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B Lenz's law questions from a dad of a smart son

  1. Nov 3, 2016 #1
    if this is in the wrong place, i apologize.

    you guys helped me out before so i was able to not sound like an idiot to my son and helped us share time and interests together

    this year in 6th grade he did a great science fair project on the duality of light and did the double slit experiment and he built an electroscope and did the photoelectric effect experiment.. it all came out very nice..

    he is already started talking about next years fair and he is very intrigued by lenzs law the magnet through the copper pipe.. i've been looking for books for him that would be good ..any direction there actually would be welcome?

    but also as cool as it is.. how can he work it in to a science fair project what would he do and show? there has to be more than just a magnet through a pipe .. and what practical applications would it have? any direction advice or ideas of places online to check out for ideas or other things to go with it
  2. jcsd
  3. Nov 3, 2016 #2


    Staff: Mentor

    A magnet through a pipe can be plenty for a science fair project. Like, actually measure how much time it takes using different pipes or different magnets. Have him make real hypotheses about how different pipe lengths, diameters, materials will change the time. Something like that can really teach the essence of science.
  4. Nov 3, 2016 #3
    To zero in a bit, focus in wall thickness of pipe. Making a speedometer in the pipe will be good achievable project beyond a stopwatch method.

    I guess you could use different grade of metal in the pipes, sourcing that might be hard to do tho.

    I would keep pipe length and outer diameter constant....or make inner diameter constant and vary outer diameter, or.......
  5. Nov 3, 2016 #4
    thank you both so much.. he said something about wanting to do other experiments with it to show something about electromagnetism.. it was late last night he woke me up to tell me.. and this is not in my sphere of knowledge .. but he wants me to be involved with him in his passions and i want to be involved as you can well imagine..mentioned faradays law too but it was really lenz and the magnet through the pipe he was really on about.. do they go together?

    what other stuff would go with it?.. and what could he maybe be trying to show with different combinations of different experiments?..or i guess i should say what are the possibilities? i'm trying to learn but i'm old and my job keeps me a bit busy and a lot wore out

    on an aside i looked up thick wall copper pipes ..like i saw in some videos.. i love my son but those really thick ones are just out of my price range wow.
  6. Nov 3, 2016 #5
    Less expensive you could focus on measuring the speed and just use one pipe. Then use the data to model the motion. You could even add mass to the magnet with non magnetic materials.
  7. Nov 3, 2016 #6


    Staff: Mentor

    Yes, they definitely go together. Lenz's law basically just explains the negative sign in Faraday's law.

    Good point, you should be able to do a good project with reasonable price materials. If changing thickness is cost prohibitive then do some other variable, like material. Maybe measure the resistance of different materials and have him make his hypothesis based on resistance.
  8. Nov 4, 2016 #7
    thanks again

    i really need to find stuff in laymans terms sometimes..to be honest i had to drop out of school pretty young to help support my family .. i just do physical labor.. i just have jobs.. he can do better than jobs he can have a career.. do things he likes rather than just things to pay the bills ya know

    he's smart top of his class.. i wanna do everything to help that i can but he's smarter than me.. i'm sure he knows that but he likes us working together and sees me trying

    so anyway he told me he wanted to do things to show magnetism and electricity were two aspects of the same thing (his words) show a magnet passing through coiled wires making electric.. the magnet in the pipe .. and make some homopoler motors? anyway it sounds fun and interesting and i already know how cool looking the magnet in the pipe looks

    he really likes physics i mean really really.. i mean since kidergarden he's been telling me he wants to be a theoretical physicist

    oh i did find some pipe i could afford like in the 20 30 dollar range.. the first ones i saw were very very thick and in the many hundred dollar range.. but if i get him a 3/4 inch magnet ball .. what size pipe would i get 1 inch inside diameter? I could surprise him with this stuff for Christmas he would love it
  9. Nov 4, 2016 #8


    Staff: Mentor

    I don't have any practical experience with this, so hopefully another forum regular can make suggestions. The magnetic field of a dipole magnet falls off quite sharply, so you would want a pipe with an inner diameter just enough larger than the magnet that it can fall freely.
  10. Nov 4, 2016 #9


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    Pipes / tubes are readily (?) available in copper, s/steel and galvanised steel in various bores. These days, you can get very strong magnets for not much money.
    Also, it would not be hard to make a long slot down the length of a tube (Dremmel?) and note the effect when eddy currents are suppressed.
    If this is a School based exercise then it may be possible to borrow some data logging equipment for making and recording the drop times. Failing that, times could be measured and displayed on an oscilloscope. (Also probably available from School)
  11. Nov 4, 2016 #10
    Cheaper than pipe, aluminium window frame, its rectangular. I get free scraps from a dumpster outside a window and door frame shop.
  12. Nov 4, 2016 #11
    thanks everyone there is a lot to think about here

    can't tell you how much i appreciate it
  13. Nov 5, 2016 #12
    And he could compare the speed of the magnet in a Copper pipe with the same magnet in a plastic pipe (those are very low cost).
  14. Nov 5, 2016 #13

