List three sources of magnetic fields.

Four circular loops of radii $1 \; CM$,$2 \; CM$,$3 \; CM$,and $4 \; CM$ carry the same electric current. At the center of which loop will the magnetic field be the strongest?

Choose the incorrect statement.

When a light,straight conducting wire is suspended perpendicular to a strong magnetic field and an electric current is passed through it,the magnetic force acting on it is proportional to which of the following?

$A$ magnetic compass needle is placed in the plane of paper near point $A$ as shown in the figure. In which plane should a straight current-carrying conductor be placed so that it passes through $A$ and there is no change in the deflection of the compass? Under what condition is the deflection maximum and why?

Which of the following is used in a fire alarm?