Magnetism & Matter PYQs Set 5 (PDF)

Set 5 of 6 · Questions 121-150 of 176

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Mere Teacher · Previous Year Questions JEE Main Physics · Magnetism & Matter · Set 5 (Q121-150)
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121. $A soft ferromagnetic material is placed in an external magnetic field. The magnetic domains:$ (1) $decrease in size and changes orientation.$ (2) $may increase or decrease in size and change its orientation.$ (3) $increase in size but no change in orientation.$ (4) $have no relation with external magnetic field$ 122. $When two soap bubbles of radii a and b (b > a) coalesce, the radius of curvature of common surface is:$ (1) $\frac{ab}{b - a}$ (2) $\frac{a + b}{ab}$ (3) $\frac{b - a}{ab}$ (4) $\frac{ab}{a + b}$ 123. $A bar magnet having a magnetic moment of 2 \times 1 0^{4} J T^{- 1} is free to rotate in a horizontal plane. A horizontal magnetic field B = 6 \times 1 0^{- 4} T exists in the space. The work done in taking the magnet slowly from a direction parallel to the field to a direction 6 0^{o} from the field is$ (1) $12 J$ (2) $6 J$ (3) $2 J$ (4) $0.6 J$ 124. $If a diamagnetic substance is brought near the north or the south pole of a bar magnet, it is$ (1) $repelled by the north pole and attracted by the south pole$ (2) $attracted by the north pole and repelled by the south pole$ (3) $attracted by both the poles$ (4) $repelled by both the poles$ 125. $A magnet hung at 45° with magnetic meridian makes an angle of 60° with the horizontal. The actual value of the angle of dip is$ (1) $tan^{- 1} (\frac{3}{2})$ (2) $tan^{- 1} (6)$ (3) $tan^{- 1} (\frac{2}{3})$ (4) $tan^{- 1} (\frac{1}{2})$ 126. $Core of electromagnets are made of ferromagnetic material which has$ (1) $high permeability and high retentivity$ (2) $low permeability and low retentivity$ (3) $high permeability and low retentivity$ (4) $low permeability and high retentivity.$ 127. $The force acting per unit length when a very long straight conductor is carrying a steady current of 1 A and the direction of the current is from south to north is (The horizontal component of the earth's magnetic field at the place is 3 \times 10^{- 5} T and the direction of the field is from the geographical south to geographical north.)$ (1) $3 \times 10^{- 5} Nm^{- 1}$ (2) $1 \times 10^{- 5} Nm^{- 1}$ (3) $0$ (4) $1.5 \times 10^{- 5} Nm^{- 1}$ 128. $A rod, of length L at room temperature and uniform area of cross section A, is made of a metal having a coefficient of linear expansion α / ℃ . It is observed that an external compressive force F, is applied on each of its ends, prevents any change in the length of the rod, when its temperature rises by ΔTK. Young's modulus, Y, for this metal is:$ (1) $\frac{F}{Aα ∆ T}$ (2) $\frac{F}{Aα ( ∆ T - 273 )}$ (3) $\frac{F}{2 Aα ∆ T}$ (4) $\frac{2 F}{Aα ∆ T}$ 129. $Earth's magnetic field always has a horizontal component except at$ (1) $equator$ (2) $magnetic poles$ (3) $a latitude of 60^{\circ}$ (4) $an altitude of 60^{\circ}$ 130. $A bar magnet of magnetic moment 2 Am^{2} lies aligned with the direction of a uniform magnetic field of 0.3 T . The amount of work required by an external torque to turn the magnet so as to align its magnetic moment normal to the field direction is$ (1) $0.15 J$ (2) $0.3 J$ (3) $0.6 J$ (4) $1.2 J$ 131. $An iron bar having a cross sectional area of 2 \times 10^{- 5} m^{2} and magnetising