12th Standard Physics Supplementary 2021: Online Practice Test

Explanation. The field of an infinite uniformly charged plane is \(E=\frac{\sigma}{2\varepsilon_0}\), independent of distance, so it is uniform. The fields of a point charge, shell and line all decrease with distance.

Explanation. The teeth of the parrot fish, made of tightly woven crystal fibres, serve as the biomimetic model for ultra durable synthetic materials. Lotus leaf inspires self-cleaning surfaces, and the Morpho butterfly and peacock feather inspire structural colour.

Explanation. For an ideal transformer \(\frac{I_s}{I_p}=\frac{N_p}{N_s}\), so \(I_s=6\times\frac{410}{1230}=2\) A.

Explanation. Electromagnetic waves are transverse (E and B are perpendicular to the direction of propagation), are produced by accelerating charges and need no medium. So "longitudinal" is false.

Explanation. Since \(R\propto A^{1/3}\), \(R_{Cu}=3.6\times\left(\frac{64}{27}\right)^{1/3}=3.6\times\frac{4}{3}=4.8\) fermi.

Explanation. Electric flux \(\Phi_E=EA\); E is in \(\text{NC}^{-1}\) and A in \(\text{m}^2\), so the unit is \(\text{Nm}^{2}\text{C}^{-1}\).

Explanation. Total energy \(\frac{Q^2}{2C}\). When shared equally, electric energy \(\frac{q^2}{2C}=\frac{1}{2}\cdot\frac{Q^2}{2C}\), so \(q=\frac{Q}{\sqrt{2}}\).

Explanation. The magnetic length of a bar magnet is about \(\frac{5}{6}\) of its geometrical length, so the ratio is \(\frac{5}{6}\approx0.833\).

Explanation. In reverse breakdown a Zener diode keeps the voltage across it nearly constant over a wide range of current, so it is mainly used as a voltage regulator.

Explanation. Refractive index \(n=\frac{V_a}{V_w}\) (also equal to \(\frac{\lambda_a}{\lambda_w}\), since frequency does not change).

Explanation. The least distance of distinct vision (near point) of a normal eye is 25 cm.

Explanation. \(I=\frac{\xi}{R+r}\Rightarrow 0.2=\frac{2.1}{10+r}\Rightarrow 10+r=10.5\), so \(r=0.5\ \Omega\).

Explanation. \(\frac{1}{2}mv^2=qV\Rightarrow v=\sqrt{\frac{2qV}{m}}\). Force \(F=qvB=qB\sqrt{\frac{2qV}{m}}=\sqrt{\frac{2q^3B^2V}{m}}\).

Explanation. Fringe width \(\beta=\frac{\lambda D}{d}\). If d becomes 2d, β stays the same only if D becomes 2D.

Explanation. When a metal is heated, its free electrons gain enough thermal energy to escape; this is thermionic emission.