Voiceover: All bodies and systems possess
a property called temperature.
Most commonly temperature is used to refer
to how hot or cold something is,
but the real sciency definition of temperature
is that it's a measure of the average kinetic
energy of the particles in a system.
I've got a system and I'm filling it
with little individual particles.
If we think about this microscopically
each little particle in the system
is moving in some way whether in rotation
or in a straight line, or curving,
or by kind of a combination of these means.
All of these little particles are moving.
The energy of motion is called the kinetic energy.
All of these moving particles have kinetic energy.
The faster those little particles are moving,
the greater their kinetic energy.
If each of those little particles in the system
has greater kinetic energy,
that means the system as a whole
has a larger amount of total energy,
and we would say that it has a greater temperature
because, again, temperature is a measure
of the average kinetic energy of those particles.
Because knowing the amount of energy in a system
can be really useful in chemistry and in physics,
we've developed temperature scales
to help us quantify or measure the amount of
this value, this value of energy.
The three scales most widely used are
the Kelvin scale, the Celsius scale,
and the Fahrenheit scale.
For all of these scales I'm going to draw
a little thermometer,
one for Kelvin.
Then we have a thermometer for Celsius,
and then another thermometer for the Fahrenheit scale.
The two scales used most in the physical sciences
are probably the Celsius scale and the Kelvin scale.
As a point of comparison here on these thermometers,
the freezing point of water occurs
at zero degrees Celsius.
We have zero degrees Celsius.
That's where water freezes.
Then the boiling point of water occurs
at 100 degrees Celsius.
So the boiling point of water occurs
at 100 degrees Celsius.
That's where water turns into steam.
I'm going to write H20 here real quick
just so we don't get confused
that we're talking about the freezing
and boiling point of water.
Now when we use the Kelvin scale,
we find that water's freezing point
is 273.15 Kelvin.
Then we find that water boils at 373.15 Kelvin.
They differ fundamentally in the zero point.
The Celsius and Kelvin scales differ
in the zero points that they use,
but between water's freezing point,
and water's boiling point,
we have a span of 100 temperature units
for both scales.
So although they differ in the zero points that they use.
They use the same size unit,
or the same magnitude of unit
to measure the temperature.
Converting then between the two scales
only really requires that we make an adjustment
for the two different zero points.
This is what I mean.
If we want to know the temperature in Kelvin,
all we need to do is take the temperature
in Celsius and add 273.15 degree units to it.
If we want to know the temperature in Kelvin
for the freezing point of water,
we take the temperature in Celsius
which would be zero,
and we add 273.15 units to it,
and that would give us 273.15 Kelvin.
Now if we want to flip that,
and if we want to find the temperature in Celsius
from Kelvin,
all we have to do is take the Kelvin figure
and subtract 273.15 to it,
or subtract 273.15 from it, excuse me.
We would see that 373.15 Kelvin minus 273.15
would give us 100 degrees Celsius.
Just as another example, let's convert
300 Kelvin to Celsius.
To start, since we're looking for Celsius,
we'll take that Kelvin value,
and we'll subtract 273.15 from it.
That's going to give us 26.85 Celsius.
So 26.85 degrees Celsius is the same thing
as 300 Kelvin.
I just want to point out really quickly
that I'm only using the degree symbol here
for Celsius,
and I'm doing that intentionally.
We don't need this symbol with Kelvin scale
because instead of calling the temperature units degrees,
we just call them Kelvin.
The only thing we need is an uppercase K.
Now converting between the Celsius
and Fahrenheit scales is a little bit more complicated.
You see in Fahrenheit, water freezes
at 32 degrees Fahrenheit,
and water boils at 212 degrees Fahrenheit.
This give us a span between the freezing point
and the boiling point of water of 180 degree units.
We're going to need to consider two
different adjustments here;
one for degree size because the units
have a different magnitude,
and the same value, or the same span
of temperature is 100 units in Celsius
and 180 units in Fahrenheit.
We're also going to need to account for
the two different zero points,
zero degrees Celsius for freezing,
and 32 degrees Fahrenheit for the freezing
point of water.
First we can say that 180 degrees Fahrenheit
is equal to 100 degrees Celsius.
Again, we can say this because both of these
magnitudes refer to the same change in total energy.
If we write this as a ratio,
we have 180 over 100 which just reduces down
to nine over five.
So the ratio of Fahrenheit to Celsius
is nine to five.
Now we need to think about the two different
zero points.
Because 32 degrees Fahrenheit is equal
to zero degrees Celsius,
we can find the Celsius temperature
if we take the temperature in Fahrenheit
and we subtract 32 degrees from it.
This makes sense because 32 degrees Fahrenheit
minus 32 degrees Fahrenheit would give us
zero degrees Celsius.
Now we just need to apply the unit ratio,
so just like any dimensional analysis problem,
we need to cancel out the degrees Fahrenheit.
If we put the degrees Fahrenheit on the bottom here,
so nine degrees Fahrenheit,
we can cancel out the Fahrenheit
leaving us with just degrees Celsius.
To find the temperature in Celsius,
we take the temperature in Fahrenheit
subtract 32 from it,
and multiply it by a ratio of five to nine.
Then we can also manipulate this formula
if we want to start with Celsius.
All we have to do is solve
for the temperature in Fahrenheit.
To start we would divide both sides
by five over nine,
or that's the same thing as multiplying
by the reciprocal.
Then to finish it off, we would just add 32,
so plus 32 is equal to the temperature in Fahrenheit.
Now if we want to start with temperature in Celsius,
we can move to temperature in Fahrenheit,
or we could start with temperature in Fahrenheit
and move to temperature in Celsius.
To practice this let's go from Celsius
to Fahrenheit.
It turns out that these temperature scales
actually cross paths at a temperature
which is kind of a fun fact.
If we plug in negative 40,
let's go from negative 40 degrees Celsius
to Fahrenheit.
We'll find that TF is equal to negative 40
times nine-fifths plus 32.
We can reduce this term here,
so five and negative forty reduces to negative eight.
So negative eight times nine plus 32.
That's negative 72 plus 32.
So the temperature in Fahrenheit
would equal negative 40 as well.
So negative 40 degrees Celsius
is the same thing as saying
negative 40 degrees Fahrenheit.
That's kind of just a fun fact.
And another observation from this little factoid
is that Celsius and Fahrenheit scales
can both have negative or positive values.
We see that both can be at negative 40.
These can both have negative values.
That's actually a point where they differ
from the Kelvin scale.
The Kelvin scale can only have a positive value.
It turns out that the absolute coldest temperature
is zero Kelvin.
So zero Kelvin is absolute zero.
The reason we can't get any colder
is that at this point, no particles would have
any kinetic energy.
That means no motion at all.
We said that temperature is a measure
of the kinetic energy,
and the coldest that you can get is
no kinetic energy whatsoever.
It turns out that the laws of physics
specifically the uncertainty principle
just don't allow for this.
We can get close like within a billionth of a Kelvin,
but we can't get all the way there.
Because Kelvin scale always has a positive value,
it becomes a little handier in various formulas,
so to use as the standard, the SI unit for temperature.
I'll show you in future videos why absolute zero
happens at negative 273.15 degrees Celsius,
but I'm starting to run out of time in this one
so I'm going to have to save it for later.
I'll talk about that with Charles's Law in the future.