By increasing the number of turns $3$ times in a toroidal solenoid,the magnetic field:

Commercial electric motors do not use

An alternating electric current has a frequency of $50 \ Hz$. How many times does it change its direction in one second (in $times$)?

In the arrangement shown in the figure,there are two coils wound on a non-conducting cylindrical rod. Initially,the key is not inserted. Then the key is inserted and later removed. Then

$A$ current $I$ passes through a circular loop $C$ and a solenoid $AB$ as shown below:
$(i)$ What is the polarity of: $(a)$ the face of the loop you are looking at? $(b)$ the end $B$ of the solenoid?
$(ii)$ What is the direction of the magnetic field at the centre of the loop?

Identify the poles of the magnet in the given figures $(1)$ and $(2).$