378 + Interview Questions in PROPERTIES-OF-BULK-MATTER IN GENERAL AWARENESS Page 8 InterviewSolution

351.	Why are clear nights colder than cloudy nights ?
Answer» Clouds reflect the heat radiations falling on them. So on a cloudy night, the radiations from earths surface reaching the clouds are relfected back so the temperature of earth does not fall. But on a clear night, the radiations from the earth surface escape out so, earths temperature falls. that is why a clear night is colder than a cloudy night.

Discussion

352.	A large block of ice `5 m` thick has a vertical hole drilled through it and is floating in the middle of a lake. What is the minimum length of a rope required to scoop up a bucket full of water through the hole? Relative density of ice `=0.9`.
Answer» Let `rho` and `rho_(1)` be the density of ice and see water respectively. so, `rho//rho_(1)=0.9` Let H be the thickness of ice block, of crosso section area A. The weight of ice block = `A H rho g` Let x be the thickness of iceblock above the surface of water `:.` Thickness of ice block inside the water `=(H-x)` By the principle of floatation, `AH rhog=(H-x) A rho_(1) g` or `x=H(1-rho//rho_(1)) = 5(1-0.9)=0.5 m`.

Discussion

353.	A ice box of styrofoam (thermal conductivity = `0.01` J `ms^(-1)K^(-1)`) is used to keep liquid cool. It has a total wall area, including lid, of `10.8 m^(2)` and wall thickness of `2.0` cm. A bottle of water is placed in the box and filled with ice. If the outside temperature is `30^(@)C`, the rate of flow of heat into the box is (in `J//s)`A. 16B. 14C. 12D. 10
Answer» Correct Answer - C Here `K=0.01 J ms^(-1)K^(-1), A=0.8 m^(2)`, `Delta x=2 cm =2xx10^(-2)m` `Delta T=30^(@)C, dQ//dt=?` `(dQ)/(dt) =KA (Delta T)/(Delta x)=0.01xx0.8xx(30)/(2xx10^(-2))` `=(8)/(1000)xx(30)/(2)xx100 =12 J//s`

Discussion

354.	How do you arrange `Cu, Al and Ag` in the order of increasing thermal conductivity?
Answer» Correct Answer - Al, Cu,Ag.

Discussion

355.	The only possibility of heat flow in a thermos flask is through its cork which is `75 cm^(2)` in area and 5 cm thick. Its thermal conductivity is `0.0075 cal//cm sec^(@)C`. How lolng will 500 g of ice at `0^(@)C` in thermos flask will is `40^(@)C` and latent heat of ise is `80 cal.//gram`.
Answer» Correct Answer - `[2.47 hr.]` `Q=mL =KA((Delta T)/(Delta x))(Delta t)` `500xx80=0.0075xx75xx((40-0))/(5)Delta t` On solving, `Delta t=8.9xx10^(3) s= 2.47 hr`

Discussion

356.	Two moles of oxygen is heated at a constant pressure from `0^(@)C`. What must be the gas for the volume to be doubled ? The specific heat of oxygen under these condition is `0.218 cal g^(-1) K^(-1)` .
Answer» Correct Answer - `[3809 cal]` Here, intial temperature `=0+273=273 K` At constant pressure, `(V_(2))/(T_(2))=(V_(1))/(T_(1))` or `(2V_(1))/(T_(2))=(V_(1))/(273)` or `T_(2)=2xx273=546 K` Rise in temperature, `Delta T=T_(2)-T_(1)=546-273=273K` Mass of 2 mole of oxygen, `m=2xx32=64 g` Heat required `Q=mc_(p)Delta T=64xx0.218xx273=3809 dcal`

Discussion

357.	What is terminal velocity? Discuss the factors on which it depends.
Answer» Terminal velocity of the body is the maximum constant velocity acquired by the body while falling freely in a viscous medium. Terminal velocity `(upsilon)` of a body of radius r, density `rho` moving through a viscous medium of viscosity `eta` and density `sigma` is given by `upsilon = 2/9 (r^2)/(eta) (rho - sigma)g` Thus, the value of `upsilon` depends upon r,`rho, sigma` and `eta`.

