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Heavy Water is an excellent moderator due to its high moderating ratio and low absorption cross section for neutrons.
A mass defect refers to the difference in mass between of an atom and the sum of the masses of the protons, neutrons, and electrons of the atom. The "missing" mass is the energy released by the formation of the atomic nucleus.
Who indirectly determined the mass of the electron by measuring the charge of the electron ?
Millikan determined the mass of the electron by measuring the charge of the electron.
Resistance of a wire at 20°C is 20 Ω and at 500°C is 60 Ω. At what temperature its resistance is 25 Ω ?
R_{T}=60 ohms @ T=500^{o}C
Also, R_{T′}=20 ohms @ T′=20^{o}C
Resistance at a given temperature R_{T}=R_{T′}[1+α(T−T′)]
Let resistance of wire is 25 ohm at temperature T′′.
Photo cell is used to convert photon energy(i.e. light energy) into chemical energy.
A nichrome wire 50cm long and square millimeter crosssection carries a current of 4A when connected to a 2V battery. The resistivity of nichrome wire in ohm meter is
Two coils are placed close to eachother. The mutual inductance of the pair of coils depends upon
The coupled flux of two coils system is used to define the mutual inductance between the coils. The mutual inductance between the coils is
So it is defined as the proportionality between the emf generated in coil 2 due to the current flows in coil 1. Thus It depends on the relative position and orientation of two coils.
The p.d. across an instrument in an A.C. circuit of frequency f is V and the current flowing through it is I such that V = 5 cos 2π amp. The power dissipated in the instrument is
V = 5cos2πft [ω = 2πf]
V = 5cosωt
=5sin(ωt+π2) and i = 2sinωt
Power = Vrms×irms×cosφ=0
(Since φ = π/2, therefore cosφ = cos(π/2)=0)
The current flowing in a coil of self inductance 0.4 mH is increased by 250 mA in 0.1 s. The e.m.f. induced will be
As current is increasing, induced e.m.f. is in opposite direction, therefore, e =  1 mV.
The capacitance is proportional to the surface area of the conducting plate and inversely proportional to the distance between the plates. Hence, it depends upon the size of the conductor, and not upon the nature of conductor, thickness or colour.
If size of the earth is reduced to half, then no. of days in one year will be
Time period is not dependent on radius.
The capacity of a condenser with two identical plates separated by a distance 'd' in air is 'C'. If the distance of separation is reduced to d/2, the capacity is
The capacity of a parallel plate condenser is given as,
When the separation between the plates is halved then the new capacity of the condenser is given as,
Thus, the new capacity of parallel plate condenser is 2C.
Figure shows two large cylindrical shells having uniform linear charge densities + λ and  λ . Radius of inner aylinder is 'a' and that of outer cylinder is 'b'. A charged particle of mass m , charge q revolves in a circle of radius r. Then its speed 'V' is:
[Neglect gravity and assume the radii of both cylinders to be very small in comparison to their length]
Using Gauss's law for electrostatics,
Electric field at charged particle would be given as,
So centripetal force on particle is,
The energy emitted per second by a black body at 27°C is 10 J. If the temperature of the black body is increased to 327°C, the energy emitted per second will be
Vapour is injected at a uniform rate in a closed vessel which was initially evacuated. The pressure in the vessel
When the vapour is injected into the evacuated vessel, the pressure of the vessel starts to increase. As the pressure is increased and reaches the saturated vapour pressure, the vapour starts to condense beyond this. Therefore, after a particular instant, the pressure of the container becomes constant.
So, the pressure increases initially and then becomes constant.
At what temperature, the root mean square velocity of a hydrogen molecules is equal to that of an oxygen molecules at 47°C?
_{}
The diameter of a copper sphere is 6 cm. It is melted to form a wire of uniform crosssection. If the length of the wire is 36 cm then find the radius of the wire.
If a long hollow copper pipe carries a current, the produced magnetic field will be
If the current flows on the surface of a long hollow copper pipe. Then the magnetic field inside is zero. Hence, there shall be magnetic field produced outside the pipe.
A ball of mass m strikes elastically a wall at an angle of 30° with velocity ν. The change in momentum is
Change in momentum (P)=2mv cosθ
P=2mv cos 30°
P=2mv√3/2
P=√3mv
In a surface tension experiment with a capillary tube, water rises upto 8 mm. If the same experiment is repeated on an artificial satellite, which is revoling around the earth, water will tise in capillary tube upto a height of
Two solid pieces, one of steel and the other of aluminium when immersed completely in water have equal weights. When the solid pieces are weighed in air
Apparent weight in air
If pressure inside two soap bubbles are 1.01 and 1.02 atm respectively, then the ratio between their volumes is (Take atmospheric pressure = 1 atm)
Pressure × Radius = Surface Tension
Here the pressure is Gauge Pressure, i.e the difference between absolute pressure and atmospheric pressure
Hence ratio of gauge pressures is 1:2
Accordingly ratio of radii is 2:1
Since Volume∝radius^{3}
Hence ratio of volumes is 8:1
An incompressible fluid flows steadily through a cylindrical pipe which has radius 2 R at point A and radius R at point B. If velocity at point A is ν, then its velocity at point B is
ATQ, equation of continuity,
Area is inversely proportional to flow rate,
therefore if area is reduced by four times
the flow rate becomes 4 times = 4v
The potential energy of a molecule on the surface of a liquid compared to one inside the liquid is
Water level rises in a capilary tube, if the angle of contact is
If the angle of contact is less than 90 ,then the level of water rises in concave meniscus.
