A solenoid of length 0.4 m and having 500 turns of wire carries a current of 3 A . A thin coil having 10 , turns of wire and of radius 0.01 m carries a current of 0.4 A . The torque required to hold the coil in the middle of the solenoid with its axis perpendicular to the axis of solenoid.
Which of the following graph represents the variation of magnetic flux density $B$ with distance $r$ for a straight long wire carrying an electric current?




Three charges, each of $+4 \mu \mathrm{C}$, are placed at the corners $A, B, C$ of a square $A B C D$ of side 1 m . What is the magnitude (in $\mathrm{NC}^{-1}$ ) of electric field at point $O$ towards point $D$ ?
36 cells each of internal resistance $0.5 \Omega$ and emf 1.5 V each are used to send current through an external circuit of $2 \Omega$ resistor. For getting maximum current, $n$ cells are combined in series in each row. If total number of such rows are $m$, then the value of $(n / m)$ will be