Needles N₁, N₂ and N₃ are made of a ferromagnetic, a paramagnetic and a diamagnetic substance respectively. A magnet when brought close to them will

• attract all three of them

• attract N₁ and N₂ strongly but repel N₃

• attract N₁ strongly, N₂ weakly and repel N₃ weakly

• attract N₁ strongly, but repel N₂ and N₃ weakly

Relative permittivity and permeability of a material are ε_r and µ_r, respectively. Which of the following values of these quantities are allowed for a diamagnetic material?

• ε_r = 0.5, µ_r = 1.5

• ε_r = 1.5, µ_r = 0.5

• ε_r = 0.5, µr = 0.5

• ε_r = 1.5, µ_r = 1.5 

The length of a magnet is large compared to its width and breadth. The time period of its width and breadth. The time period of its oscillation in a vibration magnetometer is 2 s. The magnet is cut along its length into three equal parts and three parts are then placed on each other with their like poles together. The time period of this combination will be

• 2 s

• 2/3 s

• 2√3 s

• 2/√3 s

The materials suitable for making electromagnets should have

• high retentivity and high coercivity

• low retentivity and low coercivity

• high retentivity and low coercivity

• low retentivity and high coercivity

Two long conductors, separated by a distance d carry current I₁ and I₂ in the same direction. They exert a force F on each other. Now the current in one of them increased to two times and its direction reversed. The distance is also increased to 3d. The new value of the force between them is

• −2F 

• F/3

• −2F/3

• -F/3