Which of the following statements regarding solenoid are correct?

(i) The wire wound with a number of turns in the form of helix is known as solenoid.

(ii) The magnetic flux density is directly proportional to the number of turns, current and absolute permeability of the solenoid.

(iii) The magnetic field outside the solenoid is zero.

The correct answer is option 4): ((i), (ii) and (iii))

Concept:

• A solenoid is a device comprised of a coil of wire, the housing and a moveable plunger (armature).
• When an electrical current is introduced, a magnetic field forms around the coil which draws the plunger in. More simply, a solenoid converts electrical energy into mechanical work.
• A solenoid is an instrument consisting of copper coiling over a cylinder designed to create a strong magnetic field inside the coil because of the flow of current through the coil. By wrapping the same wire many times around a cylinder, the magnetic field due to the flow of current can become quite strong. The wire wound with a number of turns in the form of a helix is known as a solenoid.
• Strength of the magnetic field in a solenoid will be directly proportional to the number of turns and amount of current flowing through a wire and will be inversely proportional to its length
• \(B = \frac{{{\mu _0}NI}}{L}\)
• B is Magnetic field strength, N is Number of turns, I is the Current, L is Length of solenoid, and μ₀ is Permeability of air
• The magnetic field lines also  present outside the solenoid
•  The flu outside the solenoid is much less compared to when flux inside the solenoid. Hence the magnetic field outer region of solenoid is so weak commonly considered as zero.