field of 2400 A / m produce a magnetic flux of 2.4 π \times 10^{- 5} Wb . What will be the value of permeability (μ) and susceptibility (χ) of the bar (Given (μ_{0} = 4 π \times 10^{- 7})$ (1) $μ=5\times 1 0^{- 4},χ=1249π$ (2) $μ=5π\times 1 0^{- 4},χ=1249π$ (3) $μ=5π\times 1 0^{- 4},χ=1249$ (4) $μ=5\times 1 0^{- 4},χ=1249$ 132. $If θ_{1} and θ_{2} be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip θ is given by$ (1) $ta n^{2} θ = ta n^{2} θ_{1} + ta n^{2} θ_{2}$ (2) $co t^{2} θ = co t^{2} θ_{1} - co t^{2} θ_{2}$ (3) $ta n^{2} θ = ta n^{2} θ_{1} - ta n^{2} θ_{2}$ (4) $co t^{2} θ = co t^{2} θ_{1} + co t^{2} θ_{2}$ 133. $A bar magnet has total length 2 l = 20 units and the field point P is at a distance d = 10 units from the Centre of the magnet. If the relative uncertainty of length measurement is 1 %, then uncertainty of the magnetic field at point P is:$ (1) $4 %$ (2) $15 %$ (3) $5 %$ (4) $10 %$ 134. $Two identical metal wires of thermal conductivities K_{1} and K_{2} respectively are connected in series. The effective thermal conductivity of the combination is:$ (1) $\frac{2 K _{1} K _{2}}{K _{1} + K _{2}}$ (2) $\frac{K _{1} + K _{2}}{2 K _{1} K _{2}}$ (3) $\frac{K _{1} + K _{2}}{K _{1} K _{2}}$ (4) $\frac{K _{1} K _{2}}{K _{1} + K _{2}}$ 135. $The magnetic field at the center of current carrying circular loop is B1. The magnetic field at a distance of 3 times radius of the given circular loop from the center on its axis is B2. The value of B1/B2 will be$ (1) $9 : 4$ (2) $12 : 5$ (3) $8 : 1$ (4) $5 : 3$ 136. $A planet has magnetic dipole moment of 27 \times 10^{22} Am^{2} . If the radius of the planet is 300 km, what would be the magnetic field at its equator? \frac{μ _{0}}{4 π} = 10^{- 7}$ (1) $1 T$ (2) $27 T$ (3) $11 T$ (4) $30 T$ 137. $A wire X of length 50 cm carrying a current of 2 A is placed parallel to a long wire Y of length 5 m. The wire Y carries a current of 3 A. The distance between two wires is 5 cm and currents flow in the same direction. The force acting on the wire Y is:$ (1) $1.2 x 1 0^{- 5} N directed towards wire X.$ (2) $1.2 x 1 0^{- 4} N directed away from wire X.$ (3) $1.2 x 1 0^{- 4} N directed towards wire X.$ (4) $2.4 x 1 0^{- 5} N directed towards wire X.$ 138. $A load of mass M kg is suspended from a steel wire of length 2 m and radius 1.0 mm in Searle's apparatus experiment. The increase in length produced in the wire is 4.0 mm. Now the loads is fully immersed in a liquid of relative density 2. The relative density of the material of load is 8. The new value of increase in length of the steel wire is:$ (1) $3.0 mm$ (2) $4.0 mm$ (3) $5.0 mm$ (4) $Zero$ 139. $Two short bar magnets each of magnetic moment of 9 Am^{2} are placed such that one is at x = - 3 cm and the other at y = - 3 cm . If their magnetic moments are directed along positive and negative X -directions respectively, then the resultant magnetic field at the origin is$ (1) $100 T$ (2) $10 T$ (3) $0.1 T$ (4) $0.001 T$ 140. $A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip pointing down at 30^{\circ} with the horizontal. The horizontal component of the earth's magnetic field at the place is 0.3 G . Then the magnitude of the earth's magnetic field at the location is$ (1) $\frac{3}{5} G$ (2) $3 G$ (3) $\frac{20}{3} G$ (4) $\frac{2}{3} G$ 141. $The vertical component of the earth's magnetic field is 6 x 10-5 T at any place where the angle of dip is 37°. The earth's resultant