# Cant understand those lines in a text about solid state physics

1. Mar 24, 2010

### nhrock3

"we want to calculate the heat enrgy of a gas molecule
n - is the number of molecules per volume
v_th - is the average speed of gas molecule
L - is the distance which travels one molecule in a certain time

we have six directions for which they travel
+-x +-y +-z

so in each time n/6 of the molecules travel in every one of those 6 dircetion."

i cant see how?
we have n molecules why they divided evenly
it could be the in the +z direction move 5/6n of the molecules

??

2. Mar 24, 2010

### waht

Gas molecules are energetic and bounces off of walls and gazillions of other gas molecules. As a result they constantly change direction. That's why it's impractical to assign one specific direction because in the next moment a gas molecule will bounce off something and fly in the opposite direction. That's why the average of 6 possible directions is taken.

3. Mar 24, 2010

### nhrock3

thanks :)
next question about missundertood text:(question 2)
"the total amount of molecules which hit an area unit in a certain time
equals 1/6 of the number of molecules which present in the rectangular box
the area of its base is L=V_th*1
so
we have 1/6 * n *L=(n*V_th)/6
"
a bout this
L=V_th*1
V_th is the speed .sped times 1 cannot be area?

how they got
1/6 * n *L*1=(n*V_th)/6
?
n is the number of molecules per volume ,L is the distance which travels in some time
so i cant understand what is the result of multiplying dencity by distance
its undefined.

n*V_th is speed time density another undefined unit
??

Last edited: Mar 24, 2010
4. Mar 24, 2010

### nhrock3

(question 3)
formula question:"
calculate the density n and its the material energy of a gas which is contained in presure 5 and termperature 325k"

i am not asking you to solve this homw work question
my question is about the formula
i know that
$$\frac{mv_{th}^2}{2}=\frac{3}{2}kT$$
the left side represent the total energy
or
$$E=\frac{3}{2}kT$$
so i used this formula
but in the solution they use
$$E=n\frac{3}{2}kT$$

5. Mar 24, 2010

### dlgoff

I'm not sure this is helpful, but if you are wanting to determine the density of an ideal gas, n, wouldn't you want to use p=nkT where p is the pressure, T is the temperature, and k is Boltzmann's constant?

6. Mar 25, 2010

### nhrock3

i need to calculate the energy too
my dilema is about the formula they used its not like the formal formulaa