# 575 - Physics Questions Answers

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

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

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

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

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