Define Solenoid. What is the behaviour of a current-carrying solenoid?  List the main application of a solenoid.

A coil of many circular turns of insulated copper wire wound on a cylindrical insulating body, such that its length is greater than its diameter, is called a solenoid. 

The field lines around a current-carrying solenoid are similar to that produced by a bar magnet. This means that, when current is flowing through the solenoid, the magnetic field line pattern resembles exactly those of a bar magnet with fixed polarity, i.e. North and South poles at its ends, and it acquires the directive and attractive properties similar to a bar magnet. Hence, the current-carrying solenoid behaves as a bar magnet.  

Uses of a current-carrying solenoid:  

1. It is used to form a temporary magnet called an electromagnet as well as a permanent magnet.  
2. The strong magnetic field produced inside a solenoid can be used to magnetise a piece of a magnetic material like soft iron when placed inside the coil.