$A$ current passing through a circular coil of two turns produces a magnetic field $B$ at its centre. The coil is then rewound so as to have four turns and the same current is passed through it. The magnetic field at its centre now is

Six very long insulated copper wires are bound together to form a cable. The currents carried by the wires are $I_1 = +10 \text{ A}, I_2 = -13 \text{ A}, I_3 = +10 \text{ A}, I_4 = +7 \text{ A}, I_5 = -12 \text{ A}$ and $I_6 = +18 \text{ A}$. The magnetic induction at a perpendicular distance of $10 \text{ cm}$ from the cable is (Given: $\mu_0 = 4\pi \times 10^{-7} \text{ T m/A}$) (in $\mu\text{T}$)

At a distance of $10\, cm$ from a long straight wire carrying current,the magnetic field is $0.04\, T$. At a distance of $40\, cm$,the magnetic field will be....$T$

Two long straight parallel wires are separated by a distance $2d$. Each wire carries a current $I$ in the same direction. The magnetic field at a point $P$ midway between them is

The dimensions of the ratio of magnetic flux $(\phi)$ and permeability $(\mu)$ are

Two parallel wires carry currents $i_1$ and $i_2$ $(i_1 > i_2)$. When the currents are in the same direction,the magnetic field at a point midway between the wires is $10 \, \mu T$. When the direction of $i_2$ is reversed,the magnetic field at that point becomes $30 \, \mu T$. What is the ratio $\frac{i_1}{i_2}$?