Tuesday, August 3, 2010

Web Trawling for Physics Fun

So I’ve been searching the net for interesting discussion topics since it feels like we’ve already maxed out our first chapter. After much surfing I found a website that had some interesting challenges and thought experiments.


I’ve picked a few that relate somewhat to our recent chapter and thought it would be fun to see what everyone thinks.

1. Can One Boil Water With Boiling Water?
A pot of water is brought to a steady boil on a stove and then a thin plastic cup of room temperature water is suspended in the boiling water so that no part of the cup touches the pot (see above figure). Will the water in the cup start to boil if you wait long enough?
My thinking is Yes. The heat would transfer through the plastic cup as it would have a higher heat capacity of water anyway.

2. Does a neutrally buoyant balloon rise or fall as the temperature increases?
Consider a balloon filled with helium gas and then weighted so that it remains motionless in the center of a sealed box of air at room temperature and atmospheric pressure. If the box is slowly and uniformly warmed so that the temperature everywhere inside increases by a small amount, determine whether the balloon will rise, fall, or remain in the same place.

As a guess the balloon would remain in the same place. Although the increase of thermal energy of the gas would cause the number of collisions to increase the collisions would be happening at all sides of the balloon generating no net movement.

3. Equilibration of Two Birthday Balloons
Consider two identical spherical birthday balloons, one of which is inflated to 2/3 its maximum diameter and the other inflated to 1/3 its maximum diameter. What happens when the openings of the two balloons are connected to each other by a straw so that air can flow back and forth between the two balloons?

The easy answer would be that they equilibrate and stop there and I’m thinking the elasticity of the 2/3 balloon would cause the air to move from that one to the 1/3 balloon but that it would continue to ‘donate’ its air to the 1/3 balloon until it was empty. My basis for that is the initial rush of air would cause the 1/3 balloon to expand more than required and create a vacuum which in turn sucks in more air. Maybe...

I’m probably wrong on all three accounts, but let me know what you think.

Also I was looking at TED.com and found a video of Richard Feynman (won the Nobel Prize for particle physics and quantum mechanics) discussing how the ‘jiggling’ of atoms can explain many things. Check it out:


  1. 1) I too would consider that the water in the cup could boil. I suppose it depends though on exactly what temperature the water is at in the pot (are we assuming 100 degrees celcius?), how much water is in this particular pot, how large the cup is, how much water is in the cup, and the exact specific heat capacity of the cup. Again though, it is possible.

    2) Your answer would be right I believe provided that your assumption is correct. I think that if the box was heated from below we might see some rise in the balloon due to the rising of the heat initially.

    3) With the balloons, yes there would be an initial rush of air from the large balloon to the small balloon, but then the same would happen in the reverse direction and keep doing so until the equilibrium is found with both balloons at half size. I say this because the balloons themselves are elastic, and so will be trying to also reach the point of lowest energy (least stretched) and so will keep compressing their air inside them until they are both at equal size.

  2. Nice! This is exactly the sort of thing that this course structure is designed to encourage.