Physics

According to de-Broglie hypothesis, the wavelength associated with moving electron of mass ′m′ is ′λ′e. Using mass energy relation and Planck’s quantum theory, the wavelength associated with photon is ′λ′p. If the energy (E) of electron and photon is same then relation between ′λ′e and ′λ′p is

λp ∝ λe λp ∝ λ2e λp ∝ √λe λp ∝ 1/λe Solution: The correct answer is A λp∝λe λe denotes the wavelength of the electron and λp denotes the wavelength of the proton. $ {{E}_{p}}=hv=\frac{hc}{{{\lambda...

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Photodiode is a device

(A) which is always operated in reverse bias (B) which is always operated in forward bias (C) in which photo current is independent of intensity of incident radiation (D) which may be operated in...

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Two unknown resistances are connected in two gaps of a meter-bridge. The null point is obtained at 40 cm from left end. A 30Ω resistance is connected in series with the smaller of the two resistances, the null point shifts by 20 cm to the right end. The value of smaller resistance in Ω is

12 24 48 36 Solution: The correct answer is B. 24 let the resistance at right be R and at left be r $ \frac{r}{R}=\frac{{{l}_{x}}}{{{l}_{R}}}=\frac{40}{60} $ $ \frac{r+30}{R}=\frac{60}{40} $ $...

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When one end of the capillary is dipped in water, the height of water column is ′h′. The upward force of 105 dyne due to surface tension balanced by the force due to the weight of water column. The inner circumference of the capillary is (Surface tension of water = 0.07 N/m)

1.5 cm 2 cm 2.5 cm 3 cm Solution: The correct answer is A. 1.5 cm $ Upward\text{ }force\,is:\,\,\,F=105~dyne~=105\times {{10}^{-5}}~N $ $ Surface\text{ }tension\text{ }of\text{ }water~T=7\times...

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The observer is moving with velocity ′v0’ towards the stationary source of sound and then after crossing moves away from the source with velocity ′v0’. Assume that the medium through which the sound waves travel is at rest. If ′v′ is the velocity of sound and ′n′ is thee frequency emitted by the source then the difference between apparent frequencies heard by the observer is

A. 2nv0/v B. v/2nv0 C. v/nv0 D. nv0/v Solution: The correct answer is A. 2nv0/v $ \text{Making use of the }doppler\text{ }effect $ $ Case\text{ }I:~Observer\,is\text{ }approaching\text{ }the\text{...

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A disc of moment of inertia ‘I1’ is rotating in horizontal plane about an axis passing through a centre and perpendicular to its plane with constant angular speed ′ ω ′ 1 . Another disc of moment of inertia I2 having zero angular speed is placed coaxially on a rotating disc. Now both the discs are rotating with constant angular speed ω2 . The energy lost by the initial rotating disc is

Solution: The correct answer is D. $ Initial\text{ }angular\text{ }momentum\text{ }of\text{ }the\text{ }system~is\,given\,by: $ $ {{L}_{i}}={{I}_{1}}{{\omega }_{1}}+{{I}_{2}}\omega \prime ~ $ $...

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A ceiling fan rotates about its own axis with some angular velocity. When the fan is switched off, the angular velocity becomes (1/4)th of the original in time ′t′ and ′n′ revolutions are made in that time. The number of revolutions made by the fan during the time interval between switch off and rest are (Angular retardation is uniform).

8n/15 4n/15 16n/15 32n/15 Solution: The correct answer is C. 16n/15 $ {{(\omega /4)}^{2}}={{\omega }^{2}}-2\alpha ~n(2\pi ) $ $ \therefore 2\alpha n(2\pi )={{\omega }^{2}}-\frac{{{\omega }^{2}}}{16}...

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The ratio of magnetic fields due to a bar magnet at the two axial points P1 and P2 which are separated from each other by 10cm is 25:2. Point P1 is situated at 10cm from the centre of the magnet.Points P1and P2 are on the same side of magnet and distance of P2 from the centre is greater than distance of P1 from the centre of magnet,then magnetic length is

5 cm 20 cm 15 cm 10 cm Solution: The correct answer is D. 10cm We know that the magnetic field at a distance d from the magnet's centre on the axis is given as follows for a magnet of length 2l and...

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A series combination of N1capacitors (each of capacity C 1 ) is charged to potential difference ′ 3 V ′ . Another parallel combination of N 2 capacitors (each of capacity C 2 ) is charged to potential difference ‘V’. The total energy stored in both the combinations is same. The value of C 1 in terms of C 2 is

Solution: The correct answer is A. $ For\,series\,combination,\,equivalence\,capacitance\,=\,\frac{{{C}_{1}}}{{{N}_{1}}} $ $ ch\arg e,\,\,q=(3V)\frac{{{C}_{1}}}{{{N}_{1}}} $ $...

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The moment of inertia of a ring about an axis passing through the centre and perpendicular to its plane is ‘I’. It is rotating with angular velocity ′ω′. Another identical ring is gently placed on it so that their centres coincide. If both the rings are rotating about the same axis then loss is kinetic energy is

Iω2/2 Iω2/4 Iω2/6 Iω2/8 Solution: The correct answer is Iω2/4 $ Initial\text{ }angular\text{ }momentum,\,{{L}_{1}}=I\omega  $ $ {{K}_{1}}=\frac{1}{2}I{{\omega }^{2}} $ We know that the block's...

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The electric field intensity at a point near and outside the surface of a charged conductor of any shape is ‘E1’. The electric field intensity due to uniformly charged infinite thin plane sheet is ‘E2’. The relation between ‘E1’ and ‘E2’ is

A) 2E1 = E2                                             B) E1 = E2 C) E1 = 2E2                                             D) E1 = 4E2 Solution: The answer is C) E1 = 2E2      The electric field...

