Physics

According to the band theory of solids, semiconductors have a completely filled valence band and a totally empty conduction band at absolute zero temperature, separated by a narrow forbidden energy gap (band gap). This band gap is small enough to allow thermal excitation of electrons from the valence band to the conduction band at room temperature, enabling electrical conductivity. Option B correctly describes this: a completely filled valence band, a totally empty conduction band, and a narrow forbidden band. Option A is incorrect because it states a partly filled valence band, which is not true for semiconductors. The valence band is fully occupied at 0 K. Option C incorrectly states a partially filled conduction band, which is not typical for intrinsic semiconductors. Option D incorrectly states a partly filled valence band and a wide forbidden band, which does not describe semiconductors.

The modulus of elasticity (also known as Young's modulus) is defined as stress divided by strain. Stress has units of force per unit area (N/m²), and strain is dimensionless, so the modulus of elasticity has units of N/m², which is equivalent to Pascal (Pa).

The energy required to magnetize and demagnetize a material is represented by the area of the hysteresis loop. This area corresponds to the energy loss per cycle due to magnetic hysteresis.

In intrinsic semiconductors, increasing temperature excites more electrons from the valence band to the conduction band, thereby increasing the number of charge carriers and thus increasing electrical conductivity.

The energy stored in a stretched wire is equal to the work done to stretch it, which is given by the formula: Energy = (1/2) × Force × Extension. Here, the force is 20 N and the extension is 1 mm (0.001 m). Therefore, the energy stored = 0.5 × 20 × 0.001 = 0.01 J. This matches option A. The original explanation correctly calculated the energy but the correct answer was incorrectly marked as B. The correct answer is A.

When a force acts parallel to the direction of motion, the work done is maximized because work is calculated as the product of force, displacement, and the cosine of the angle between them. Here, the angle is zero degrees, so cosine is 1, giving maximum work.

In a simple cubic crystal structure, one atom is located at each of the eight corners of the cube. Each corner atom is shared among eight adjacent cubes, resulting in one atom per unit cell.

Kinetic energy (K.E.) is given by K.E. = p²/(2m), where p is momentum and m is mass. Since all particles have the same momentum, the particle with the smallest mass will have the highest kinetic energy. Among proton, electron, neutron, and alpha particle, the electron has the smallest mass, so it has the highest kinetic energy.

Light from the Sun takes approximately 8 minutes to reach the Earth, as the average distance is about 150 million kilometers and light travels at about 300,000 kilometers per second.

The property where the angular position of cells remains the same in a crystal sample refers to the external symmetry of form, which describes the consistent geometric arrangement and symmetry of crystal faces.