MOVING CHARGES AND MAGNETISM — Physics STD 12 Science — Question

→Solenoid : "A helical coil consisting of closely wound turns of insulated conducting wire is called a solenoid."
→If the length of a solenoid is very large as compared to its radius, the solenoid is called long solenoid.
→Solenoid consists of a long wire wound in the form of a helix where the turns are closely spaced. Hence, each turn can be considered as a circular loop.
→The total magnetic field of solenoid is the vector sum of the fields due to all the turns.
→Enamelled wires are used for winding to make turns insulated from each other.



(a) The magnetic field due to a section of the solenoid which has been stretched out for clarity. Only the exterior semicircular part is shown. Notice how the circular loops between neighbouring turns tend to cancel.



(b) The magnetic field of a finite solenoid.
→Figure shows the magnetic field lines for a finite solenoid.
→It is clear from the figure (a) that the magnetic field of the circular loop between two neighbouring turns cancel out each other (in opposite direction)
→From the figure (b), it can be seen that the magnetic field is uniform, strong and in the direction of its axis at the mid point P inside the solenoid.
→While the magnetic field at the outer mid point Q is weak and almost parallel to the axis of the solenoid.
→ As the solenoid is made longer it looks like a long cylindrical metal sheet.
→Here we assume that for a long solenoid, the magnetic field outside the solenoid is zero, while inside the magnetic field is parallel to the axis every where.