Interpretation: The mean speed of the emerging beam relative to the initial value has to be calculated.
Concept introduction: The expression for the mean speed is derived from Maxwell Boltzmann distribution. The mean speed for the gas particles is represented as,
vmean= (8RTπM)1/2

Answer to Problem 1B.3P
The speed of the emerging beam relative to the initial value is vx=0.47〈vx〉initial_.
Explanation of Solution
The expression for the velocity of the emerging beam is,
vx=Ka∫0vxf(vx)dvx (1)
Where,
- • K is the constant of proportionality.
- • vx is the velocity.
The value of f(vx) is given by,
f(vx)=(m2πkT)1/2e−mv2/2kT
Where,
- • m is the mass.
- • k is the Boltzmann constant.
- • T is the temperature.
The normalised function is written as,
vxa∫0vxdvx=Ka∫0f(vx) dvx1=Ka∫0f(vx) dvx
Substitute the value of f(vx) in the above equation.
1=K(m2πkT)1/2a∫0e−mv2/2kT dvx (2)
The equation (2) is evaluated by defining an error function.
mv2x2kT=n2v2x=2kTmn2vx=(2kTm)1/2n
Or,
dvx=(2kTm)1/2dn (3)
Substitute equation (3) in equation (2) to find n.
1=K(m2πkT)1/2(2kTm)1/2b∫0e−n2dn
Here, b=(m2kT)1/2×a
1=K(2kTm)1/2(m2πkT)1/2b∫0e−n2dn=(Kπ)1/2b∫0e−n2dn
According to error function,
erf(z)=2π1/2z∫0e−n2dn
Therefore,
1=(Kπ)1/2b∫0e−n2dn=12√K erf(b)=2erf(b)
To calculate the mean velocity of the beam, substitute the values in equation (1).
vx=K(m2πkT)1/2a∫0vxe−mv2x/2kTdvx=K(m2πkT)1/2(−kTm)a∫0ddvx(e−mv2x/2kTdvx)
vx=−(kT2mπ)1/2(e−mv2x/2kT−1) (4)
The initial velocity, a=〈vx〉initial and the initial velocity is (2kT/mπ)1/2. The expression for average magnitude of one dimensional velocity in the x direction is,
〈vx〉initial=2∞∫0vxf(vx) dvx=2∞∫0vx(m2πkT)1/2 e−mv2/2kTdvx=(m2πkT)1/2(2kTm)=(2kTmπ)1/2
For a=∞ the value of constant, b=∞ and the value of error function, b erf(b)=1. Substitute the value of a and b erf(b)=erf(1/π1/2) in equation (4).
vx=(2kTmπ)1/2×1−e−1/πerf(1/π1/2) (5)
The value of erf(1/π1/2) is,
erf(1/π1/2)=erf(0.56)=0.57
The value of e−1/π is 0.73.
Substitute the values in equation (5) to calculate mean velocity.
vx=(2kTmπ)1/2(1−0.730.57)=0.47〈vx〉initial_
Thus, the value of mean velocity of emerging beam is vx=0.47〈vx〉initial_.
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Chapter 1 Solutions
PHYSICAL CHEMISTRY-WEBASSIGN
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