Discussion

358.	What is the effect of temperature on thermal conductivity of substances?
Answer» The thermal conductivity of oils and pure metals decreases with rise of temperature whereas that of alloys and water increases with rise of temperature.

Discussion

359.	Water is heated from below but not from top. Why?
Answer» When water is heated from below, its density decreases, it becomes light and rises up. Cooler water from above comes down and takes the position vacated by hot water. So convential currents are set up and the whole water gets heated. When water is heated from top, it will conduct very small amount of heat to the bottom part of water because water is a poor conductor of heat. hence water will not be heated properly.

Discussion

360.	A spherical ball contracts in volume by 0.01% when subjected to a normal uniform pressure of 100 atmospheres. Calculate the bulk modulus of the meterial.
Answer» Correct Answer - `1.013 xx 10^(11) Nm^(-2)` `Delta (V)/(V) = (0.01)/(100), p = 100 atm.` ` = 100 x 0.76 xx 13.6 xx 10^3 xx 9.8 N m^(-2)` `:. B = -(P)/(Delta V//V) = (100 xx 0.76 xx 13.6 xx 10^3xx 9.8)/(0.01//100)` ` =1.013xx 10^(11) N m^(-2)`

Discussion

361.	When the load on a wire is slowly increased from `3kgwt` to `5 kg wt`, the elongation increases from `0.61` to `1.02 mm`. The work done during the extension of wire is
Answer» Here, `F_(1)=3kg f=3 xx 9.8 N, Delta l_(1) = 0.61 xx 10^(-3)m` `F_(2)=5kg f=5 xx 9.8 N, Delta l_(2) = 1.02 xx 10^(-3)m` Work done, `W=1.2 F_(2) Delta l_(2) - 1/2 F_(1)Delta l_(1)` `=1/2 [(5 xx 9.8) xx (1.02 xx 10^(-3))` `-(3 xx 9.8) xx (0.61 xx 10^(-3))]` `=16.023 xx 10^(-3)J`.

Discussion

362.	A particle is placed at the origin and a force F=Kx is acting on it (where k is a positive constant). If `U_((0))=0`, the graph of `U (x)` verses x will be (where U is the potential energy function.)A. B. C. D.
Answer» Correct Answer - A `F=-(dU)/(dx)=kx` or `U_((x))=int_(0)^(x)-kxdx=-(1)/(2)kx^(2)` Which is the equation of a parabola symmetrical to - y axis.

Discussion

363.	Compute the fractional change in volume of a glass slab, when subjicted to a hydrautic pressure of 10 atmosphere. Bulk modulus of elasticity of glass = `37 xx 10^(9) Nm^(-2)` and 1 atm = `1.013 xx 10^(5)Pa`.
Answer» Here, `p=10 atm, = 10 xx 1.013 xx10^(5) Pa, B = 37 xx 10^(9)Nm^(-2)` Volumetric strain = `(Delta V)/(V) = (p)/(B) = (10 xx 1.013 xx 10^(5))/(37 xx 10^(9)) = 2.74 xx 10^(-5)` `:.` Froctional change in volume = `(Delta V)/(V) = 2.74 xx 10^(-5)`.

Discussion

364.	What is the value of isothermal bulk modulus of elasticity of air at NTP?
Answer» Isothermal Bulk modulus of elasticity of air is equal to pressure of air, so, `B_(i) = P` = one atmosphere = `1.01 xx 10^(5)N//m^(2)`

Discussion

365.	A uniform pressure p is exerted on all sides of a solid cube of a meterial at temprature `t^(@)C`. By what amount should the temperature of the cube be raised in order to bring its original volume back to the value it had before the pressure was applied ? K is the bulk modulus and `alpha` is the coefficient of linear expansion of material of solid cube.
Answer» Bulk modulus of elasticity `K = (P)/(Delta V//V) or (DeltaV)/(V) = P/K` But `(DeltaV)/(V) = gamma Delta t` or `Delta t = (Delta V)/(V) xx 1/(gamma) = P/K xx 1/(gamma) = (P)/(K xx 3 alpha)`.

