CBSE Class 10 Science 2019 · Paper 31/1/1 · Question 21

A solenoid is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. (i) Current-carrying solenoid: inside, the field lines are parallel straight lines along the axis (a uniform field); outside, they come out of one end (N), curve round and enter the other end (S), as for a bar magnet. The end at which the current appears anticlockwise is N. (ii) Bar magnet: the field lines emerge from the N pole and merge at the S pole outside the magnet, and run from S to N inside it; they are closed curves, never cross, and are crowded near the poles. Distinguishing features (any two): (1) The solenoid's field exists only while a current flows, whereas the bar magnet's field is permanent. (2) The strength of the solenoid's field can be changed by changing the current, whereas that of a bar magnet is fixed. (3) The direction of the solenoid's field (its polarity) is reversed by reversing the current, whereas the poles of a bar magnet are fixed.