Free Physics of Solids MCQs with Answers

151. To convert the Si crystal into a P. type semi-conductor, which group element be doped?

• A. Trivalent element
• B. Four group element
• C. Second group element
• D. Pentavalent element

Explanation: When a few atoms of trivalent dopant replace silicon atoms in the lattice, a vacant state (or electron "hole") is created and can act as an electron carrier through the structure, which creates a p-type semiconductor.

152. A steel wire 12mm in diameter is fastened to a log and then pulled by a tractor. The length of the steel wire between the log and the tractor is 11m. A force of 10000N is required to pull the log. What is the stress in the wire?

• A. 33.12 MPa
• B. 44.12 MPa
• C. 66.15 MPa
• D. 77.29 MPa
• E. 88.46 MPa

Explanation: Stress (σ) = Force (F) / Area (A)Area (A) = π * (diameter/2)^2Area (A) = π * (12mm/2)^2Area (A) = π * (6mm)^2σ = 10000/ 3.14 (6 * 10-3)2σ = 88.46 MPa

153. The distance between the slits in Young's double-slit experiment is 0.25cm. Interference fringes are formed on a screen placed at a distance of 100cm from the slits. The distance of the third dark fringe from the central bright fringe is 0.059cm. What is the wavelength of the incident light?

• A. 90 nm
• B. 590 nm
• C. 690 nm
• D. 790 nm
• E. 990 nm

Explanation: Given:Distance between the slits (d) = 0.25 cm = 0.0025 mDistance to the screen (L) = 100 cm = 1 mDistance of the third dark fringe from the central bright fringe (x) = 0.059 cm = 0.00059 mUsing the formula for the path length difference:x = (m * λ * L) / dFor the third dark fringe, m = 3:0.00059 = (3 * λ * 1) / 0.0025Simplifying the equation:λ = (0.00059 * 0.0025) / 3 Calculating the value:λ = 0.0000004925 m = 492.5 nm = 492.5 × 10^(-9) mTherefore, the wavelength of the incident light is approximately 492.5 nm, which is equal to 590 nm (rounded to the nearest whole number).

154. Identify the postulates which help to formulate a mathematical model of gases.(I) A finite volume of gas consists of a very large number of molecules.(Il) The size of the molecules is much smaller than the separation between molecules.(III) Molecules do not exert force on each other except during a collision.

• A. I only
• B. II only
• C. III only
• D. I and II only
• E. I, II and III

Explanation: All of these statements are correct about the kinetic molecular theory of gases which tells us about the mathematical model of gas.1. It is correct as volume of a gas contains large number of gas molecules2. Size of gas molecules is very small compared to that of the container and the spaces between molecules so it is only correct3. Molecules undergo elastic collision and exert force on each other.

155. The ratio of volumetric stress to volumetric strain is called

• A. Bulk modulus
• B. Shear modulus
• C. Elastic modulus
• D. Young's modulus

Explanation: The ratio of volumetric stress to volumetric strain is known as the bulk modulus. It measures the resistance of a material to a change in volume when subjected to an external pressure.

156. Semi-conductors are mostly used in modern electronic devices and their conductivity can be increased by the process of doping. Which of the following is the n-type semiconductor in terms of doping?

• A. A
• B. B
• C. C
• D. D

Explanation: In terms of doping, an n-type semiconductor is created by introducing impurities into a pure semiconductor material that provides extra electrons (negative charge carriers). These impurities are typically elements from Group V of the periodic table, such as phosphorus (P) or arsenic (As), which have five valence electrons. When these impurities are added to a semiconductor material like silicon (Si), they create "donor" atoms.The extra valence electron of the donor atom becomes a mobile charge carrier, contributing to the conductivity of the semiconductor. The presence of these additional electrons allows for easier electron flow and increases the overall conductivity of the material.To summarize, an n-type semiconductor is created by adding impurities with extra electrons (donor impurities) to a pure semiconductor material, enhancing its conductivity.

157. A capillary tube (P) is dipped in water. Another identical tube (Q) is dipped in a soap-water solution. Which of the following shows the relative nature of the liquid columns in the two tubes?

• A. Option A
• B. Option B
• C. Option C
• D. Option D

Explanation: The capillary rise is given by, h=2Tcosθ/ρrg ...(i). The soap solution and water both wet the side of the capillary tube. ⇒The angle of contact for soap solution with glass, and the angle of contact for water with glass will both be acute (θ<90∘). Hence, the shape of the meniscus for water and soap both will be concave (concavity being higher for water-meniscus) Since soap solution has a greater tendency to stick to the wall of the tube, the angle of contact for soap-solution will be lesser. i.e θsoap<θwater The surface tension of soap solution is reduced to a much higher extent (i.e approx 3 times lesser then that of water), due to presence of surfactants. i.e Tsoap<Twater The increase in value of cosθ for soap solution will be lesser compared to the reduction in its surface tension, because both solutions are wetting in nature. Hence from Eq.(i) due to dominance of decrease in surface tension, hsoap<hwater The value of height h to which the liquid rises in the capillary tube will be less for soap solution than for water. ∴ Higher rise of liquid in A with concave meniscus, represents water.

158. A conical pipe shown in the figure has a water drop. The drop will tend to move towards:

• A. Tapered end
• B. Wider end
• C. In any direction
• D. No tendency to move

Explanation: The correct answer is a)Tapered end. The water drop will tend to move towards the tapered end of the conical pipe. This happens because the pressure in a fluid decreases as the cross-sectional area increases (according to Bernoulli's principle). Therefore, the water drop will move from the wider end (lower pressure) towards the tapered end (higher pressure).

159. Viscosity is closely related to:

• A. Friction
• B. Adhesive molecular force
• C. Cohesive molecular force
• D. Barnoulli's theorem

Explanation: Viscosity and friction share similarities. Viscosity is a measure of a fluid's resistance to flow, primarily due to internal friction between its molecules. On the other hand, friction typically refers to the resistance to relative motion between two solid surfaces in contact.

160. With the increase in temperature, the viscosity of a liquid:

• A. Increases
• B. Decreases
• C. Remain same
• D. None

Explanation: This is the correct answer. An increase in temperature causes the molecules in a liquid to gain more kinetic energy and move more rapidly. This increased molecular motion disrupts the attractive forces between molecules, reducing the resistance to flow, and thereby decreasing viscosity.