Physics

Holes, which are the absence of electrons acting as positive charge carriers, can exist in conductors, insulators, and semiconductors. However, they are most significant and commonly discussed in semiconductors. Since holes can theoretically exist in all these materials, the correct answer is 'All of the above'.

When two protons are brought closer, the electrostatic repulsive potential energy between them increases because like charges repel each other, raising their potential energy.

A ferromagnet becomes fully magnetized when subjected to a direct current (D.C.) at its peak value, as this provides a steady magnetic field that aligns the magnetic domains. Alternating current (A.C.) continuously changes direction, preventing full magnetization.

One megawatt hour (MWh) is equal to 1,000,000 watts multiplied by 3600 seconds, which equals 3.6 x 10^9 joules. This is confirmed by multiple authoritative sources. Therefore, the correct answer is option C: 36 x 10^9 J (which is equivalent to 3.6 x 10^10 J, but the intended meaning is 3.6 x 10^9 J). The originally marked answer A (36 x 10^6 J) is incorrect as it equals 3.6 x 10^7 J, which is much smaller than the actual value.

The strain energy per unit volume (also called strain energy density) stored in an elastic material under stress without exceeding the elastic limit is given by (1/2) × stress × strain. Among the given options, 'Stress x strain' (option C) is the closest to the correct formula, although it ideally should include the factor 1/2. The other options are incorrect as they do not represent the energy per unit volume.

A vacant or partially filled band in a solid is called the conduction band because it contains electrons that are free to move and conduct electricity. The valence band is typically fully filled, the forbidden band refers to the energy gap, and 'empty band' is not a standard term.

Work done on a body results in a change in its total mechanical energy, which includes both kinetic energy (K.E.) and potential energy (P.E.). Therefore, the work done equals the change in K.E. plus the change in P.E.

The energy stored in a stretched wire within the elastic limit is the elastic potential energy given by (1/2) × force × extension. Using Hooke's law, the force F is proportional to the extension E with spring constant k = EA/L, where A is the cross-sectional area and L is the original length. The energy stored is therefore (1/2)kE² = (1/2)(FE/L)E = FE²/(2L). Among the given options, FE²/L (option C) is the closest and dimensionally consistent expression representing the energy stored (ignoring the factor 1/2). Hence, option C is correct.

Crystals have a regular, repeating atomic structure that can diffract X-rays or other waves, acting like a diffraction grating. This property is fundamental in crystallography for analyzing crystal structures.

The slit spacing (d) of a diffraction grating is the reciprocal of the number of lines per unit length. Given 500 lines per mm, the slit spacing d = 1/500 mm = 0.002 mm = 2 x 10^-3 mm. This matches option D. The original answer C (2 x 10^-5 mm) is incorrect. The explanation is that the slit spacing is the inverse of the number of lines per mm, so for 500 lines/mm, d = 1/500 mm = 0.002 mm = 2 x 10^-3 mm.