Question

Class X Sciences
[papers from previous years]

(a) What is a solenoid? Draw a sketch of the pattern of field
lines of the magnetic field through and around a current carrying
solenoid.
(b) Consider a circular loop of a wire lying in the plane of
the table. Let the current pass through the loop clockwise. Apply
the right hand rule to find out the direction of the magnetic
field inside and outside the loop.

Answer 1

a)
A solenoid is a current carry conductor coil closely wound in cylindrical shape.   Often inside the space of a solenoid a ferro magnetic material in a cylindrical shape, is kept to increase the magnetism of the solenoid. When current is passed through the solenoid , It generates a nearly uniform magnetic field inside the coil.   Magnetic fields inside a solenoid

   Magnetic field lines through a current carrying conductor loop is along its axis at its center.   As the solenoid of big length and small radius has a number of coils carrying the same current and have no gaps in between the windings, the magnetic lines add up. 

   The resulting magnetic field lines in side the solenoid become more or less uniform and aligned along the axis of the solenoid.  At the edges they tend to diverge towards the opening.  Outside the solenoid the effects of the neighboring windings cancels out the magnetic field to zero.

Use as an electromagnet

     Thus the cylindrical area inside a solenoid is an electromagnet by itself.  We can also wrap a solenoid around a ferromagnetic material like iron, say, over half or full length of the bar.  The bar can be bent in a shape of a "U"or a "horse shoe" or a "circle".  Then the air gap between the  two ends can be used as a magnetic field.

   The empty solenoid can be bent in a shape of a horse shoe also.

Strength

    The strength of the electromagnet above is increased by adding more number of windings to the solenoid, increasing the number of windings per unit length, increasing the current, decreasing the diameter of solenoid.

     It can be increased also by introducing a ferro-magnet of high paramagnetic property inside the solenoid.

    It can be increased by adding another solenoid of larger diameter around the solenoid.
    The strength of an electromagnet can be increased by immersing the solenoid and surroundings in side a paramagnetic fluid of high permeability μ.

         B = μ₀ μ n i ,        n = windings/unit length,  i = current
=========================================
2)

the magnetic field  as per the right hand thumb rule:  is downward into the table.  we curl the fingers in the direction of the current, and the thumb points to the direction of magnetic field.