What is a solenoid ? Draw a neat diagram and name its various components.

Answer: A solenoid is a long cylindrical coil made of many closely wound turns of insulated copper wire. When current flows through it, it behaves like a bar magnet.

Diagram (solenoid and its magnetic field):

   field lines outside: N -----> curve round -----> S
         _______________________________________
   S    |  (   (   (   (   (   (   (   (   (    |    N
        | ---------------------------------->   |   ← field lines inside:
        | ---------------------------------->   |      straight, parallel
        |  )   )   )   )   )   )   )   )   )    |      (uniform field)
        |_______________________________________|
          |  turns of insulated copper wire   |
          |  wound on a cylinder (core)       |
     end A|                                   |end B
          |______ + | - _______ ( key ) ______|
               battery            switch

Labels: (1) insulated copper wire, (2) turns of the coil, (3) cylindrical core (cardboard/air or soft iron), (4) two ends of the coil, (5) battery, (6) plug key/switch, (7) magnetic field lines with N and S poles.

Components and working:

• Coil of insulated copper wire wound closely in many turns in the form of a cylinder.
• Core on which the wire is wound (air/cardboard tube; with a soft iron core it becomes an electromagnet).
• Battery and key connected to the two ends of the coil to pass current.

When current is passed, the magnetic field lines inside the solenoid are straight and parallel, i.e. the field inside is uniform. One end of the solenoid behaves as the north pole and the other as the south pole, just like a bar magnet. The strength of the field increases with the number of turns and with the current.