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Cracks, holes, and hot air

August 19th, 2013 · 31 comments

Writes our submitter in Philadelphia: “We do not have air-conditioned hallways in my building, so there’s been an ongoing debate on my floor about how open the hallway windows need to be for maximum airflow.” Suddenly…SCIENCE!

Dear Neighbor - MORE air comes into the building when the window is cracked slightly. NOT fully open. Its simple physics. Love your neighbor.   Simple Physics? Please elaborate.   Smaller Hole = Greater vac suction See: Pressure diff hot air formula (?)  Please be sure to calculate ?P in ALL orifice conditions in your fluid model

related: A/C, windows, and Kelvin’s law of thermodynamics

FILED UNDER: It's science! · neighbors · Philadelphia · signed with love · temperature

31 responses so far ↓

  • #1   Red Delicious

    Call Mythbusters, done deal.

    Aug 19, 2013 at 5:45 pm   rating: 29  small thumbs up

    • #1.1   dmf

      Or, like many things on Mythbusters, you could just call an existing equation.

      Aug 20, 2013 at 7:19 am   rating: 11  small thumbs up

    • #1.2   The Elf

      And miss out on the very sexy Jamie Hyneman demonstrating the principle using some sort of explosive? Perish the thought!

      Aug 20, 2013 at 7:45 am   rating: 34  small thumbs up

    • #1.3   RedDelicious

      Quite, Elf. Quite.

      Aug 27, 2013 at 1:29 am   rating: 0  small thumbs up

  • #2   Ely North

    The only part of this that interests me is “all orifice conditions in your fluid model.”

    Aug 19, 2013 at 6:27 pm   rating: 40  small thumbs up

    • #2.1   The Elf

      It’s one of those things that sounds very, very dirty but is in fact completely not. Oh, baby, talk nerdy to me.

      Aug 20, 2013 at 6:08 am   rating: 17  small thumbs up

    • #2.2   The Elf

      Also sounds a little too dirty: “Smaller hole = greater vac suction”. I’m gonna have to trust you on that.

      Aug 21, 2013 at 6:28 am   rating: 3  small thumbs up

  • #3   Neeners

    “Simple” and “physics” should never be used in the same sentence.

    Aug 19, 2013 at 6:41 pm   rating: 40  small thumbs up

    • #3.1   Neeners

      I think it’s sweet how there is promotion of “love thy neighbor” in the notes too! I bet he wears a sweater and changes into sneakers after writing this note.

      Mr. Rogers would approve.

      “Won’t you please, won’t you please, please won’t you be….my neighbor?”

      Aug 19, 2013 at 7:02 pm   rating: 8  small thumbs up

    • #3.2   Jessi

      @Neeners, I work with a girl (20 years old) who has no idea who Mr. Rogers is. Can you believe that?

      Aug 20, 2013 at 1:48 am   rating: 10  small thumbs up

    • #3.3   Neeners

      Blasphemy, you should get that show on dvd and school her on what she has missed.

      Aug 20, 2013 at 5:06 pm   rating: 6  small thumbs up

  • #4   misscris

    Well, actually Neeners – there is a comma between love and your neighbour – so they mean love from your neighbour (sorry for being a pedant! lol)

    Aug 19, 2013 at 7:41 pm   rating: 11  small thumbs up

    • #4.1   Neeners

      That takes his note to a whole other level.

      Aug 20, 2013 at 5:06 pm   rating: 2  small thumbs up

  • #5   Kacey Hammer

    Reminds me of something my mom said before she passed away. During our visits, we would hang on every word she said, trying to soak up all her wisdom. The conversation turned to cooking. My mom said, “you know why water boils when you add salt, right? It’s just science.” At that she rolled her wheelchair away to emphasize the wisdom bomb she just dropped. OK then.

    Aug 19, 2013 at 8:05 pm   rating: 43  small thumbs up

    • #5.1   dmf

      I’m sure your mom was nice, but that made no sense.

