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Course: Physics library>Unit 18

Lesson 3: Measuring magnetic fields

Inverse cube law (method of oscillation)

Here we compare the speed of compass oscillation vs. distance from a magnet. What kind of relationship do we observe. Is it linear? Created by Brit Cruise.

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• When Brit counts, he depresses the counter until the click. While depressing the counter increment button, Brit brings the steel closer to the compass; after the click, he retreats. Does the proximity of the steel tally/click counter significantly influence the magnet?
• Interesting question! I would say no as the steel counter wasn't magnetized. It's possible it made a minute difference, though it would be smaller than the margin of error involved in my hand-eye coordination. This is indeed a very *rough* experiment, similar to a sketch on a piece of paper.

I'd love to see an advanced version of this experiment with 100's of data points. I'd post the video here if someone wants to make it!
• What is that thing in brits hand?
(1 vote)
• Hi Tanmay,

He is holding a mechanical counter. At the peak of every oscillation he pushes the button which advances the count.

He could have counted in his head but the counter makes it easier to see...

Regards,

APD
• Clearly the swings per second is not linear with distance and wouldn't it be that when the magnet doesn't really affect the compass that the swings per second would basically be 0 and that once the magnet starts affecting it that the number of swings per second goes up in a parabolic way and thus swings per second = distance * y * x^2 +/- z?
y = coefficient of x^2
x = square root of the place on the parabola -/+ +/- z(notice how the minus-plus cancels with the plus minus and so you are basically if z is negative adding the absolute value of z and if z is positive subtracting z)
z = the shift of the parabola from f(x) = x^2
• so every time the magnet gets close to the thing idk whats the name of it it spins more why ?
(1 vote)
• Yes. Because the magnetic field is stronger the closer you are from the magnet
• it is cool of how fast you have to do it
(1 vote)
• what is the speed in this circumstance?
(1 vote)
• Will the amount of times he has to click the machine change if there is a bigger magnet?
(1 vote)
• Yeah since it has a greater magnetic field for each point, which means the compass is distubed longer for each point (more oscillations)
(1 vote)
• what happened at the beginning?
(1 vote)
• He had got a disturb compass to see how many times did he have to click to prevent compass to completely stop at the magnets side
(1 vote)
• how does he make the compass spin?
(1 vote)
• If you got the compass to swing at precisely one swing per second, couldn't you use the swinging part of the compass as a pendulum for a clock?
(1 vote)

Video transcript

- [Man] Breathe! (counter clicking at a slow tempo) (pencil thudding the table) (gears cranking) (counter clicking rhythmically slightly faster) (pencil scratching) (counter clicking at a moderate tempo) (counter clicking rapidly) (counter clicking more rapidly) (pencil scraping) (thudding) (pencil scraping)