1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Kick-Off Demo - Grade 11 Physics

  1. Sep 9, 2007 #1
    Hello everyone,

    I am in Grade 11 and since the past week was the first week of school, my physics teacher did a few demos to start the new year. He did one "magical" demo and he said that we would find out later the physics behind it. I was wondering if anyone could offer some insight as to how the demo worked; I am eager to know. Thank you!

    My teacher used a copper tube, bought from Canadian Tire (a warehouse/tool shop in Canada), with no hidden holes or anything. He was standing, and he held the copper tube with one hand. With the other bare hand, he dropped a camera battery (or another kind of battery) down the tube, and he took his hand away the tube after he dropped the battery. Since the tube was held vertically, the battery dropped to the ground in 1 to 2 seconds.

    This is the interesting part. Afterwards, he picked up the same battery, and he dropped it the same way. However, this time, the battery was in the tube dangling around (The sound can be heard) for at least 10 to 15 seconds before dropping to the ground.

    Does anyone know how my teacher kept the battery in the tube for a longer time? Thanks again!
    Last edited: Sep 9, 2007
  2. jcsd
  3. Sep 9, 2007 #2


    User Avatar
    Gold Member

    Welcome to PF, Vertciel.
    Your description of the setup is a bit vague. My first thought is that he hid a magnet in his hand.
  4. Sep 9, 2007 #3
    Thanks for the reply and welcome Danger!

    I rewrote the second paragraph and I hope that it is clearer now. If you have any questions, please be sure to ask!
  5. Sep 9, 2007 #4


    User Avatar
    Homework Helper
    Gold Member

    That's what I was thinking. You sure you didn't see a magnet, vertciel?
  6. Sep 9, 2007 #5
    Hmm, I don't recall seeing a magnet. I will ask some other people in my class if they saw one or not.

    If there wasn't a magnet, what could it be?

    Thanks so far for your help!
  7. Sep 9, 2007 #6
    Did the teacher say the thing was a battery?

    If yes, then I have no idea what happened.

    If no, and you assumed it was a battery, your assumption was probably wrong, and there are actually two different objects.

    The first is simply a piece of metal, and fell through the tube unhindered.

    The second is a magnet, and while copper is not magnetic, the motion of the magnet trough it generates eddy currents (http://en.wikipedia.org/wiki/Eddy_current), the fields of which attract the magnet and slow its fall.
  8. Sep 10, 2007 #7


    User Avatar
    Science Advisor
    Homework Helper

    Did he rest the tube on the floor in the second case?
  9. Sep 10, 2007 #8
    My guess: your teacher dropped the battery + or - side up the first time, and - or + side up the second, both activating an electromagnet. The former expelled the ferrous battery, and the latter levitated it somewhat.
  10. Sep 10, 2007 #9
    I remember seeing this exact demonstration. I'm pretty sure that it used a piece of metal and a magnet seperately and the magnet induces a current that opposes the magnet's motion a la lenz's law.

    Edit for the OP's interest: http://en.wikipedia.org/wiki/Lenz's_law
  11. Sep 10, 2007 #10
    Hmm, a battery? A magnet will have the desired effect due to Lenz's Law (see above poster), because of the magnetic field conducting charge flow. To do so with a battery would require a metal and copper wire loops to be involved. Are you sure it was just a battery?
  12. Oct 3, 2007 #11
    Thank you very much for your help!

    @Lateral: The teacher seemed to be pretty honest about the battery being a normal battery, not some special one.
  13. Oct 3, 2007 #12


    User Avatar

    did he hold his thumb over the top end of the tube so that a bit of a vacuum was created in the tube above the battery? what is the inner diameter of the tube and what is the outer diameter of the battery?
  14. Oct 3, 2007 #13
    :rofl: I like that.
  15. Oct 4, 2007 #14


    User Avatar
    Gold Member

    Seconded. Nice approach, Rbj.
    I'll just slink over to my corner now and kick myself for not thinking of that. :biggrin:
  16. Oct 4, 2007 #15
    I'm guessing a flat round battery almost the same diameter as the inside of the tube?
  17. Oct 4, 2007 #16


    User Avatar

    i dunno, maybe the OP has ruled out the thumb-over-the-top-hole trick.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook