568 - Physics Questions Answers

A body emits radiant energy 1600Js‾¹ when it is at temperature 273 °C. if its temperature decreases to 273 K them it emits radiant energy at the rate of

(1) 0

(2) 800 Js‾¹

(3) 400 Js‾¹

(4) 100 Js‾¹

Asked By: HIMANSHU MITTAL
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Joshi sir comment

Temprature of second case is half of the temprature of first case so according to stefan's law radiant power will become 1/16 part = 100

A copper cube of each side 40 cm floats on mercury. the density of copper cube is 3.2g/cc and of mercury is 13.6g/cc at 27° C. the coefficient of volume expansion of Hg and linear expansion of copper are 1.8 * 10^(-4) /°C and 3 * 10^(-6) /°C respectively. Calculate the increase in height i.e. how much block sink further when the temperature rises from 27°C to 100°C?

Asked By: HIMANSHU MITTAL
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Joshi sir comment

dipped length of the cube in first case = m/Ad  here m, A, d are mass of cube, area of the cube and density of mercury respectively

after increasing temperature dipped length in the second case = m/A(1+2αt)[d/(1+γt)]

values are m = VD = 403 3.2, A = 402, α = 3 * 10-6, d = 13.6, γ = 1.8 * 10-4, t = 73

all values are in CGS

now solve then substract 1st length from 2nd length   

A thermally insulated piece of metal is heated by supplying a constant power P. Due to this, the temperature of the metal starts varying with time as T= at^1/4 + To(T not)

The heat capacity of the metal as a function of temperature is

(1) 4PT³/a^4

(2) 4P(T-To)³/a^4

(3) 4PT²/a³

(4) 4P(T - To)²/a³

Asked By: HIMANSHU MITTAL
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Joshi sir comment

P = dQ/dt = mcdT/dt

or mcdT = Pdt

or mc = Pdt/dT

or mc = Pd/dT[(T-T0)4/a4]

here mc is heat capacity

now solve 

Two absolute scales A and B have triple points of water defined to be 200 A and 350 B. What is the relation between Ta and Tb?

(1) Ta = Tb/2

(2) Ta = 7Tb/4

(3) Ta = 2Tb

(4) Ta = 4Tb/7

Asked By: HIMANSHU MITTAL
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Joshi sir comment

273.15 K = 200 A

and 273.15 K = 350 B

so A = 273.15 K/200 and B = 273.15 K/350

so Ta A = TbB

or Ta/T= B/A = 200/350 = 4/7 

so 4th option

The absolute zero temperature is

(1) -273° C

(2) -273.15 K

(3) -273.15° F

(4) -273.15° C

Asked By: HIMANSHU MITTAL
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Joshi sir comment

it is -273.15 0

2 moles of a diatomic gas are enclosed in a vessel. when a certain amount of heat is supplied, 50% of the gas molecules get dissociated, bit there is no rise in temperature. What is the heat supplied if the temperature is T?

(1) RT

(2) RT/2

(3) 11RT/2

(4) 5RT

Asked By: HIMANSHU MITTAL
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Joshi sir comment

internal energy of 2 mole diatomic gas = 2*5/2 RT = 5RT

now after 50% disociation in monoatomic form total internal energy

of 1 mole diatomic gas = 1*5/2 RT = 2.5 RT

and that of 2 mole monoatomic = 2*3/2 RT = 3RT  ( 2 mole monoatomic gas will be formed by the dissociation of 1 mole diatomic gas)

so internal energy change = 1/2 RT

This internal energy increment will be equal to the heat provided 

A gaseous mixture consists of 16g of helium  and 16g of oxygen. the ratio Cp/Cv of the mixture is

(1) 1.59

(2) 1.62

(3) 1.4

(4) 1.54

Asked By: HIMANSHU MITTAL
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Joshi sir comment

16 gms He = 4 mole,   16 gms O2= 1/2 mole

so total freedom n = [4(3) + 1/2(5)] / [4 + 1/2]  = 3.22222   here 3 and 5 are freedoms for mono and di atomic gases  

so γ = 1 +  2/3.2222222 = 1.621

A vessel of volume 0.3 m³ contains helium at 20.0°C. the average kinetic energy per molecule for the gas is

(1) 6.07 * 10^(-21) J

(2) 7.3 * 10³ J

(3) 14.6 * 10³ J

(4) 12.14 * 10^(-21) J

Asked By: HIMANSHU MITTAL
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Joshi sir comment

Use the formula 3/2 kT   

k is boltzmann constant = 1.38* 10-23

 

4 moles of an ideal monoatomic gas is mixed with 6 moles of a  non reactive diatomic ideal gas. the molar heat capacity of the  mixture at constant volume is (assume no vibration) nearly

(1) 27 J mol‾¹K‾¹

(2) 17 J mol‾¹K‾¹

(3) 7 J mol‾¹K‾¹

(4) 37 J mol‾¹K‾¹

 

Asked By: HIMANSHU MITTAL
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Joshi sir comment

at constant volume work done = 0

degree of freedom = 4(3) + 6(5) / 4 + 6 = 12+30/10 = 4.2   so γ = 1 + (2/4.2) 

so molar heat capacity = R/(γ-1)

consider an ideal gas contained in a vessel. if the intermolecular interaction suddenly beings to act, which of the following will happen?

(1) the pressure decreases

(2) the pressure increases

(3) the pressure remain unchanged

(4) the gas collapses

Asked By: HIMANSHU MITTAL
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Joshi sir comment

if the intermolecular interaction suddenly beings to act, then the pressure increases because due to intermolecular interaction gase will work as real gas 

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