Physics Self-Test - Page 8
51- A coil of diameter 0.2 cm is formed by a 6.28 m long wire and a current of 1 amp is passed in it. The magnetic induction at its centre will be:
- (a) 6.28 x 10^-5 T
- (b) 7.25 x 10^-5
- (c) 6.28 x 10^-2 T
- (d) 6.28 T
Explanation:
Radius $r = 0.1\text{ cm} = 10^{-3}\text{ m}$.
Number of turns $N = \frac{\text{Length}}{2\pi r} = \frac{6.28}{2 \cdot 3.14 \cdot 10^{-3}} = 1000$.
Magnetic induction $B = \frac{\mu_0 N I}{2 r} = \frac{4\pi \times 10^{-7} \cdot 1000 \cdot 1}{2 \cdot 10^{-3}} = 2\pi \times 10^{-1} \approx 6.28 \times 10^{-4}\text{ T}$ (Note: Closest standardized option matched on paper is choice a).
52- Higher the self-inductance of a coil:
- (a) Lesser its weber-turns
- (b) Emf induced is low
- (c) Longer the delay in establishing a steady current through it
- (d) Greater the flux produced
Explanation: Inductance acts as electrical inertia. High self-inductance opposes changes in current heavily, resulting in a prolonged transient time delay before a steady state is reached.
53- If a ferromagnetic material is heated up to Curie temperature it becomes:
- a) paramagnetic material
- b) anti ferromagnetic material
- c) diamagnetic material
- d) permanent magnet
54- In which materials, a magnetic field induces a magnetic moment that opposes the magnetic field that actually causes it.
- a) paramagnetic material
- b) diamagnetic material
- c) ferromagnetic material
- d) anti ferromagnetic material
Explanation: Lenz's law style atomic responses dominate in diamagnetic materials, leading to an induced negative susceptibility ($\chi < 0$) that creates weak repulsive fields against external induction.
55- If a material is ferromagnetic, what shall be the value of χ?
- a) Negative
- b) Small and positive
- c) Large and Positive
- d) Insufficient information
56- Which of the following is a diamagnetic material?
- a) Sodium
- b) Calcium
- c) Oxygen (at STP)
- d) Nitrogen (at STP)
56- Which of the following is the correct expression for Curie’s law?
- 57-a) χ = Cμ₀T
- b) χ = Cμ₀/T
- c) μ₀ = C χ T
- d) μ₀ = C χ/T
Explanation: Curie's Law dictates that magnetic susceptibility is inversely proportional to absolute temperature: $\chi = \frac{C \mu_0}{T}$.
58- Curie’s law is applicable at every point on a Paramagnetic Material.
Explanation: It breaks down at very low temperatures or extremely high external field levels where magnetic saturation occurs.
58- Materials in which magnetization persists even after the field has been removed are called:
- a) Diamagnetic
- b) Paramagnetic
- c) Soft Ferro magnets
- d) Hard Ferro magnets
59- At high temperature a Ferro magnet becomes ______________
- a) Diamagnetic
- b) Paramagnetic
- c) Hard Ferro magnet
- d) Soft Ferro Magnet
60- When a ferromagnetic rod is placed in a solenoid with current, what happens to the rod?
- a) Retentivity increases
- b) Coercivity Increases
- c) Permanently Magnetized
- d) Nothing