What is the principle of an electric motor?

How can it be shown that a magnetic field exists around a wire through which a direct current is passing?

What is the shape of the magnetic field lines inside a solenoid?

Explain briefly different methods of producing induced electromotive force $(emf)$.

What are magnetic field lines? How is the direction of magnetic field at a point determined? Mention two important properties of magnetic field lines.

$(a)$ In a pattern of magnetic field lines due to a bar magnet,how can the regions of relative strength be identified?
$(b)$ Compare the strength of the magnetic field near the poles and the middle of a bar magnet.