# 462 - Physics Questions Answers

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

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

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

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

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

in the previous case if north pole is brought near the mettalic ring  why the direction is reversed. we have to consider the  drn of magnetic field of magnet or not. if yes then which drn N-S or S-N.

Joshi sir comment

If observer is in the side of magnet, then in the given case form N in the ring, it will give you anticlockwise direction.

Due to this current produced magnetic field will be towards the coming magnet, and since north pole of magnet is coming towards ring so this field will oppose the motion of magnet

The south pole of a magnet is bought near a metallic ring ,the direction induced current in the ring will be?  Explain it clearly.

Joshi sir comment

If observer is in the side of magnet, then in the given case form S in the ring, it will give you clockwise direction

can you give me an idea of solving the parallel LC circuit? Also to find the phase difference between the I and E?

Joshi sir comment

1st make a 3 line circuit in which ac source, resistor with an inductor and resistor with a capacitor will be there in the three lines then divide total  current "I"

from AC into the two legs, equations will be

e = L di1/dt + i1R  (1)

e =q2 / C + i2 R     (2)                                        here dq2 / dt = i2

now solve

Why earth moves in orbital motion around the sun ...? why not in circular motion...?

Joshi sir comment

Since every heavenly body like sun, earth, moon are rotating freely in free space, there is no body which is placed on a base, so rotation of every body depends on all the surrounding bodies. It means its track will also depend on the attraction forces given by all the surrounding bodies.

Two cylinders are twisted by the same external torque.If the ratios of their modulous of rigidity , radii & lengths are 1:2, 2:3 & 3:4 respectively. Then find the ratio of  angle of twist ??

Joshi sir comment

Let the angle rotated due to torque = θ and angle of twist (shear) = δ

then arc covered = rθ   (here r is radius)

now for twist of wire δ = rθ/l  (here l is length)

according to formula η = F/Aδ  and torque ζ = kFr    (here k is constant of proportion)

we get ζ = kηAδr

Use the formula and get the answer, you have to calculate ratio of δ's so k is not required

how far from earth we can see earth as a whole elliptical?{we can clearly see the earth

Joshi sir comment

for getting the complete vision we should draw tangents at the extreme of major and minor axis of the earth, this will give a point of intersection for which height from centre of earth will be R√2 so height from earth surface will be R√2 - R = R(√2-1)

here R is used for radius of earth

This is approximate calculation only

Solution by Joshi sir

for getting the complete vision we should draw tangents at the extreme of major and minor axis of the earth, this will give a point of intersection for which height from centre of earth will be R√2 so height from earth surface will be R√2 - R = R(√2-1)

here R is used for radius of earth