The website's policy is of course against smoking, but there is still an interest in the physics of smoke - why does cigarette smoke inside the car want to come out?
Bernoulli's law states that for a flow with a stream function the energy along the line is constant. In simple words, if it is possible to point to a water stream particle along its entire path (there is no circulation or mixing), then along this path the energy of the particle is constant, and it only changes its shape. This is assuming that we have not externally added or subtracted from its energy by a pump, turbine or friction. The energy can change form from static pressure energy on the sides to velocity energy or height energy (potential height). If one of these factors increases, the sum of the others must decrease. In the case of cigarette smoke, the pressure inside the vehicle will always be higher than outside, because the flow velocity inside is low.
Air (and with it the smoke) flows from high to low pressure. The faster the car drives, the faster the smoke will come out of the window, because the pressure difference will be greater. This is also the reason why sometimes you open a window at home and the air does not enter. The air does not want to enter a place with high air pressure than a place of low air pressure. In that case we will have to open another window, preferably on the other side of the house. We will notice that in the air conditioners in the car, if you put internal circulation, a fan is turned on. But if you put in external circulation, the air comes from outside. That is, if the window is opened, the air in this condition will enter at a much greater rate than with the windows closed - the air refuses to enter the car with the high pressure. A nice experiment that many are familiar with and demonstrates the idea well is to take a pan (hair dryer) and turn it on when it is facing up. So put a ping pong ball above the hot air, and the ball will bounce in the air but not fall to the sides, because of the pressure differences.
Why will pumps always be at the bottom of the well and not at the top?
In short, it is easy to push water and hard to pull it. When we drink water with a straw, we do not draw water but create a negative pressure up the straw and it is the one that draws the water. The longer the straw is, we will have to lower the pressure up the straw, and when we reach a water column of 10 meters (equivalent to the pressure of one atmosphere of water) we will not be able to extend the straw any more. In fact, even if we create a complete vacuum at the end of the straw, we will not be able to raise more water in a column that is higher than ten meters. The water moves up the straw because of the atmospheric pressure at the water level, so a complete vacuum in the straw will give a maximum pressure difference of one atmosphere. That's why pipeline planners would prefer to put the pump at the bottom of the reservoir, and give the water the added pressure before rising to the ground.
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An excellent question that I did not bring a reference to. If the car was completely sealed then it would be very difficult for the air to get out. Why is it difficult and not impossible? Because the window provides a two-way path of air: in and out.
Hello Jonathan.
Shouldn't the described process (air release from the car) end following sufficient air release that will equalize the pressures?
Error correction: the diffusivity of the material is not multiplied by four, but by two. I chose to use a first degree equation which is linear. Multiply by four when building a network of measurement points on the computer and treat diffusion as a surface flux.
Another point for thought for Yehuda: if the smoke was driven by diffusion, the gas would spread in all directions and it is not like that.
Answer to Judah
Diffusion is a very slow process that occurs for statistical reasons only. A substance with a high concentration will move to a low concentration, and a substance from a low concentration will move to a place with a high concentration. The amount coming from the higher concentration is greater than the one coming from the place with the lower concentration and an imbalance is created that tends towards the lower place. Diffusion is affected by many factors, we will mention two of them: temperature, the distance between the particles. When the first increases the diffusion is better because the particles move faster and when the second increases the diffusion decreases. Those who want a detailed explanation should send an e-mail, but we will content ourselves with the fact that there is a factor called activation energy for the particular substance that undergoes diffusion, and it is affected by temperature to the first degree. In addition, the diffusivity of a material (the quality of its ability to pass the message on to the volume around it) depends on the temperature in an inverse exponential way. But because the temperature in the equation is absolute (Kelvin) the effect of diffusivity is small. Raising 2.718 (exponential or natural log) to the power (one divided by minus 298 Kelvin) is very similar to raising 2.718 to the power (one divided by minus 238 Kelvin). The math is simple: the expression in parentheses is almost equal to zero, so the general expression is 1. We will continue with the answer: smoke coming out of a car moves in a linear path, assuming that one window is open and there are no turbulences. In this simple case, we will multiply the measurement time of the diffusion by 4 and double the diffusivity and extract a root. We will get the linear distance that the smoke traveled. There is no point in moving the formula to two dimensions because most of the smoke moves towards the window. I did not solve the equation by placing 40 cm between the cigarette and the window, but I have no doubt that with the car stationary, the resulting time is very long for a jet of thick smoke.
And here is the point of the response: the smoke moves because of pressure differences. When you want to connect two streams together, accelerate the first and connect a pipe of the second into the first. The speed of the first one creates a low pressure (low static rather than dynamic pressure) and this acts as a pump for the second fluid to join it.
Note to readers: I would be very happy to make it easier on myself with the abbreviated explanations in the articles and raise the scientific level a little. If people are interested in this I would love to hear from you.
Hello to Jonathan Shanhav!
First of all your articles are interesting and thought provoking.
Regarding this article, the smoke strives to get out of the vehicle not only because of the opening of the windows but also because of the diffusion phenomenon of the smoke with its surroundings. Can you tell us about the rate of spread resulting from this phenomenon? I had trouble understanding the formula that explains this rate of expansion.
Thanks in advance
And in appreciation
Sabdarmish Yehuda
I am publishing my email again, so that readers can ask questions in any field and at any level. If the questioner does not want the question and the answer to be published on the website, this must be stated in the letter.
jonix123@yahoo.com
Yonatan Shanhav