magnetic field at 3 that place will be (Given tan 37° = \frac{3}{4})$ (1) $8 x 1 0^{- 5} T$ (2) $6 x 1 0^{- 5} T$ (3) $5 x 1 0^{- 4} T$ (4) $l x 1 0^{- 4} T$ 142. $Assertion: The magnetic field lines are continuous and form closed loops.Reason: Magnetic monopole does not exist.The correct option among the following is$ (1) $(A) is true, (R) is true and (R) is the correct explanation for (A)$ (2) $(A) is true, (R) is true but (R) is not the correct explanation for (A)$ (3) $(A) is true but (R) is false$ (4) $(A) is false but (R) is true$ 143. $In an environment, brass and steel wires of length 1 m each with areas of cross section 1mm² are used. The wires are connected in series and one end of the combined wire is connected to a rigid support and other end is subjected to elongation. The stress required to produce a net elongation of 0.2 mm is, [Given, the Young's Modulus for steel and brass are, respectively, 120 \times 10^{9} N / m^{2} and 60 \times 10^{9} N / m^{2}]$ (1) $1.2 \times 10^{6} N / m^{2}$ (2) $8 \times 10^{6} N / m^{2}$ (3) $1.8 \times 10^{6} N / m^{2}$ (4) $0.2 \times 10^{6} N / m^{2}$ 144. $A coil in the shape of an equilateral triangle of side l is suspended between the pole pieces of a permanent magnet such that B is in plane of the coil. If due to a current i in the triangle a torque τ acts on it, the side l of the triangle is$ (1) $\frac{2}{3} (\frac{τ}{Bi})$ (2) $2 (\frac{τ}{3 Bi})^{1/2}$ (3) $\frac{2}{3} (\frac{τ}{Bi})^{1/2}$ (4) $\frac{1}{3} \frac{τ}{Bi}$ 145. $Two identical bar magnets are fixed with their centers at a distance d apart. A stationary charge Q is placed at P in between the gap of the two magnets at a distance D from the center O as shown in the figure. The force on the charge Q is$ (1) $zero$ (2) $directed along OP$ (3) $directed along PO$ (4) $directed perpendicular to the plane of paper$ 146. $A bar magnet of length ‘ l ’ and magnetic dipole moment ‘ M ’ is bent in the form of an arc as shown in figure. The new magnetic dipole moment will be$ (1) $\frac{2}{π} M$ (2) $\frac{M}{2}$ (3) $M$ (4) $\frac{3}{π} M$ 147. $A bar magnet of length 14 cm is placed in the magnetic meridian with its north pole pointing towards the geographic north pole. A neutral point is obtained at a distance of 18 cm from the center of the magnet. If B_{H} = 0.4 G the magnetic moment of the magnet is (1 G = 10^{- 4} T)$ (1) $2.880 \times 10^{3} J T^{- 1}$ (2) $2.880 \times 10^{2} J T^{- 1}$ (3) $2.880 J T^{- 1}$ (4) $28.80 J T^{- 1}$ 148. $A bimetallic strip consists of metals A and B . It is mounted rigidly as shown. The metal A has highercoefficient of expansion compared to that of metal B . When the bimetallic strip is placed in a cold both, it will$ (1) $Bend towards the right$ (2) $Not bend but shrink$ (3) $Neither bend nor shrink$ (4) $Bend towards the left$ 149. $A perfectly diamagnetic sphere has a small spherical cavity at its centre, which is filled with a paramagnetic substance. The whole system is placed in a uniform magnetic field B \to . Then the field inside the paramagnetic substance is$ (1) $Much large than \| B \to \| and parallel to B \to$ (2) $B \to$ (3) $Much large than \| B \to \| but opposite to B \to$ (4) $Zero$ 150. $The coercivity of a magnet is 5 \times 10^{3} A / m . The amount of current required to be passed in a solenoid of length 30 cm and the number of turns 150 , so that the magnet gets demagnetised when inside the solenoid is _____A.$