Hey it's professor Dave, let's learn how to do temperature conversions.
Temperature is a big part of the human experience. If you want to make small
talk in public, just bring up the temperature that day and you're good to
go, but just like mass, length, and time, we can use different units to measure
temperature, and we need to be able to convert between these units just as we
would with any other set of units. Most people are familiar with just one
temperature scale, which will depend on where they live in the world, but there
are three that are commonly used, and these are Fahrenheit, Celsius, and Kelvin.
Fahrenheit is used only in the United States and a handful of US territories.
The rest of the world uses Celsius. And the scientific community will typically
use Kelvin when doing calculations, as this is the SI unit for temperature.
Why would we need to convert between temperature units? Well we could convert
between Fahrenheit and Celsius to figure out whether it was hotter in California
or Italy on any given day. Or we might need to convert a temperature from one
of those scales into Kelvin in order to do scientific calculations. So let's
learn about the equations that will allow us to make these conversions.
In order to put these temperature scales into context, we must understand the
reference values that each scale uses. Both Fahrenheit and Celsius are defined
using the freezing point and boiling point of water. Celsius sets the freezing
point of pure water at zero, and the boiling point at a hundred, splitting up
the space between into 100 equal parts called degrees. Fahrenheit, a little less
intuitively, sets zero as the freezing point of brine, which is a salt water
solution. That puts the freezing point of pure water at 32 and the boiling point
at 212, splitting up the space between them into
180 degrees. So to understand how to convert between these, we must realize
that Fahrenheit climbs 180 degrees in the same space that Celsius climbs 100,
which requires that Fahrenheit temperatures climb at 9/5 the rate of
Celsius temperatures. But there is also a 32 degree discrepancy in where these
segments begin, and this also has to be accounted for. That means that these will
be the two equations we can use when converting in one direction or the other.
Let's test them by plugging in the freezing and boiling points of water to
ensure that we get valid results. And we can see that each conversion goes
precisely as we would expect. Now let's try a different example. Let's say that
on a particular day it's 95 degrees Fahrenheit in Los Angeles and 32 degrees
Celsius in Rome. Where is it hotter? Well let's take our Fahrenheit temperature,
plug it in here, and we get 35 degrees Celsius. Just to be thorough, let's
convert the 32 degrees Celsius into Fahrenheit as well, and we should get
about 90. So it looks like California is the winner in this round. As we mentioned
earlier, Kelvin is the SI unit for temperature, and it is derived from the
Celsius scale, in that one degree Celsius is precisely equal to 1 Kelvin.
But Kelvin is an absolute temperature scale, meaning zero Kelvin is absolute zero, or
the lowest temperature possible. Believe it or not, there is a lowest possible
temperature, because temperature is a measure of available heat energy, so if
there is no heat energy whatsoever, you are left with absolute zero. We are
used to below zero temperatures in Fahrenheit and Celsius if it is very
cold, but this is impossible in Kelvin, and for good reason, because if we do
calculations using certain equations and we plug in negative values or zero for
temperature, it will ruin our math, because it doesn't make sense to use
negative values when discussing heat energy. We need a temperature scale that
will always be positive so that we can demonstrate the proportional change in a
quantity due to temperature change. As it turns out, absolute zero is about
negative 273 Celsius, which means we can use these simple equations to go between
Celsius and Kelvin, which we will frequently need to do in chemistry.
So that same hot day in Los Angeles from before, measured in Kelvin would be
around 308. And now we should be able to convert between Fahrenheit, Celsius, and
Kelvin temperature scales. Let's check comprehension.
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