$A$ long solenoid of $50\, cm$ length having $100$ turns carries a current of $2.5\, A$. The magnetic field at the centre of the solenoid is $...... \times 10^{-5}\, T$. $(\mu_{0} = 4\pi \times 10^{-7}\, T\, m\, A^{-1})$

The magnetic energy stored in a long solenoid of area of cross-section $A$ in a small region of length $L$ is

$A$ long solenoid has $70 \text{ turns } cm^{-1}$ and carries current $I$. An electron moves within the solenoid in a circle of radius $2.5 \text{ cm}$ perpendicular to the solenoid axis. If the speed of the electron is $4.4 \times 10^6 \text{ m s}^{-1}$, then the current $I$ in the solenoid is (Take $\mu_0 = 4 \pi \times 10^{-7} \text{ SI unit}$, mass of electron $= 9 \times 10^{-31} \text{ kg}$, charge of electron $= 1.6 \times 10^{-19} \text{ C}$) (in $\text{ mA}$)

$A$ solenoid $60 \;cm$ long and of radius $4.0\; cm$ has $3$ layers of windings of $300$ turns each. $A$ $2.0 \;cm$ long wire of mass $2.5\; g$ lies inside the solenoid (near its centre) normal to its axis; both the wire and the axis of the solenoid are in the horizontal plane. The wire is connected through two leads parallel to the axis of the solenoid to an external battery which supplies a current of $6.0\; A$ in the wire. What value of current (in $A$) in the windings of the solenoid can support the weight of the wire? $(g=9.8\; m \,s ^{-2})$

The correct curve between the magnetic induction $B$ along the axis of a long solenoid due to current flow $i$ in it and distance $x$ from one end is:

What is a solenoid? Provide information about the magnetic field through a qualitative discussion.