Free Physics of Solids MCQs with Answers

251. When force is applied on a body, then the elastic potential energy is stored in it. On removing the force, this energy

• A. Converts into kinetic energy
• B. Converts into heat energy
• C. Remains as potential energy
• D. Transforms into sound energy

Explanation: When force is applied to a body, it stores energy as elastic potential energy. Upon removal of the force, this energy is predominantly converted into kinetic energy, causing the body to move or return to its original shape. This is because the stored energy is released, leading to motion. Other forms of energy transformation, like heat or sound, may occur, but are not the primary result of removing the force. The incorrect options either suggest energy remains as potential energy or transform into forms like heat or sound, which are secondary effects.

252. On stretching a wire, the elastic energy stored per unit volume is

• A. Fl / 2AL
• B. FA / 2L
• C. FL / 2A
• D. FL / 2

Explanation: The elastic energy stored per unit volume in a stretched wire can be derived from the formula for elastic potential energy, which is (1/2)F * l. When considering energy per unit volume, we must divide by the volume (A*L for a wire with cross-sectional area A and original length L). This gives us the expression Fl / 2AL. Option A is correct because it accounts for the force, extension, cross-sectional area, and original length. The other options either incorrectly include or exclude necessary terms, leading to incorrect expressions for elastic energy per unit volume.Elastic energy stored per unit volume = (1/2) × stress × strain= (1/2) × (F/A) × (l/L)= Fl / 2AL

253. Which of the following, the most suitable material for making permanent magnet is

• A. Aluminium
• B. Bismuth
• C. Copper
• D. Nickel

Explanation: The correct answer is Nickel. Nickel is a ferromagnetic material, which means it can be magnetised and retain its magnetisation over time, making it an excellent choice for creating permanent magnets. In contrast, Aluminium, Bismuth, and Copper are not suitable for making permanent magnets. Aluminium and Copper are non-magnetic materials, while Bismuth exhibits diamagnetism, which means it actually repels magnetic fields and cannot retain magnetisation.

254. In the case of bar magnet, lines of magnetic induction

• A. Start from the north pole and end at the south pole
• B. Run continuously through the bar and outside
• C. Emerge in circular paths from the middle of the bar
• D. Are produced only at the north pole like rays of light from a bulb

Explanation: The correct answer is that lines of magnetic induction run continuously through the bar and outside. Magnetic field lines form closed loops, indicating that they do not have a beginning or an end. Outside the magnet, they travel from the north pole to the south pole, while inside, they continue from the south pole back to the north pole. This continuous loop structure is fundamental to understanding magnetic fields. Option A is incorrect because it suggests that field lines start and end at the poles, which is misleading. Option C is incorrect as it suggests emergence in circular paths from the middle, which does not happen. Option D is incorrect because it implies that field lines are only produced at the north pole, which is not true, as they form loops.

255. Magnetic induction is a

• A. Scalar quantity
• B. Vector quantity
• C. Both (a) and (b)
• D. None of the above

Explanation: The correct answer is that magnetic induction is a vector quantity. This is because it has both magnitude and direction. The direction of magnetic induction is the direction of the magnetic field. Scalar quantities, on the other hand, only have magnitude but no direction, which does not apply to magnetic induction. Therefore, options A and C are incorrect. Option D is incorrect as it suggests that none of the options are correct when option B is indeed correct.

256. Magnetic field intensity is defined as

• A. Magnetic moment per unit volume
• B. Magnetic induction force acting on a unit pole
• C. Number of lines of force crossing per unit area
• D. Number of lines of force crossing per unit volume

Explanation: Magnetic field intensity, also known as magnetic flux density, is correctly defined as the number of magnetic lines of force crossing per unit area. This is a measure of the strength of the magnetic field at a given point. Option C is correct because it accurately describes this concept. Options A and D are incorrect because they confuse the definition with magnetisation and incorrectly define it per unit volume. Option B describes a concept related to magnetic force, not field intensity.

257. If the magnetic flux is expressed in weber, then magnetic induction can be expressed in

• A. Weber/m²
• B. Weber/m
• C. Weber-m
• D. Weber-m²

Explanation: Magnetic induction, also known as magnetic flux density (B), is defined as the magnetic flux (Φ) per unit area (A). It is expressed in tesla (T), but can also be represented in terms of weber per square meter (Weber/m²) since 1 T = 1 Weber/m². Therefore, the correct answer is Weber/m². The other options, such as Weber/m, Weber-m, and Weber-m², are not applicable for magnetic induction as they do not correctly represent the physical quantity of magnetic flux density.

258. A magnetic needle is kept in a non-uniform magnetic field. It experiences

• A. A force and a torque
• B. A force but not a torque
• C. A torque but not a force
• D. Neither a torque nor a force

Explanation: A magnetic needle placed in a non-uniform magnetic field experiences both a force and a torque. The non-uniform field exerts a force on the needle due to the field's gradient, which can move the needle through space. Additionally, the field exerts a torque on the needle, which tends to align the needle with the field lines. This is because a magnetic needle behaves like a magnetic dipole, and in a non-uniform field, the forces on the north and south poles of the dipole are unequal, causing both a linear force and a rotational effect. Option B is incorrect because a torque is also experienced due to the magnetic field's non-uniformity. Option C is incorrect because the force is present as well. Option D is incorrect because both a force and a torque are exerted on the needle.

259. Magnetic lines of force due to a bar magnet do not intersect because

• A. A point always has a single net magnetic field
• B. The lines never cross each other
• C. The lines need magnetic lenses to be made to intersect
• D. None of the above

Explanation: The correct answer is that a point always has a single net magnetic field. Magnetic field lines are conceptual tools used to depict the direction and strength of a magnetic field. At any particular point in a magnetic field, there is a unique direction of the magnetic field, which is why the lines do not intersect. If they did, it would imply that at the intersection point, there are two different directions of the magnetic field, which is not possible. Option B repeats the question without providing a reason. Option C introduces a concept not related to the properties of magnetic field lines. Option D dismisses the possibility of a correct answer among the given choices.

260. For protecting a sensitive equipment from the external magnetic field, it should be

• A. Placed inside an aluminium case
• B. Placed inside an iron case
• C. Wrapped with insulation around it when passing current through it
• D. Surrounded with fine copper sheet

Explanation: The correct answer is to place the equipment inside an iron case. Iron is a ferromagnetic material, which means it can absorb and redirect magnetic field lines, thus providing effective magnetic shielding. This property makes it suitable for protecting sensitive equipment from external magnetic fields. Option A, using an aluminium case, is incorrect because aluminium is not effective for magnetic shielding due to its non-ferromagnetic nature. Option C suggests using insulation, which is incorrect because insulation is used for preventing electrical conduction, not for blocking magnetic fields. Option D, using a copper sheet, is also incorrect because while copper is conductive and can provide electromagnetic interference shielding, it is not effective for shielding against static or low-frequency magnetic fields.