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DorelXD
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Hello! It's been around two months since I started to learn about the knetic theory of ideal gases. But I haven't managed to completely understand it. What I don't understand is how we derive the formula for the presure that the gas molecules exerts on a wall's surface.
Here's what I've managed to understand; first we begin by studying a single molecule. The change in momentun when a molecule hits a wall is (in modulus): [itex]2mv_x[/itex] . So, the force exerted by a single molecule is: [itex]f=\frac{2mv_x}{\Delta t}[/itex]. The total force exerted by all the molecules that collide with the wall within a time interval /Delta t is: [itex] F=\Sigma f=\Sigma \frac{2mv_x}{\Delta t}[/itex]
Different molecules have different velocities. Ley [itex]n=\frac{N}{V}[/itex] be the number of particles per unit of volume. [itex]n_1[/itex] molecules have a x-compomnent [itex]u_1[/itex], [itex]n_2[/itex] molecues have a x-component [itex]u_2[/itex], and so on. It's obvious that:
[itex]n=n_1+n_2+n_3+...n_i[/itex]
Now, for a molecule to collide with a wall, it must get to the wall within the time interval [itex]\Delta t[/itex]. So it must be within a distance [itex] v_x\Delta t[/itex]. Given the fact that the area of the wall is [itex]A[/itex], for a molecule to colide with a wall it must be found in the volume determined by the area [itex]A[/itex] and the distance [itex] v_x\Delta t[/itex]. This volume is: [itex]v_x\Delta tA[/itex]. The number of mlecules found in this volume is:[itex]n_iv_x\Delta tA[/itex], but only half of them will hit the wall, because of some fancy mathematics which I'll hopefully understand in a few years: [itex]\frac{1}{2}n_iu_i\Delta tA[/itex].
The force exerted on the wall by a certain category of molecules that have a certain velocity will be, [itex]\frac{n_i}{2}u_i\Delta tA[/itex] times the force exerted by one molecule, [itex] \frac{2mu_i}{\Delta t}[/itex]: [itex]n_iAmu_i^2[/itex];
The total force exerted by all the molecules will be:
[tex]F=\Sigma f=\Sigma f(u_1)+ \Sigma f(u_2)+...\Sigma f(u_i)=Am(n_1u_1^2+n_2u_2^2+...+n_iu_i^2)[/tex]
Now, the average squared velocity on the x direction, [itex]\overline{u^2}[/itex] is: [itex]\frac{n_1u_1^2+n_2u_2^2+...+n_iu_i^2}{n}=\overline{u^2}[/itex], so replacing the parentheses
we obtain: [itex]F=nAm\overline{u^2}[/itex]
So far, so good. I understand that we need to replace the sum of that velocities with the average velocity because we can't find the value of each velocity, but I don't get what are we doing next. Everything I wrote before, I understood from a physics book. Now, after that, the book replaces [itex]n[/itex] with [itex]\frac{N}{V}[/itex]. I know that we defined to be like that, but It dosen't seem right to replace it. I mean, in our expresion for the force we did the sum for the molecules that hit the wall. Why all that molecules are contained in one unit of volume? I don't understand.
I'm sorry for my English and I hope you guys will help me to finally understand this theory. If you have another approach, I'm willing to listen. If you find anything wrong in what I said, or don't understand, please let me know. I really hope that you'll help me understand.
Here's what I've managed to understand; first we begin by studying a single molecule. The change in momentun when a molecule hits a wall is (in modulus): [itex]2mv_x[/itex] . So, the force exerted by a single molecule is: [itex]f=\frac{2mv_x}{\Delta t}[/itex]. The total force exerted by all the molecules that collide with the wall within a time interval /Delta t is: [itex] F=\Sigma f=\Sigma \frac{2mv_x}{\Delta t}[/itex]
Different molecules have different velocities. Ley [itex]n=\frac{N}{V}[/itex] be the number of particles per unit of volume. [itex]n_1[/itex] molecules have a x-compomnent [itex]u_1[/itex], [itex]n_2[/itex] molecues have a x-component [itex]u_2[/itex], and so on. It's obvious that:
[itex]n=n_1+n_2+n_3+...n_i[/itex]
Now, for a molecule to collide with a wall, it must get to the wall within the time interval [itex]\Delta t[/itex]. So it must be within a distance [itex] v_x\Delta t[/itex]. Given the fact that the area of the wall is [itex]A[/itex], for a molecule to colide with a wall it must be found in the volume determined by the area [itex]A[/itex] and the distance [itex] v_x\Delta t[/itex]. This volume is: [itex]v_x\Delta tA[/itex]. The number of mlecules found in this volume is:[itex]n_iv_x\Delta tA[/itex], but only half of them will hit the wall, because of some fancy mathematics which I'll hopefully understand in a few years: [itex]\frac{1}{2}n_iu_i\Delta tA[/itex].
The force exerted on the wall by a certain category of molecules that have a certain velocity will be, [itex]\frac{n_i}{2}u_i\Delta tA[/itex] times the force exerted by one molecule, [itex] \frac{2mu_i}{\Delta t}[/itex]: [itex]n_iAmu_i^2[/itex];
The total force exerted by all the molecules will be:
[tex]F=\Sigma f=\Sigma f(u_1)+ \Sigma f(u_2)+...\Sigma f(u_i)=Am(n_1u_1^2+n_2u_2^2+...+n_iu_i^2)[/tex]
Now, the average squared velocity on the x direction, [itex]\overline{u^2}[/itex] is: [itex]\frac{n_1u_1^2+n_2u_2^2+...+n_iu_i^2}{n}=\overline{u^2}[/itex], so replacing the parentheses
we obtain: [itex]F=nAm\overline{u^2}[/itex]
So far, so good. I understand that we need to replace the sum of that velocities with the average velocity because we can't find the value of each velocity, but I don't get what are we doing next. Everything I wrote before, I understood from a physics book. Now, after that, the book replaces [itex]n[/itex] with [itex]\frac{N}{V}[/itex]. I know that we defined to be like that, but It dosen't seem right to replace it. I mean, in our expresion for the force we did the sum for the molecules that hit the wall. Why all that molecules are contained in one unit of volume? I don't understand.
I'm sorry for my English and I hope you guys will help me to finally understand this theory. If you have another approach, I'm willing to listen. If you find anything wrong in what I said, or don't understand, please let me know. I really hope that you'll help me understand.
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