In our lecture, we discussed a relation between pressure and Mean Free path of a

Question

In our lecture, we discussed a relation between pressure and Mean Free path of air molecules. This is also in your lecture note. Calculate and put in the following table pressure vs. mean free path of molecules in air around room temperature for pressures = 1atm, 10-1atm, 10-2atm, …etc. until 1mTorr. Note: we stated before that fab equipment in many areas utilize vacuum to achieve higher mean free path. Based on the trend shown in your calculation, can you appreciate why?

Pressure(atm)

1

10-1

10-2

10-3

10-4

10-5

10-6~1mTorr

MFP (cm)

Pressure(atm)

1

10-1

10-2

10-3

10-4

10-5

10-6~1mTorr

MFP (cm)

Explanation / Answer

At room temperature we use following formula for calculation of mean free path (MFP)

MPF (l ) = 5 x 10-3 / P

Here pressure is in torr

Calculation of MFP at Pressure for 1 atm.

1 atm = 760 torr

MFP = 5 x 10-3 / 760

= 6.60 x 10-6 cm

2).

P = 10-1 atm

            MFP = 5 x 10-3 / (10-1 x 760 )

=6.60 E-5

3)

P = 10-2

MFP = 5 x 10-3 / (10-2 x 760 )

= 6.60 x 10-4

4)

MFP = 5 x 10-3 / (10-3 x 760 )

= 6.60 x 10-3

5)

MFP = 5 x 10-3 / (10-4 x 760 )

= 6.60x 10-2

6)

MFP = 5 x 10-3 / (10-5 x 760 )

= 6.60 x 10-1

7)

MFP = 5 x 10-3 / (10-6 x 760 )

=6.6 cm

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