# B Boyles law, thermal physics

1. May 24, 2017

### Kajan thana

Hi guys,
From Boyles Law, we know that Volume is inversely proportional to Pressure give that the temperature is kept.
My question, if we decrease the volume, the pressure will increase due to rate of change of momentum ( we can say those two are proportional), then why can we not claim that there is increase in average molecular speed?

2. May 24, 2017

The reason the pressure goes up for the same temperature (and same average molecular speed) is that the density is increased when you decrease the volume. Thereby, you have more particles per given time imparting momentum to the walls of the container. $P=\frac{n}{V} RT$ . Notice the density $\frac{n}{V}$ has increased as you decrease $V$.