12th Standard Physics March 2026: Online Practice Test

Explanation. After touching, each ball carries \(\frac{q_1+q_2}{2}\). Since \(\left(\frac{q_1+q_2}{2}\right)^2 \ge q_1q_2\), the force (proportional to the product of charges) becomes more than before (equal only if \(q_1=q_2\)).

Explanation. \(R=\frac{\rho L}{\pi r^2}\), so with same \(\rho\) and L, \(R\propto\frac{1}{r^2}\). \(\frac{r_A^2}{r_B^2}=\frac{R_B}{R_A}=\frac{1}{3}\), hence \(\frac{r_A}{r_B}=\frac{1}{\sqrt{3}}\).

Explanation. Current \(I=\frac{q\omega}{2\pi}\), so \(\mu=I\pi r^2=\frac{q\omega r^2}{2}\). Angular momentum \(L=mr^2\omega\). Hence \(\frac{\mu}{L}=\frac{q}{2m}\).

Explanation. \(\varepsilon=L\frac{\Delta I}{\Delta t}\), with \(\Delta I=4\) A: \(L=\frac{8\times0.05}{4}=0.1\) H.

Explanation. A stationary charge produces only an electric field and a uniformly moving charge produces steady fields; only an accelerating charge radiates electromagnetic waves.

Explanation. In a medium the refractive index varies with wavelength (dispersion), so the speed \(v=c/n\) depends on the wavelength of light.

Explanation. Fringe width \(\beta=\frac{\lambda D}{d}\). If d becomes 2d, D must become 2D to keep \(\beta\) unchanged.

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

Explanation. Binding energy \(=0.042\times931\approx39.1\) MeV. Per nucleon \(=39.1/7\approx5.6\) MeV.

Explanation. A solar cell converts light energy directly into electrical energy by the photovoltaic effect.

Explanation. According to the Standard Model, particles acquire mass through the Higgs field, whose quantum is the Higgs particle (boson).

Explanation. \(U=-\vec{p}_m\cdot\vec{B}=-(0.4\times2)=-0.8\) J.

Explanation. Different equipotential surfaces have different potentials, the field is always perpendicular to them, and no work is done in moving a charge between two points on the same surface. So all statements hold (statement (c) refers to points on the same surface).

Explanation. Along the optic axis there is no double refraction: the ordinary and extraordinary rays travel with the same velocity, so the ratio is 1 : 1.

Explanation. \(d=\sqrt{2Rh}\Rightarrow h=\frac{d^2}{2R}=\frac{64^2}{2\times6400}=\frac{4096}{12800}=0.32\) km.