Question

A soft iron bar is enclosed by a coil of insulated copper wire as shown in figure. When the plug of the key is closed, the face B of the iron bar marked as:
 

 A: N-pole
 B: S-pole
 C: N- pole if current is large
 D: S-pole if current is small

Answer 1

Hint :In the figure, we are given a solenoid. In a solenoid, we can find the developed polarity by identifying the direction of the current flow in the solenoid, that is, by checking whether it is clockwise or anticlockwise. We can solve this problem by keeping this point in our mind.

Complete Step By Step Answer:
A solenoid is a device that comprises a coil of a conducting material like copper, and a movable armature. When electric current is supplied to the solenoid, it forms a magnetic field around the coil and the armature is moved. In this way, the solenoid converts electrical energy to mechanical energy.
 The speciality of soft iron is that it is a highly ferromagnetic material which allows maximum field lines to pass through it. When current is supplied to solenoid, it makes a magnetic field inside and outside of it. The solenoid magnetizes the iron bar and the field strengthens, thus developing a polarity on the sides of the solenoid, depending on the direction of current.
 From the figure, we can see that the current at B is clockwise. Thus a north polarity is developed.
Hence, option A is the correct answer in the given options.

Note :
Ampere’s right hand rule for current is used in this case. That is, when we curl the fingers in the direction of the current, our thumb extends to the magnetic north pole. As the direction of current is clockwise at B, so on curling the fingers clockwise, we get that the north pole is developed at B and the south pole is developed at A.