The magnetic field at the centre of a coil of n turns, bent in the form of a square of side 2 l, carrying current i, is :

1.  $\frac{\sqrt{2}{\mathrm{\mu }}_0\mathrm{ni}}{\mathrm{\pi l}}$                               2.  $\frac{\sqrt{2}{\mathrm{\mu }}_0\mathrm{ni}}{2\mathrm{\pi l}}$

3.  $\frac{\sqrt{2}{\mathrm{\mu }}_0\mathrm{ni}}{4\mathrm{\pi l}}$                               4.  $\frac{2{\mathrm{\mu }}_0\mathrm{ni}}{\mathrm{\pi l}}$

A circular coil A  has a radius \(R\) and the current flowing through it is \(I.\) Another circular coil B has a radius \(2R\) and if \(2I\) is the current flowing through it, then the magnetic fields at the centre of the circular coil are in the ratio of (i.e. $B_A$ to $B_B$):
1. \(4:1\)                             
2. \(2:1\)
3. \(3:1\)                             
4. \(1:1\)

A straight wire of diameter 0.5 mm carrying a current of 1 A is replaced by another wire of 1 mm diameter carrying the same current. The strength of the magnetic field far away is :

1. Twice the earlier value

2. Half of the earlier value

3. Quarter of its earlier value

4. Unchanged

A wire carrying current i is shaped as shown. Section AB is a quarter circle of radius r. The magnetic field is directed :

                                                      
1. At an angle $\mathrm{\pi }/4$ to the plane of the paper

2. Perpendicular to the plane of the paper and directed in to the paper

3. Along the bisector of the angle ACB towards AB

4. Along the bisector of the angle ACB away from AB

Two long straight wires are set parallel to each other. Each carries a current i in the same direction and the separation between them is 2r. The intensity of the magnetic field midway between them is- 

1. ${\mu }_{0}i/r$                            
2. Zero
3. $4{\mu }_{0}i/r$                             
4. ${\mu }_{0}i/4r$

The earth’s magnetic field at a given point is $0.5\times {10}^{-5}Wb-m^{-2}.$ This field is to be annulled by magnetic induction at the center of a circular conducting loop of radius 5.0cm. The current required to be flown in the loop is nearly :

1. 0.2 A                  

2. 0.4A

3. 4A                      

4. 40A

A part of a long wire carrying a current i is bent into a circle of radius r as shown in the figure. The net magnetic field at the centre O of the circular loop is

1. $\frac{{\mu }_{0}i}{4r}$                                

2. $\frac{{\mu }_{0}i}{2\mathrm{\pi r}}\left(\mathrm{\pi }+1\right)$

3. $\frac{{\mu }_{0}i}{2r}$                                

4. $\frac{{\mu }_{0}i}{2\mathrm{\pi r}}\left(\mathrm{\pi }-1\right)$

What is the magnetic field at point \(O\) in the figure?

The magnetic moment of a current (i) carrying circular coil of radius (r) and number of turns (n) varies as :

1. $1/r^2$                            

2. $1/r$

3. r                                  

4. $r^2$

If the current is flowing in the south direction along a power line, then what will be the direction of the magnetic field above the power line (neglecting the earth's field)?