# iGCSE Physics (0625)-Unit 2. Thermal physics-2.1.2 Molecular model- Paper 4

### Question

(a)       In terms of the momentum of molecules, explain how a gas exerts pressure on the walls of its container.        

(b) A fixed mass of gas of volume V1 is at a pressure p1. It is compressed to a volume V2.

1. Complete the equation for the final pressure p2 of the gas when the gas is compressed at constant temperature.

p2 =                                                                                                                                                              

2. State and explain how the final pressure compares with p2 when the temperature of the gas increases during compression.

statement …………………………………………………………………………………………………………..

explanation …………………………………………………………………………………………………………    

[Total: 9]

Ans:

(a)

molecules strike walls

momentum (of molecules) changes / momentum = mass× velocity

force = rate of change of momentum

pressure = (sum of) force(s) / area / pressure = rate of change of momentum / area

(b)

(i)

(p2=)p1V1 / V2

p1V1 =p2V2 )

(ii)

greater
molecules move faster / have greater KE / molecules have greater momentum
(leads to) more frequent / harder collisions (with walls) / great rate of change of momentum