What is the source of a magnetic field?

When an electric current is passed through a solenoid,how can we determine which end behaves as a North pole and which as a South pole? Explain.

If an electron revolves around a nucleus in a circular orbit of radius $R$ with frequency $n$,then the magnetic field produced at the centre of the nucleus will be

Describe and illustrate the magnetic field lines produced by a current-carrying circular loop.

$A$ current $I$ flows in a wire bent in the shape of a quarter ring of radius $R$ as shown in the figure. The magnetic field induction $B$ at point $O$ is

The current of $5 \ A$ flows in a square loop of side $1 \ m$ placed in air. The magnetic field at the centre of the loop is $X \sqrt{2} \times 10^{-7} \ T$. The value of $X$ is . . . . . . .