Which of the following affets the elasticity of a substance?
A wire can be broken by applying a load of 200 N.The force required to break another wire of the same length and same material, but double in diameter, is
We know that, E = dΦ/dt,
E = d/dt {2t²  3t + 1}
= (4t  3)
at t = 0.25 sec,...
E = (4×0.25  3) = 2 V
A body starts from rest. What is the ratio of the distance travelled by the body during the 4th and 3rd second
For a projectile, the ratio of maximum height reached to the square of flight time is (g = 10 ms⁻^{2})
From the above relation we can see that magnification is directly proportional to the focal length of objective lens.
The potential energy of a particle executing simple harmonic motion at a distance y from its equilibrium position is
Potential Energy = 1/2 k y^{2}
ω^{2 }= k/m
k = mω^{2}
Potential Energy = 1/2 (mω^{2 }) y^{2}
Potential Energy = 1/2 mω^{2}y^{2}
The frequency of a light wave in a material is 2 x 10^{14} H z and wavelength is 5000 Å. The refractive index of material will be
Time of exposure for a photographic print is 10 second, when a lamp of 50 Cd is placed at 1 m from it. Then another lamp of luminous intensity I is used, and is kept at 2 m from it. If the time of exposure now is 20 s, the value of I in (in Cd) is :
Intensity ∝ time
= 100 Cd
When a sphere of moment of inertia I about an axis through its centre of gravity and mass m rolls down from rest, on an inclined plane without slipping, its kinetic energy is
Rolling motion= Pure translatory motion + Pure rotatory motion
Rolling motion= 1/2 mν^{2}+1/2Iω^{2}
When light travels from one medium to another which are separated by a sharp boundary, the characteristic of light which does not change, is
Color and energy depend on frequency and frequency does not change. The light photons are the same as the photons at the beginning. Frequency is a parameter of the source of emission of light, not the medium it is travelling through.
A wheel is rotating at 900 r.p.m. about its axis When power is cut off it comes to rest in 1 minute The angular retardation in rad/s^{2} is
A body is rotating with angular velocity 30 rad/s. If its kinetic energy is 360 J, then its moment of inertia is
Kinetic Energy(KE) = 360J
Angular Speed = ω = 30rad/sec
A ballet dancer of moment of inertia I is spinning about the vertical axis with an angular velocity of 20 rads^{⁻1}. If she suddenly changes her angular velocity to 10 rads^{⁻1}, then new moment of inertia would be
Here, angular momentum is conserved.
Initial angular momentum = Final angular momentum
I×20=I′×10
where, I′ is the new moment of inertia.
Then,
I′= 2 I
A fan is making 600 revmin^{⁻1}. If it makes 1200 revm^{⁻1}, then increase in its angular velocity is
Initial angular velocity= ω_{1 }= 2πn_{1}
Final angular velocity= ω_{2} = 2πn_{2}
Increment in angular velocity ω = 2π(n_{2 }−n_{1})
The period of spring oscillating simple harmonically
The period of a simple harmonic oscillator is given by
Any type of semiconductor either p or n type both are neutral i.e. uncharged.
At 27°C a gas is suddenly compressed such that its pressure becomes 1/8th of original pressure. Temperature of the gas will be (γ = 5/3)
Since gas is compressed suddenly, gas is undergone an adiabatic process
In adiabatic process,
The length and breath of a metal sheet are 3.124 m and 3.002 m respectively. The area of this sheet up to four significant figures is
Area of metal sheet =3.124×3.002 = 9.378248
Now, the result must have significant figures equal to the least of figure being multiplied.
So, area of metal sheet =9.378 m^{2}
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre.
The gravitational force is given as:
If the two slits in Young's experiment have width ratio 1:4, the ratio of intensity at maxima and minima in the interference pattern in
When the source of disturbance is a slit (i.e. line source) then the wavefront is cylindrical because all the points are equidistant from the source and they lie on the surface of the cylinder.
According to wave theory, light is a form of energy which travels through a medium in the form of
Standing waves are produced in a 10 m long stretched string. If the string vibrates in 5 segments and the wave velocity is 20 m/s, the frequency is
There are 5 loops in 10 metre.
Length of one loop = 2m
Length of two loops = 2×2=4m
Now, λ= 4m, v= 20m/sec
From formula, v= vλ
20= v × 4
v= 5 Hz
An isolated particle of mass m is moving in a horizontal plane along the xaxis at a certain height above the ground. It suddenly explodes into two fragments of masses m/4 and 3m/4. An instant later, the smaller fragment is at y = 15 cm. The larger fragment at this instant is at
Before explosion particle was moving along xaxis i.e. it has no ycomponent of velocity. Therefore the centre of
mass will not move in ydirection or we can say y_{cm }= 0.
Now, y_{cm }=
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