Yeah.. we are done
Do it again... look at the extra guy who jumped earlier..
Jump!
That's all folks for now.
I think there's a lot of history and interesting facts with regards to SI units and it will be a shame if we just study the chapter on measurement without reading some of these facts.
Fun Fact 1: There are only 6 base SI units instead of 7.
From year 3 or year 5 Physics lessons we would have know that there are 7 base SI units known as m, kg, s, K, mol, A, Cd. But Candela is strictly not a base unit as we can express it in terms of other SI units. The current definition for
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.So Candela is expressed in terms of watt per steradian. Watt is the unit for Power and Power and that, as we know, can be expressed as kg m^2 s^-3 so it is not a base unit. What about steradian? What kind of unit is that? We shall come to that later.
Then why is it in the list of 7? Because of historical reasons which I shall explain later
Fun Fact 2: History of SI units
To talk about this, I shall quote the famous lower secondary science story about units. In the past, people use their feet and arm as a unit of measurement. Obviously there were many disagreement on what is the length of a feet or arm. Fast forward to a couple of centuries later, Physics were in its infancy stage (hence candela got voted in), there is a great importance to standardise the way we measure things in order to let science develop further. So people start to sit together in committees to decide on the International Standard units.
Why not call it IS units? Because at that point of time, the French were a lot more influential than the British hence the language of science is French instead of English. So SI is the abbreviation for International Standard in French.
Read more here
Fun Fact 3: How do we define SI units
Today, we know a lot more about Physics than 100 years ago. So we could be a lot more precise and accurate when we define our SI units. Personally, I believe that if we could define our SI units so precisely such that an Alien from Alpha Centauri will be able to understand our units of measurement exactly, then we would have succeed in our mission to standardise units. Anyway, here's how the rest of the 6 SI base units are defined.
Meter - The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.See here for more details
Second - The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.
Ampere - The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10^-7 newton per meter of length.
Kelvin - The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
Mole - The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12
Kilogram - The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
Fun Fact 4: Why kilogram's definition is a prototype?
The fact is, most definitions started off as a prototype. Like a prototype meter etc placed somewhere while all other copies were made with reference to this prototype. But scientists are not happy with this current definition and sooner or later, it would be replaced with something more precise and physical in its definition.
Fun Fact 5: Is plane angle and solid angle an example of a unit that cannot be expressed using base SI units?
Unfortunately, no. You see, the purpose of calling them base units is that everything else can be expressed as those units, so if you can find a physical unit that cannot be expressed using the 6 base units, then we will have 7 base units instead. The fact is, radians or steradians (or degrees if you like) is actually a ratio. The definition of a radian or steradian is by ratio of lengths, well described in Wikipedia here and here. So the unit for radian is m/m = 1, a dimensionless unit (like all ratios)
If we look back at the Candela case, we can always define it using a complete sphere instead hence avoiding steradians altogether (the total brightness of the light source around all directions instead of a particular direction). To make matters worse, the ratio for these plane and solid angles rely on a fundamental assumption that we live in a Euclidean space where parallel lines do not meet. Einstein already proved that our Universe is curved and the angles of a triangle do not add up to 180 degrees in such a world (so naturally Euclid was wrong about parallel lines doesn't meet and other 'anomalies' in the real universe).
The reason why we keep units for these dimensionless quantities is so that we can remind ourselves that these dimensionless quantities mean something to us. That it is actually a ratio of 2 quantities that we have a common understanding of. There are other dimensionless units around our Physical worlds besides radians and steradians. If there is a chance, I might be able to share a bit more in class.
-----------------------------------
So, units and measurement can be very interesting. It is not just only about getting the units right and putting spaces between your units. We could catch a glimpse of the development of Physics by looking at Base SI units and its definition. The more we know about the universe, the better we can describe it precisely and accurately.
Have you read about the official kilogram losing it's mass? We need a new definition for the kilogram fast or we will all gain weight if the official kilogram is lighter!
Guess who we meet on the way to the next station...
McChicken with clues...
The whole class was there at Lau Pa Sat... should have taken a class photo there...
And have our lunch dinner there too
Waiting to cross the road safely (except for the photographer)
Nothing better to do than to take photos of students crossing road...
At the next station...
All of them survived the smelly bridge
Collecting water from the sea for the games later...
Should have recycled it once more as 1 side is still plain white...
More portraits, forgot to brighten up this pic..
The A700 exposure is more consistent and dependable compared to the A100 which at times tend to underexpose a bit using multisegment metering.
Ah well... NUS term started today, class starts tomorrow... means night lessons once again... JIA YOU!
Guess what, the map that they got from the tourist was in German!
Finally made it to the place
And the task is elastic collision of similar spheres, aka pool
The new rules for the game
Mr Expert coaches Ms Innocent
Nice pose..
Don't know what the term means but here's some random pics...
Orange for House A, Orange for Alpha, Alpha for A... cool...
Paper crane folding
On the streets
Through the tunnel
To take the MRT
And of course, PJ again...
W & G of J4M 07
Ok, I'm asked to take photos of other houses beside my own house.. so here goes
Red house
Subscribe to:
Posts (Atom)
Posts