      Aug 20, 2013 at 7:16 am   rating: 5  small thumbs up

    • #5.2   Lil'

      Thanks for making me choke on my breakfast, Kacey. That is hilarious.

      Aug 20, 2013 at 7:30 am   rating: 13  small thumbs up

    • #5.3   Marty

      Kacey, she was correct! It is SCIENCE! “It takes less energy to raise the temperature of the salt water 1°C than pure water.” You can read the science behind that here:

      She was one smart cookie. :D

      Aug 20, 2013 at 12:11 pm   rating: 3  small thumbs up

    • #5.4   Chinchillazilla

      Technically your mom was right. Adding small particles of anything to water will make it boil faster when heated, although I don’t remember why. I failed chemistry.

      Aug 21, 2013 at 9:04 pm   rating: 1  small thumbs up

    • #5.5   redheadwglasses

      dmf, salt water alters the boiling temp of water (raises it), but not by enough to make any difference in cooking.

      But the story cracked me nevertheless. : )

      Aug 21, 2013 at 9:05 pm   rating: 2  small thumbs up

  • #6   AP

    Not as surprising as you’d think, non-science people generally don’t have graph paper lying around for random notes.

    Aug 19, 2013 at 10:40 pm   rating: 8  small thumbs up

  • #7   Dr. Sheldon Cooper

    The model only works for spherical chickens in a vacuum, I’ve heard.

    Aug 20, 2013 at 9:40 am   rating: 23  small thumbs up

    • #7.1   FeRD bang

      Frictionless spherical cow (or chicken) with zero mass, in a vacuum.

      Mustn’t forget to handwave away all that messy weight(/mass) and friction, or the math just gets ugly. (Not to mention, vaguely applicable to reality. Can’t have that! There’s a reason it’s called Theoretical Physics.)

      …Practical Physics, the kind that troubles with fiddly things like friction and pressure and gravity and mass, we refer to as “Engineering”.

      Aug 20, 2013 at 10:28 pm   rating: 6  small thumbs up

  • #8   wait..what?

    And here is your science.

    The equation for boiling point elevation is dT = KbM where dT is the temperature change, Kb is the ebullioscopic constant, and M is Molality of the solution. The Molality is equal to the moles of solute (salt) divided by the kilograms of solvent (water). By increasing the amount of moles (amount) of salt, you increase the temperature change.

    The Kb of water is 0.512. To raise the boiling point of 1 liter of water (1kg) by 2oC, you would have to add nearly 230 grams of table salt. The amount of salt you add when cooking (such as pasta) doesn’t really change the temperature by much at all, its more for flavor.

    Boiled down, it doesn’t make water boil faster. It increases the temperature of the water but not by much.

    Aug 20, 2013 at 9:58 am   rating: 9  small thumbs up

    • #8.1   Singedrac

      Maybe she was talking about nucleation points.

      Aug 20, 2013 at 10:09 am   rating: 4  small thumbs up

    • #8.2   Nikki

      What you do need to know is that if you put the salt in before the water is boiling, it can pit your pot, depending on what material it is made of. True story. Also science!

      Aug 20, 2013 at 11:37 am   rating: 3  small thumbs up

  • #9   Raichu

    Or, you know, just get a fan for your own apartment…

    Aug 20, 2013 at 11:20 am   rating: 4  small thumbs up

  • #10   spacenomyous

    trojan man!!!

    Aug 20, 2013 at 2:34 pm   rating: 2  small thumbs up

    • #10.1   Nony

      More like, Staedtler Man.

      Aug 20, 2013 at 6:37 pm   rating: 0  small thumbs up

  • #11   Poltergeist

    Is there science I can use decipher what the hell that word is after “Hot Air”?

    Aug 20, 2013 at 5:28 pm   rating: 3  small thumbs up

    • #11.1   No

      I think that it says “rises”

      Aug 20, 2013 at 7:54 pm   rating: 2  small thumbs up

  • #12   Tesselara

    Passive-Aggressive Physics. Best. Thing. Ever.

    Aug 20, 2013 at 6:06 pm   rating: 9  small thumbs up


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