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Welcome to Speed Physics.
Introducing the S.I. family of units. There are seven of them in all.
the metre, symbol lower case 'm' the kilogram, symbol lower case 'kg'
the second, symbol lower case 's' the Ampere, symbol capital 'A'
the Kelvin, symbol capital 'K' the mole, symbol 'mol' in lower case
the candela, symbol lower case 'cd'
So what do we know about S.I. ? Well first S.I. is an acronym, which is really
an abbreviation or short-hand for a word or group of words. It often takes the form of
a jumble of letters and sometimes numbers, with each letter representing a word. Though
this is not set in stone.
dvd digtal video disk 3D three dimensional
HST Hubble space telescope Get my drift?
So, what does S.I. stand for? System Internacional d'unites.
Which means the International System of Units.
As you might have guessed, the System Internacional is French.
Why? Because the French started the race to give the world a consistent set of units.
It goes back to the 1790's .
Before that time, in Europe all measurement was in a mess, but especially in France. There
were so many different units, for length and weight. And popular units varied in value
across the country. So if you bought a livre of potatoes in Paris, you might be short changed
if you bought the same amount in Calais.
The same was true of the aune, a unit of length. So buying seven aune of best silk had its
risks.
So King Louis 16th, the French government and the Academy of Sciences got together to
commission a new unified system of measures.
The system was administered by a small group of scientists and mathematicians headed by
Antoine-Laurent Lavoisier. You might have heard of Lavoisier, sometimes
called the Father of Chemistry - he discovered Oxygen and laid the foundation of good chemistry
practice by carefully weighing reactants and products.
The group also included the eminent mathematicians Laplace
and Legendre.
Anyway, the French Revolution happened and with it that particularly bloody episode the
Reign of Terror 1793-4.
Unfortunately King Louis 16th was guillotined in 1793 and Lavoisier soon after in 1794.
By 1799 both the metre and the kilogram had been defined.
As time went by and the news of the unit system spread, other countries got involved. Eventually
an international body was set up - the System Internacional d'unites. It was and still is
based in Paris to oversee developments.
Anyway, in the 19th century things gathered a pace. Contributions were made by many famous
scientists, among them: Carl Freidrich Gauss, James Clerk Maxwell
and William Thomson, later Lord Kelvin.
The original metre and kilogram of Lavoisier's time was extended and altered. The new system
encompassed - the centimetre, the gram and the second. It was called the C.G.S system.
However as time went by, the centimetre and the gram were deemed too small for serious
scientific work. So it was replaced by the M.K.S system, based on the metre, the kilogram
and the second.
Later amendments added to the basic three units to our present day tally of seven units.
And this is what we call the S.I. system.
The seven units are displayed here from the perspective of the quantity they measure:
quantity, length quantity, symbol l
unit, metre unit symbol, m
quantity, mass quantity symbol, m
unit, kilogram unit symbol, kg
quantity, time quantity symbol, t
unit, second unit symbol, s
quantity, electric current quantity symbol, capital I
unit, Ampere unit symbol, capital A
quantity, thermodynamic temperature quantity symbol, capital T
unit, Kelvin unit symbol, capital K
quantity, amount of substance quantity symbol, n
unit, mole unit symbol, mol
quantity, luminous intensity quantity symbol, capital I, subscript capital
V unit, candela
unit symbol, cd
Don't worry if this seems too much to swallow in one go. Each of these units will have its
own video.
Remember, if you subscribe to the channel, you will be among the first to view any release.
Any teacher will tell you, being successful at physics is like building a pyramid. If
you want to reach the top most levels, you must build on firm foundations.
And that means knowing your units.
Subscribe to Speed Physics. Buckle up and get in the fast track.
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