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Pressure

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Momentum delivered to the wall

Equation

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So the average moment\\n\\n

2,m,v_x

\\n\\nwhich is transferred to the wall in dt time by\\n\\n

\displaystyle\frac{1}{2}c_n,S,v_xdt

\\n\\nparticles gives a moment equal to:\\n\\n

\langle dp_x\rangle=\langle 2,m,v_x\displaystyle\frac{1}{2}c_n,S,v_x dt\rangle



Since the mass, the section and the time are independent of the average, it has to be

\langle dp_x\rangle= m c_nS\langle v^2\rangle dt

The factor 1/2 is due to the fact that half of the particles move towards the wall while the other half move away.

ID:(3934, 0)



Number of particles that reach the wall

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In a time dt all the particles that are at a distance equal to or less than \ langle v_x \ rangle dt will hit the wall. If the surface is S and the concentration of particles c_n , we will have that the number of particles is

dN_x=\displaystyle\frac{1}{2}c_n\langle v_x\rangle dt S

The factor 1/2 is due to the fact that half of the particles move towards the wall while the other half move away.

ID:(3935, 0)



Particles with f degrees of freedom

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Particles with f degrees of freedom

ID:(1959, 0)



Particles with five degrees of freedom

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Particles with five degrees of freedom

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Particles with three degrees of freedom

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Partículas con tres grados de libertad

ID:(1957, 0)



Pressure

Equation

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On the other hand the pressure is the force F_x per area S \\n\\n

p=\displaystyle\frac{\langle F_x\rangle}{S}

\\n\\nand the force is the variation of the moment in time\\n\\n

\langle F_x\rangle=\displaystyle\frac{\langle dp_x\rangle}{dt}

\\n\\nwe have with\\n\\n

\langle dp_x\rangle=c_nmS\langle v^2\rangle dt

\\n\\nand\\n\\n

\langle\epsilon\rangle=\displaystyle\frac{1}{2}m\langle v^2\rangle



than

p=\displaystyle\frac{2}{3}c_n\langle\epsilon\rangle

ID:(3937, 0)



Average speed of the particles

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Como la suma de los cuadrados de la velocidad en cada componente es igual al cuadrado de la magnitud\\n\\n

v^2=v_x^2+v_y^2+v_z^2



y como por simetría todas las componentes tiene que ser iguales

\langle v_x^2\rangle=\langle v_y^2\rangle=\langle v_z^2\rangle

ID:(824, 0)