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A toy cart is tied to the end of an unstretched string of length ‘l’. When revolved, the toy cart moves in horizontal circle with radius ‘2l’ and time period T. If it is speeded untill it moves in horizontal circle of radius ‘3l’ with period T1, relation between T and T1 is (Hooke’s law is obeyed)

Solution: The correct answer is D) $ T=\frac{m{{v}^{2}}}{r} $ $ Time~Period=\frac{2\pi r}{v} $ $ ~\Delta L=\frac{Fl}{AY} $ $ ~\Rightarrow T=\frac{\Delta lAY}{l} $ $ v=\sqrt{\frac{\Delta lAYr}{lm}} $...

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A cord is wound around the circumference of wheel of radius ‘r’. The axis of the wheel is horizontal and moment of inertia about it is ‘I’. The weight ‘mg’ is attached to the end of the cord and falls from rest. After falling through a distance ‘h’, the angular velocity of the wheel will be

Solution: The correct answer is (c) $ Using\text{ }law\text{ }of\text{ }conservation\text{ }of\text{ }energy.\, $ $ At\text{ }height\text{ }h:~Mechanical\text{ }Energy=~M.{{E}_{1}}=mgh $ $...

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A large number of liquid drops each of radius ‘a’ are merged to form a single spherical drop of radius ‘b’. The energy released in the process is converted into kinetic energy of the big drop formed. The speed of the big drop is [ρ = density of liquid, T = surface tension of liquid]

Solution: The correct answer is (A) Let's say there are n little drops that have fused to produce one larger drop. Before and after the merging, the liquid's mass is conserved. $ \therefore \rho...

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A hollow sphere of mass ‘M’ and radius ‘R’ is rotating with angular frequency ‘ω’. It suddenly stops rotating and 75% of kinetic energy is converted to heat. If ‘S’ is the specific heat of the material in J K kg then rise in temperature of the sphere is (M.I. of hollow sphere = 2/3 MR2)

A) Rω/4S B) R2 ω2/4S C) Rω/2S D) R2 ω2/2S Solution: The correct answer is R2 ω2/4S $ Moment\text{ }of\text{ }inertia\text{ }of\text{ }hollow\text{ }sphere~I=\frac{2}{3}M{{R}^{2}} $ $ Kinetic\text{...

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In sonometer experiment, the bridges are separated by a fixed distance. The wire which is slightly elastic, emits a tone of frequency ‘n’ when held by tension ‘T’. If the tension is increased to ‘4T’, the tone emitted by the wire will be of frequency

A) n                                                             B) 2n C) Slightly greater than 2n                         D) Slightly less than 2n Solution: The correct option is C) Slightly...

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When monochromatic light of wavelength ‘λ’ is incident on a metallic surface, the stopping potential for photoelectric current is ‘3V0’. When same surface is illuminated with light of wavelength ‘2λ’, the stopping potential is ‘V0’. The threshold wavelength for this surface when photoelectric effect takes place is

A) λ B) 2λ C) 3λ D) 4λ Solution: The correct option is D) 4λ $ Kinetic\text{ }energy\text{ }of\text{ }photoelectrons,~K.E=\frac{hc}{\lambda }-\phi ~ $ $ where\text{ }work\text{ }function,~\phi...

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In the product

$ \vec{F}=q(\vec{v}\times \vec{B}) $ $ \vec{F}=q\vec{v}(B\hat{i}+B\hat{j}+{{B}_{0}}\hat{k}) $ $ For\,q=1\,and\,\vec{v}=2\hat{i}+4\hat{j}+6\hat{k}\,and\, $ $ \vec{F}=4\hat{i}-20\hat{j}+12\hat{k} $ $...

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A convex lens ‘A’ of focal length 20 cm and a concave lens ‘B’ of focal length 5 cm are kept along the same axis with a distance ‘d’ between them. If a parallel beam of light falling on ‘A’ leaves ‘B’ as a parallel beam, then the distance ‘d’ in cm will be

(1) 25                                              (2) 15 (3) 50                                              (4) 30 Solution: Answer (2) After refraction by a convex lens, parallel beams of light...

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The stress-strain curves are drawn for two different materials X and Y. It is observed that the ultimate strength point and the fracture point are close to each other for material X but are far apart for material Y. We can say that materials X and Y are likely to be (respectively), (1) Plastic and ductile (2) Ductile and brittle (3) Brittle and ductile (4) Brittle and plastic

Answer (3) Sol. As given that fracture point and ultimate strength point is close for material $\mathrm{X}$, So, $\mathrm{X}$ is brittle in nature and both points are far apart for material $Y$...

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Two bullets are fired horizontally and simultaneously towards each other from roof tops of two buildings 100 \mathrm{~m} apart and of same height of 200 \mathrm{~m}, with the same velocity of 25 \mathrm{~m} / \mathrm{s}. When and where will the two bullets collide? \left(\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^{2}\right) (1) They will not collide (2) After 2 \mathrm{~s} at a height of 180 \mathrm{~m} (3) After 2 \mathrm{~s} at a height of 20 \mathrm{~m} (4) After 4 \mathrm{~s} at a height of 120 \mathrm{~m}

Answer (2) Sol. Let bullets collide at time t $ \begin{array}{l} x_{1}+x_{2}=100 \mathrm{~m} \\ 25 t+25 t=100 \\ t=2 s \\ \begin{array}{r} y=\frac{1}{2} g t^{2}=\frac{1}{2} \times 10 \times 2^{2} \\...

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