Science Q&A
Nothing is more exciting than getting questions from listeners. This week, Ask Science shares 3 of the more interesting queries he’s received from science fans lately.
Lee Falin, PhD
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Science Q&A
Nothing is more exciting than getting questions from listeners. While I generally respond directly to the questioner, this week I wanted to share a few of the more interesting questions I’ve received from my fellow science fans:.
Question #1: Water on the Windshield
Our first question comes from Jess:Â
“It’s been raining a lot where I live lately. The rain has been off and on, alternating between light sprinkles and torrential downpours. So the other day my girlfriend and I were sitting in my car waiting for the rain to let up a bit before making a break for it and running inside. While waiting, I was looking at the rain fall on my windshield and observed something. Even with my windshield wipers off, the areas of the windshield outside the range of the windshield wipers would coalesce into droplets on the window, while on the area where the windshield wipers had wiped when they were on, the water didn’t coalesce into droplets – wit just ran off in a sheet.
To my knowledge, the windshield hasn’t been treated with anything – especially in any way differentiating the areas of the windshield. So, my question is: Why doesn’t the rain coalesce into droplets uniformly across the whole windshield after using the wipers?”
Ask Science: Generally, when water starts forming droplets like that, it’s because it’s sitting on some kind of hydrophobic surface which is preventing it from making hydrogen bonds. Hydrogen bonding is what gives water many of its special qualities, such as being able to form a liquid at room temperature.
Most wiper blades are made of rubber, and a few are coated with teflon. Both rubber and teflon are hydrophobic, so most likely the wipers are leaving a residue of rubber and/or teflon across the area of the windshield where they pass. This results in that portion of the window being partially hydrophobic, which causes the water to bead up, since it can’t spread out very well.
More details on hydrophobic (and its opposite: hydrophilic) can be found Click here for more information.
 The astute reader will notice a flaw in this response. For more details along with the corrected response, see this episode.
Question #2: The Sun’s Fuel
Robert asks:
“I know this sounds like a stupid question, but where does the Sun store its fuel to sustain the fusion process if the fuel is mixed in with all the explosions? When I hear that the Sun has enough fuel to continue for another 4 or 5 billion years, I picture a partition in which the fuel is slowly metered out. I have watched dozens of shows about the Sun and none of them seemsto address where the fuel is stored. Please enlighten me on this.”
Ask Science:Â That’s actually a great question, Robert! The hydrogen fuel actually sits at the core of the star. The hydrogen atoms bounce around in there until they slam against each other with enough force to cause fusion, turning the hydrogen into helium. Energy is released in this process and travels outwards in the form of light, heat, and other assorted rays (x-rays, gamma rays, etc.)
However the gravity of the core, combined with the pressure of the outer layers of the sun pushing down on the core, cause almost all of the hydrogen and helium to stay in the core.
Assuming the Sun follows the same route as similar stars, eventually, the core will start to run out of hydrogen. With fewer collisions resulting in less energy being produced, the pressure inside the core will decrease, allowing the outer layers to compress. There are a few other things that happen at this point, but eventually the collapsing outer layers cause the pressure and temperature of the core to increase enough that the Sun will start using helium for fusion, creating heavier elements like carbon.
This process repeats a couple of times, resulting in heavier and heavier elements being created in the core, until the star eventually dies. Here’s a great article that explains it in more detail.
And if you prefer video, check out The Life and Death of a Star documentary.
Question #3: Pucker Up
Finally, Adam wrote in to ask the following:Â
“Dr. Falin, I have always wondered about this: Â Why is it that the air appears to be cold when you blow with your lips puckered but it appears to be warm when you blow with your mouth only slightly more open?”
Ask Science: It’s funny, I actually had never noticed that until you mentioned it, Adam. After some highly scientific research and a few experiments involving repeated hyperventilating and some drinking straws, I think the answer is that either way you exhale (puckering or otherwise), the air coming out of your mouth is in fact the same temperature. However when the faster air comes out of your mouth via puckering, its higher velocity allows it to sweep in the surrounding air along its way. Since the surrounding air is (usually) cooler than your internal body temperature, the air feels cooler.
Here are a couple of experiments you can try to see this in action:
- Do the pucker/wide mouth comparison again, but hold your finger as close to your lips as possible. This minimizes the amount of outside air that can be swept up by the higher velocity of puckered breath. You should notice that the temperature is significantly warmer than when you do the same thing with your finger far away.
- Try blowing through a drinking straw, again with your finger close to the straw’s opening. Even though you’re still using higher velocity air, the breath should feel warm.
This is a hydrodynamic phenomenon, (meaning it involves the forces of liquids pushing on each other), called entrainment. When one fluid (be it gas or water) sweeps another fluid along in its flow, we say the first fluid has entrained the second one. If you decide to research this further, I’ll warn you that nearly every branch of science uses the word “entrainment” slightly differently.
Conclusion
ConclusionThat’s all the questions I have time for this week. If you have a question you’d like to see answered in a future episode, send it to me via email at everydayeinstein@quickanddirtytips.comcreate new email, or send it to me via Twitter or Facebook.
Sun and blowing cold air images courtesy of Shutterstock.
