A coil of one loop is made from a wire of length L and thereafter a coil of two loops is made from same wire. The ratio of magnetic field at the centre of the coils will be:
1.  1 : 4
2.  1 : 1
3.  1 : 8
4.  4 : 1

A current-carrying coil (I = 5A, R = 10 cm) has 50 turns. The magnetic field at its centre will be:
1.  1.57 mT
2.  3.14 mT
3.  1 mT
4.  2 mT

For the adjoining figure, the magnetic field at a point 'P' will be:

1. $\frac{{\mu }_0}{4\pi }\odot$

2. $\frac{{\mu }_0}{\pi }\otimes$

3. $\frac{{\mu }_0}{2\pi }\otimes$

4. $\frac{{\mu }_0}{2\pi }\odot$

The magnetic field of a given length of wire for a single-turn coil at its centre is 'B'. Its value for two turns coil for the same wire will be:

1. $\frac{\mathrm{B}}{4}$

2. $\frac{\mathrm{B}}{2}$

3. $4\mathrm{B}$

4. $2\mathrm{B}$

An electron moves in a circular orbit with a uniform speed \(v\). It produces a magnetic field \(B\) at the centre of the circle. The radius of the circle is proportional to:
1. \(\sqrt{\frac{v}{B}}\)
2. \(\frac{v}{B}\)
3. \(\frac{B}{v}\)
4. \(\sqrt{\frac{B}{v}}\)