Planck's Constant - Sixty Symbols

that's an unimaginably small number what we have is zero point zero zero three four five six seven eight nine ten eleven twelve thirteen fourteen fifteen sixteen seventeen eighteen 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 663 that's planck's constant and what i'm leaving off are a load of other decimals a lot of other numbers here we've just rounded that that up slightly h conjures up quantum mechanics and it's the constant of planck that tells us that this is the smallest amount of action that we have now action is actually quite a dif difficult concept planck's constant is really it's a measure of quantization it's something that comes up in quantum mechanics and it's to do with the fact that things in in the world when you look on the small enough scale when you look closely enough you find actually they don't vary smoothly they don't vary continuously they take on sort of discrete values and so for example the energy of light given out by atom it doesn't give out light at all frequencies it gives out light at very particular frequencies and those frequencies tend to be very close together but actually there's only particular values of of energy that you can give out and planck's constant is really what dictates how close together those values are planck's constant tells us about how electrons orbit atoms um i've wondered about h and i've wondered about the physical significance of h i've never quite wondered why hey it's why it's called h or not w again i ought to know it's a similar question to see i i um i don't know i don't know by no way it's not called p because p is conventionally used for momentum and one of the key equations relating um momentum and wavelength is this so we couldn't call planck p as well so planck was a very famous german physicist considered by many to be the the sort of godfather the forefather of of um quantum mechanics do you write him oh yes yeah i don't think there's a physicist alive dead or to come that wooden red plank so this is a second year undergraduate experiment that is actually used to measure to work out what planck's constant is we have a mercury lamp here so with this of a lamp which is full filled with mercury gas and we're actually putting electricity through that to generate to excite the atoms excite the electrons in those atoms and those electrons bounce between different states and they give off light and so that's what we can see here is we can see the light coming out from the mercury lamp then we've got a series of of lenses and um slits to really um control how that where that light goes so over here we've got a prism which splits that light up into its constituent colors now unlike sunlight where you know if you take a prism and you split that up you see a broad broad spectrum here what we have are very well defined lines this is in effect a signature or fingerprint of mercury then what we do is we take those spectral lines and have to move this take that we take those spectral lines and what we have here uh it would have been helpful if i set it up can you just you can just edit this can't you yeah yeah so here we've with the prism we've broken up the light into its constituent colors we really want to focus um on one of those colors now and so let's we're going to select the grain we've got a slit here which selects the grain out we've got a lens back here which in turn focuses the light onto this photodiode so we have green light we have photons with a particular energy coming into this photodiode and what's inside this photodiode is a metal the green light comes in it releases electrons and from looking at the energy of those electrons we can work out what planck's constant is that looks like ned kelly that diode do you know who had kellies i guess i never thought quite that sense yeah so h is equal to 6.63 by 10 to the minus 34 and my first year on any undergraduate would kill me if i didn't add the unit so joule second now the reason that it appears small to us is that we are very large objects you know we weigh 80 kilos whereas atoms are something like uh 10 to the minus 27 kilos that's the mass of the proton the electron is even lighter and down at that scale the the units that we choose to express h in uh become a little bit cumbersome and inconvenient so if we were working out these things on atomic scales then h would have a much more convenient convenient and easily more easily memorable value so there's a really um fascinating story called um well about a character called mr tompkins which was written by a guy a physicist of another very important physicist called george gamma back as i believe in the 50s 40s or 50s and he mr tompkins is a bank manager who goes along to various different talks by leading physicists and scientists of the time and he sits through the lectures on invariably halfway through the lectures he falls asleep and he has a dream and the dream is usually related to the topic of the lecture and one of those very popular mr tompkins stories relates to what would happen if h wasn't this imaginably small number what if it was larger and if it were larger and it were large enough so that we could see the this quantum phenomena in the in the real world and that would mean that instead of being sort of solid objects we would have wave like characteristics so we would spread out if i were to for example to walk through the door because i'm now of wave like characteristics i would diffract and so bits of me would go in different directions

Channel: Sixty Symbols

Views: 691,757

Keywords: plancks, constant, sixty, symbols

Id: Tz2D1aahd5A

Length: 6min 16sec (376 seconds)

Published: Fri Apr 24 2009