circuit you will observe that the iron fillings align themselves in concentric circles centering the coil . In the centre of the cardboard they will appear (align themselves) as straight lines.