Discussion

366.	The effect of temperature on the value of modulus of elasticity for various substances in generalA. it increases with increase in temperatureB. remains constantC. decreases with rise in temperatureD. sometimes increases and sometimes decreases
Answer» Correct Answer - C

Discussion

367.	A meterial is more elastic if its value of modulus of elasticity is ………
Answer» Correct Answer - large

Discussion

368.	The solids are ……….. Elastic and gases are ………. Elastic.
Answer» Correct Answer - more, least

Discussion

369.	Why dies velocity increase when water flowing in broader pipe enters a narrow pipe?
Answer» In a stream line flow of a liquid, ac cording to equation of continuity, `a upsilon =a` constant where a is the ares of cross-section and `upsilon` is the velocity of the liquid flow. When pipe, the areal of cross-section of the water decreases, therefore the velocity of water incresase.

Discussion

370.	Distinguish between streamline motion and turbulent motion.
Answer» Stream line motion of a liquid is an orderly type of motion in which the liquid flows in follows the path of its perceding particle with exectly the same velocity in magnitude and direction. In stream line motion , the liquid mist flow with a velocity less than the critical velocity of the liquid. Turbulent motion of a liquid is disorderly type of motion in which the liquid flows int eddies and motion of the particles of the liquid becomes irregular at different points. In turbulent flow, the liquid moves with a velocity greater than its critical velocity.

Discussion

371.	Which of the following diagrams does not represent a streamline flow?A. B. C. D.
Answer» Correct Answer - D In a streamline flow the two streams can not cross each other .

Discussion

372.	A horizontal tube of non uniform cross section has radii fo 0.1 m and 0.05 m respectively at M and N, For a streamline flow of liquid the rate of liquid flow isA. continuously changes with timeB. greater at M then at NC. greater at N then at MD. same at M and N
Answer» Correct Answer - D

Discussion

373.	What is the fractional volume of an ice cube in a pail of water produced in an enclosure which is freely falling under gravity?
Answer» For a free fall, the effective value of ac celeration due to gravity is zero. Therefore, there will be no weight of ice cube and hence no upwar thrust on ice cube. As a rsult of it, the ice cube can float with any value of fractional volume submerged.

Discussion

374.	For a ball falling in a liquid with constant velcoity, ratio of resisitance force due to the liquid to that due to gravity isA. 1B. `(2a^2 rho g)/(9 eta ^2)`C. `(2a^2(rho - sigma)g)/(9eta)`D. none of these
Answer» Correct Answer - A

Discussion

375.	An ideal gas is expanded such that `PT^(2)=a` constant. The coefficient of volume expansion of the gas isA. `(1)/(T)`B. `(2)/(T)`C. `(3)/(T)`D. `(4)/(T)`
Answer» Correct Answer - C From ideal gas eduation `PV = n RT` or `P=(n RT)/(V)` as gas expands such that `PT^(2)=a` constant `=C` (say) so `((n RT)/(V))T^(2) =C` or `T^(3) prop V` or `T^(3) =k V` …..(i) Diffrentiating it w.r.t. T, we have `3T^(2) =k(dV)/(dT)` Dividing it by (i), we have `(3)/(T) =(1)/(V) (dV)/(dT)` Coefficient of volume expansion, `gamma=(1)/(V) (dV)/(dT) =(3)/(T)`

Discussion

376.	For an ideal liquidA. the bulk modulus is infiniteB. the bulk modulus is zeroC. the shear modulus is infiniteD. the shear modululs is zero.
Answer» Correct Answer - A::D Ideal lilquid dose not compress easily, `Delta V=0` . Therefore, `B(F)/(A)xx(V)/(Delta V)= prop`. A lilquid cannot sustain tangential force i.e. strain is infinite for a shear stress. Hence `G=0`.

Discussion

377.	The diameter of Ballll A is twice of that of B. What will be the ratio of their terminal velicities in water.
Answer» As terminal velocity `prop ("radius of ball")^(2)`, therefore, ratio of terminal velocities of A and B will be 4:1.