    Charles Link

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    @hans broden I have a suggestion that your son do an experiment with magnetic surface currents. For the case of cylindrical uniformly magnetized permanent magnets, the magnetic surface currents that cause the magnetic fields of a permanent magnet have the same geometry as the magnetic field of a current carrying solenoid (with a D.C current) without any iron core. Thereby, you could do experiments to try to estimate the surface current levels of a permanent magnet. (The magnetic surface current levels are actually so high that you will probably only be able to create magnetic fields with a solenoid that are about 1/10 as strong as a good cylindrical magnet of the same geometry.) For a link to the topic of magnetic surface currents see https://www.physicsforums.com/threads/magnetic-field-of-a-ferromagnetic-cylinder.863066/ This one happens to be a rectangular (rather than round) cylinder, but the mathematics is nearly identical.
  15. Nov 5, 2016 #14

    Charles Link

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    And a follow-on to post #13=If you do go this route, it is somewhat easy to construct a solenoid. You need insulated wire that can handle 1 ampere or thereabouts=a medium gauge would be good. The insulation doesn't need to be very thick. You should be able to wind about 10 turns per centimeter which would give ## n=N/L ## around 1000 turns/meter. (You can wind it around a thin plastic tube, etc.) A 12 volt DC power supply that can deliver I=1 ampere would probably work. The resistance of the wire is quite low so you would need a resistor in series ## R=12 \, \Omega ## or thereabouts to get ## I=1 \, ampere ##. The resistor would need to be rated at 12 watts or more. Instead of purchasing an expensive resistor to do this, the bulb for an automobile headlight would probably work ok for this application. A solenoid has surface currents and magnetic fields that makes it geometrically equivalent to a cylindrical permanent magnet. As mentioned in post #13, the surface current (per unit length) and the magnetic fields of a good permanent magnet are likely to be much stronger than the field of a solenoid. A quick calculation (using magnetization ## M=1.0 ## (SI units), very typical of a good permanent magnet) shows that surface current per unit length ## K_m=M/\mu_o=1.0 E+6 ## (approximately), ## \mu_o=4 \pi E-7 \, ## , so that the magnetic field from the permanent magnet may be 1000x stronger than that of the solenoid... ## \\ ## An alternative to this would be to put an iron core (non-permanent magnet type material) inside the solenoid to enhance the magnetic field of the solenoid. This could also be a second add-on experiment. Even with the magnetized iron core, it is the magnetic surface currents from the material that are responsible for the increased magnetic field strength. The enhancement of the magnetic field from the solenoid with an iron core could be as much as 100x or even 1000x. editing... Some quantified results of experiments with these electromagnets (solenoids with iron cores) could also make a very interesting project.
    Last edited: Nov 5, 2016
  16. Nov 5, 2016 #15

    thank you very much

    i'm running out the door right now gotta go do some work..

    i really really appreciate this and need to really look over it all good later on
  17. Nov 5, 2016 #16
    Charles what do you propose the student actually measure and experiment with, what data will they collect?

    it appears from my reading you are solving an interesting problem with theory but not doing an actual experiment.

    at best you will get a couple of data points to verify a theoretical calculation.

    what am i missing??
  18. Nov 5, 2016 #17

    Charles Link

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    The OP will need to build a solenoid with the same dimensions as a permanent cylindrical magnet, and will need to do some quantitative measurements (comparisons) of the magnetic field strengths of both. The precise details of how to measure the strength of the magnetic field (mapping out the field strength and direction) will need to be determined. Ultimately, the student should be able to determine the values (at least good estimates) of the magnetization ## M ## and the surface current per unit length ## K_m ## of the permanent magnet. editing...There are meters that measure magnetic field strengths. I don't know how affordable a good one might be, but no doubt some sort of homemade version with something like a compass might work reasonably well.
    Last edited: Nov 5, 2016
  19. Nov 5, 2016 #18
    anything that can measure magnetic field strength quantitatively will not be cheap, especially when metal pipe was out of the OP's price range. might be possible to borrow one though.

    a compass will be pretty bulky to get in to the dimensions of a permanent magnet and measuring the deflection of the needle will error prone.

    I really like your analysis just thinking within the scope of the OP.

    what I was most curious about your methodology though is sure a solenoid is built and the result compared to a permanent magnet.

    you have two data points to discuss - no graphs, trends, variables......just build and compare two numbers.
  20. Nov 5, 2016 #19

    Charles Link

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    The experiment could actually become quite detailed: Mapping the magnetic field of a permanent magnet as well as that of a solenoid. What I supplied so far was just the initial idea that could generate something quite elaborate depending upon the level of the student and the available time.
  21. Nov 5, 2016 #20
    I like your proposal and agree it could make a great investigation, I was more concerned about the sixth grader in the OP.
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