11 - Modern Physics Questions Answers

Physics >> Modern Physics >> Atomic Physics IIT JEE

A point source of light is placed at the centre of curvature

of a hemispherical surface. The radius of curvature is r

and the inner surface is completely reflecting. Find the

force on the hemisphere due to the light falling on it if

the source emits a power W.

Asked By: ANKIT SHARMA

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Physics >> Modern Physics >> Matter and Waves IIT JEE

There are N numbers of electrons present in a one-dimensional box of length L. Assuming no coulombic interactions are present, find the ground-state energy of the N-particle system. N is an even number and mass of each electron is m. All electrons have a definite momentum, p, and their collisions with the box are perfectly elastic.

Asked By: SAJAG KUMAR

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Joshi sir comment

Physics >> Modern Physics >> Matter and Waves IIT JEE

Sir

Does time exist or is it just an illusion .

Please tell.

Asked By: SHIVAM SHARMA

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Joshi sir comment

I am not a scientist yet many scientists including Einstein explain time as an illusion, so it can be said.

Physics >> Modern Physics >> Atomic Physics Others

Consider an electron trapped in an infinite well whose width is 98.5 pm. If it is in a state with n = 15, what are (i) its energy? (ii) the uncertainty in its momentum? (iii) the uncertainty in its position?

Asked By: ANKIT SONI

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Joshi sir comment

$L e t w i d t h = b F o r n t h s t a t e n \frac{λ}{2} = b \Rightarrow λ = \frac{2 b}{n} N o w E = \frac{p^{2}}{2 m} = \frac{{(\frac{h}{λ})}^{2}}{2 m} S o l v e a f t e r p u t t i n g t h e v a l u e o f λ U n c e r ta i n t y i n p o s i t i o n = b N o w u s e ∆ p . ∆ x = n h / 2 π for finding uncertainty in momentum$

Physics >> Modern Physics >> Atomic Physics Board Exam

a toy car of mass 500g travels with a uniform velocity of 25m/s for 5 seconds, the brakes are then applied and the car is uniformly retarded and comes to rest in further 10s calculate the retardation

Asked By: SARABJEET

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Joshi sir comment

use v = u + at

u = 25, v = 0 and t = 10

Physics >> Modern Physics >> Logic Gates Others

when we throw a shotput ball it covers a very less distance and when we throw a rubber ball it covers a larger distance.

the reason behind this well known to everyone that the shotput ball has more mass as compared to the rubber ball.

But when throw a paper ball it covers the least diatance why?

Asked By: SANTANU VERMA

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Joshi sir comment

for this motion 0-R = ma

so retardation = R/m

for less mass retardation will be more

in the case of shotput

mu + impulse given = mv

so v = u + {impulsegiven/m}

so more mass will gain less velocity in comparison to less mass on applying same impulse.

Physics >> Modern Physics >> Nuclear Physics Medical Exam

the half life of radium is 1620 year & its atomic weight is 226gm/mole. the number of atoms that will decay from its 1g sample per second is

1) 3.16 x 10¹⁰

2)3.6 x 10¹²

3) 3.1 x 10¹⁵

4) 31.1 x 10¹⁵

Asked By: HIMANSHU

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Joshi sir comment

for radioactivity we know that it is a first order process

so dN/dt = -λN

so dN/1 = -[0.6932/(1620*3.1*10⁷)]*(1/226)*6.023*10²³

solve now

Physics >> Modern Physics >> Nuclear Physics Medical Exam

A RADIOACTIVE SUBSTANCE EMITS n BETA PARTICLES IN THE FIRST 2 SECONDS & 0.5 n BETA PARTICLES IN THE NEXT 2 SECONDS . THE MEAN LIFE OF THE SAMPLE IS

ANS 2/(ln2) s

Asked By: SARIKA SHARMA

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Joshi sir comment

A RADIOACTIVE SUBSTANCE EMITS n BETA PARTICLES IN THE FIRST 2 SECONDS & 0.5 n BETA PARTICLES IN THE NEXT 2 SECONDS. Definitely by definition its half life will be 2 sec.

so mean life = half life / ln2 This is the relation between mean life and half life.

Physics >> Modern Physics >> Nuclear Physics Medical Exam

A NUCLEUS OF MASS NO. 220, INITIALLY AT REST , EMITS AN .α PARTICLE . IF THE Q VALUE OF THE Rxn IS 5.5MeV THE ENERGY OF THE EMITTED α PARTICLE WILL BE ??

ANS 5.4 MeV

Asked By: SARIKA SHARMA

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Joshi sir comment

5.5*216/220 = 216/40 = 5.4 MeV here 216 is the mass of nucleus after ejecting α particle.

Physics >> Modern Physics >> Nuclear Physics IIT JEE

The activity of a freshly prepared radioactive sample is 10^10 disintegrations per second, whose mean life is 10^9 sec.

The mass of an atom of this radioisotope is 10 ^ -25 kg. The mass (in mg) of the radioactive sample is

Asked By: NITIN RAO

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Joshi sir comment

dN/dt = -λN

10¹⁰ = -[1/10⁹] N λ = 1/mean life

so N = 10¹⁹disintegration

so mass of radioisotope = 10¹⁹ *10^-25= 10^-6 Kg = 1 mg

11 - Modern Physics Questions Answers

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