Discussion

378.	A cube is subjected to pressure of `5 xx 10^(5)N//m^(2)`. Each side of the cube is shortened by 1%. Find volumetric strain and bulk modulus of elasticity of cube.
Answer» Let l be the initial length of each side of cube. Final length = `(1-(1)/(100)l)=99/100 l` Initial volume `V_(i)=t^(3)` Final volume , `V_(f)=((99)/(100)l)^(3)-l^(3)` Volumetric strain = `(Delta V)=V_(f)-V_(i)` `=(1-1/100)^(3) -1 = -3/100 = -0.03` Normal stress=increase in pressure `=5 xx 10^(5) N//m^(2)` `:.` Bulk modulus of elasticity , `B=(5 xx 10^(5))/(-0.03)` `=1.67 xx 10^(7)N//m^(2)`.

Discussion

Explore topic-wise InterviewSolutions in .

Why are clear nights colder than cloudy nights ?

A large block of ice `5 m` thick has a vertical hole drilled through it and is floating in the middle of a lake. What is the minimum length of a rope required to scoop up a bucket full of water through the hole? Relative density of ice `=0.9`.

How do you arrange `Cu, Al and Ag` in the order of increasing thermal conductivity?

The only possibility of heat flow in a thermos flask is through its cork which is `75 cm^(2)` in area and 5 cm thick. Its thermal conductivity is `0.0075 cal//cm sec^(@)C`. How lolng will 500 g of ice at `0^(@)C` in thermos flask will is `40^(@)C` and latent heat of ise is `80 cal.//gram`.

Two moles of oxygen is heated at a constant pressure from `0^(@)C`. What must be the gas for the volume to be doubled ? The specific heat of oxygen under these condition is `0.218 cal g^(-1) K^(-1)` .

What is terminal velocity? Discuss the factors on which it depends.

What is the effect of temperature on thermal conductivity of substances?

Water is heated from below but not from top. Why?

A spherical ball contracts in volume by 0.01% when subjected to a normal uniform pressure of 100 atmospheres. Calculate the bulk modulus of the meterial.

When the load on a wire is slowly increased from `3kgwt` to `5 kg wt`, the elongation increases from `0.61` to `1.02 mm`. The work done during the extension of wire is

A particle is placed at the origin and a force F=Kx is acting on it (where k is a positive constant). If `U_((0))=0`, the graph of `U (x)` verses x will be (where U is the potential energy function.)A. B. C. D.

Compute the fractional change in volume of a glass slab, when subjicted to a hydrautic pressure of 10 atmosphere. Bulk modulus of elasticity of glass = `37 xx 10^(9) Nm^(-2)` and 1 atm = `1.013 xx 10^(5)Pa`.

What is the value of isothermal bulk modulus of elasticity of air at NTP?

The effect of temperature on the value of modulus of elasticity for various substances in generalA. it increases with increase in temperatureB. remains constantC. decreases with rise in temperatureD. sometimes increases and sometimes decreases

A meterial is more elastic if its value of modulus of elasticity is ………

The solids are ……….. Elastic and gases are ………. Elastic.

Why dies velocity increase when water flowing in broader pipe enters a narrow pipe?

Distinguish between streamline motion and turbulent motion.

Which of the following diagrams does not represent a streamline flow?A. B. C. D.

A horizontal tube of non uniform cross section has radii fo 0.1 m and 0.05 m respectively at M and N, For a streamline flow of liquid the rate of liquid flow isA. continuously changes with timeB. greater at M then at NC. greater at N then at MD. same at M and N

What is the fractional volume of an ice cube in a pail of water produced in an enclosure which is freely falling under gravity?

For a ball falling in a liquid with constant velcoity, ratio of resisitance force due to the liquid to that due to gravity isA. 1B. `(2a^2 rho g)/(9 eta ^2)`C. `(2a^2(rho - sigma)g)/(9eta)`D. none of these

An ideal gas is expanded such that `PT^(2)=a` constant. The coefficient of volume expansion of the gas isA. `(1)/(T)`B. `(2)/(T)`C. `(3)/(T)`D. `(4)/(T)`

For an ideal liquidA. the bulk modulus is infiniteB. the bulk modulus is zeroC. the shear modulus is infiniteD. the shear modululs is zero.

The diameter of Ballll A is twice of that of B. What will be the ratio of their terminal velicities in water.

A cube is subjected to pressure of `5 xx 10^(5)N//m^(2)`. Each side of the cube is shortened by 1%. Find volumetric strain and bulk modulus of elasticity of cube.