Question

The following diagram shows a spiral coil wound on a hollow cardboard tube AB. A magnetic compass is placed close to it. Current is switched on by closing the key. What will be the polarity at the ends A and B?

• A. A - north pole, B - south pole
• B. A - south pole, B -north pole
• C. A - south pole, B - south pole
• D. A - north pole, B - north pole

Answer 1

The correct option is A A - north pole, B - south pole

The polarity of the magnetic field (that is, which end is magnetic-north or magnetic-south) can easily be found from the Right hand thumb rule. If we hold the device in our right hand, so that the fingers curl around it in the direction of the conventional current flow, then the thumb will point to the north pole of the magnet. Also, a solenoid when suspended freely aligns itself in the north-south direction, thus behaving like a magnet. One end of the solenoid acts like a north pole and the other end the south pole.

The polarity of the solenoid AB can be changed by reversing the direction of the current. If we imagine that we are facing the coil end-on, then if the current flows in a clockwise direction, we are looking at the south pole. If the current flows in an anticlockwise direction, we are looking at the north pole.
When a magnetic compass is brought closer to A end of the solenoid, the needle deflects away from the north-south direction as like poles repel.

Hence, the polarity at A is north and polarity at B is south.