How many times does an $AC$ electric current with the frequency $50\, Hz$ change its direction in one second?

The diagram below shows a circuit containing a coil wound over a long and thin hollow cardboard tube. Copy the diagram and:
$(i)$ Show the polarity acquired by each face of the solenoid.
$(ii)$ Draw the magnetic field lines of force inside the coil and also show their direction.
$(iii)$ Mention two methods to increase the strength of the magnetic field inside the coil.

Whenever the magnetic field through a coil is changed,a potential difference is produced in the coil.

By which instrument can the presence of a magnetic field be determined?

What is the role of a fuse used in series with any electrical appliance? Why should a fuse with a defined rating not be replaced by one with a larger rating?

Two coils $C_{1}$ and $C_{2}$ are wrapped around a non-conducting cylinder. Coil $C_{1}$ is connected to a battery and a key $K$,and coil $C_{2}$ is connected to a galvanometer $G$. On pressing the key $K$,current starts flowing in the coil $C_{1}$. State your observations in the galvanometer for the following cases:
$(i)$ When key $K$ is pressed on.
$(ii)$ When current in the coil $C_{1}$ is switched off.
$(iii)$ When the current is passed continuously through coil $C_{1}$.
$(iv)$ Name and state the phenomenon responsible for the above observation. Write the name of the rule that is used to determine the direction of the induced current.