Question

A metallic ring is kept on a hard paper (which is transparent to magnetic field) such that its center coincides with the axis of a long solenoid as shown. If current is solenoid is switched on then normal reaction between ring and paper will:

A) Decreases
B) Increases
C) Remain unchanged
D) First decreases and then increases

Answer 1

1. Solenoid: It is a cylindrical coil made of wire acting as a magnet when current flows through it.
2. Solenoid behaves as an electromagnet.
3. Solenoid is coated with an iron core that increases the magnetic effect of the coil.
4. The magnetic effect in the solenoid is used to produce a straight line in motion.
5. Solenoid is designed to convert electrical energy into mechanical energy.
6. When current is allowed to cross inside the solenoid coil magnetic field is produced around the solenoid coil.

Complete answer:
Solenoid is used to control a valve electrically; it is used in door locking systems, computer printers, fuel injections gear used in cars. In hydraulic and pneumatics valves solenoid plays a vital role. According to the given figure in the question, if current in the solenoid is turned on then the normal reaction between the ring and the paper will decrease.

Hence the correct option is $\left( {\text{A}} \right)$.

Note: 1. Magnetic field lines never cross each other.
2. All the magnetic field lines of force have the same strength.
3. The density of the magnetic field decreases with the increase of the distance from the pole.
4. Magnetic field lines are imaginary lines considered for the calculations and better understandings.
5. Normal reaction is the force exerted by a surface on an object which is in contact with it which prevents the object from passing through the surface.
6. The current carrying solenoid behaves as an independently suspended object.
7. The magnetic lines of force are dense near the ends of the pole of the solenoid.