Question

A current I passes through a circular loop C and a solenoid AB, is shown below:
What is the polarity of:
(a) the face of the loop you are looking at?
(b) the end B of the solenoid?​

Answer 1

A current I passes through a circular loop C and a solenoid AB, as shown in the given figure.

To find the polarity of :  

(a) The face of the loop you are looking at.

• We see that it is a circular loop of wire and a magnet with N facing the loop is moved towards the coil.
• So, according to Electromagnetic Induction the current that will be produced in the coil will be such that there is imaginary N pole formed on the face of the coil so as to oppose the incoming magnetic N pole.
• So, for forming imaginary N pole on the face of the coil the current induced must be in anti-clockwise direction.

So, the N pole forms on the face of the coil where magnetic N pole is approaching and S pole forms on the opposite face of the coil.

(b) The end B of the solenoid.

• To find the induced N and S pole we can take help of Right Hand Thumb rule.
• Using RHT rule we see that the resultant magnetic field formed due to current inside the solenoid will be in upwards direction.
• Which means that inside the solenoid the magnetic field lines are from downward to upward direction and we know that inside a bar magnet the magnetic field lines are from S pole to N pole.

So we can conclude that in the given solenoid the face at A has 'S' pole and the face at B has 'N' pole.

Answer 2

Faraday's law:

According to Faraday's law, the polarity of the induced emf produces current whose magnetic field opposes the changes that produces the emf.

(a) The polarity of the face of the loop we're looking at:

In this experiment, the north pole of the bar magnet moves towards the loop C. The induced emf is produced in loop that opposes the motion of the magnet towards the loop.

Since, the approaching magnet increases the magnetic field lines in loop that opposes the north pole of the magnet. So, the polarity of loop is north.

(b) The polarity of the end B of the solenoid:

When bar magnet is moved towards B end of the solenoid. Same opposing induced emf effect is present in the solenoid.

Thus, the polarity at the B end is north.