You’re feeling woozy.
Maybe your forehead feels hot.
You stick a thermometer in your mouth.
It comes back 37C … ok, phew, maybe you’ve just been standing in the sun too long.
Or it comes back 38C - running a fever, call in sick.
Or worse, it comes back 40C … get yourself to the ER!
We have a lot of confidence that we’re measuring temperature accurately.
One or two little degrees can literally mean the difference between calmed nerves and a
medical scare.
Knowing temperature is important beyond your health too - whether it’s setting the grill
for the perfect burger or figuring out what hot outfit will keep you cool at a summer
cosplay event - maybe, you can often proceed informed because of our trust in modern thermometers.
Wondered what physical chemistry has done for you lately?
Then check out how these thermo-meters tell the truth.
[SPLASH]
If I told you thermometers don’t measure temperature, would you find your trust shaken?
Thermo-meter means “measurer of heat” but that’s the same thing as temperature
anyway, right?
Scientifically speaking, no.
But knowing about heat can tell you about temperature if you’re clever.
Temperature describes the average energy of the molecules in a system, while heat describes
the transfer of energy between systems that are at different temperatures.
Thermodynamics tells us how heat moves around in its first 3 laws:
0. Heat is the same regardless of how it’s measured,
1. Heat cannot be created or destroyed, and
2. Heat moves from hotter to colder.
Yeah that lists starts at zero because that part was figured out after one and two.
Armed with these laws, we can understand how thermometers work - to make them as useful
to us as possible.
Consider the bulb type - that liquid in a glass tube thing you know and love.
Imagine it’s in the freezer.
The thermometer loses heat to the surroundings.
That means the molecules in the liquid slow down, and the liquid contracts, pulling its
level down the scale.
Move it over to the sauna and the thermometer gains heat.
This causes the molecules in its liquid to move faster, so the liquid expands, and the
level moves up the scale.
When the thermometer has steadied out, both it and the surroundings are the same temperature
- which means, thanks to thermodynamics, the thermometer tells us what the temperature
“is”.
We’ve been playing around with simple up/down thermometers for hundreds of years - what
up Galileo, Fahrenheit, and Celsius!
Some clever physical chemists and engineers have given us all kinds of thermometers you
see today.
Let’s take a little tour.
Many people associate trusty bulb thermometers with the mercury.
Before we learned mercury exposure could lead to health problems, this liquid metal was
preferred because it expands and contracts at a constant rate and stays liquid over a
useful range.
And it sort of looks like the T1000, which is very cool.
But finding silvery bulb thermometers is rare these days.
That’s not exactly a new trend, because even before mercury thermometers, some early
adopters used red wine because its alcohol content protected it from freezing and it
was easier to see in glass than quicksilver.
Nowadays most bulb thermometers use ethanol that’s dyed red or sometimes blue.
One of the most common electrical thermometers are thermistors.
These are a resistor that conducts electricity differently as temperature changes.
Thermistors are made from metal oxides like manganese or say iron or semiconductor materials
like germanium or silicon, depending on the temperature range, and are often embedded
in a glass bead with wires sticking out.
These can be made at very low cost, meaning they’re often what’s inside a digital
thermometer that you might have stuck in a mouth recently, checking if you or someone
else has a fever.
It’s worth noting that electricity-based thermometers are still using thermodynamics,
but using moving electrons - electricity - gets us into the quantum realm.
Sound a little sci-fi?
Then check out this action at a distance option:
Some metalloids and metals, produce electricity when light shines on them - explaining this
got Einstein a Nobel Prize and the world got solar cells.
All objects give off a sort of light called infrared radiation, or “I-R”.
Indium, germanium, and other semiconductors will produce electricity when IR hits them.
So pointing them at say, a child’s aching ear or a simmering pot of stew, the semiconductor
detector converts the radiation readings to temperature on a digital readout.
You may also recognize infrared sensors in the form of heat-seeking cameras, used in
hunting, warfare and cryptozoology missions to track down big foot.
Measuring temperature through radiation readings reach FAR.
Satellites scanning Earth measure its temperature from thousands of miles up.
So satellite thermometers in orbit are essential to understanding our climate.
And one day they could routinely tell us about other planets’ climate so you could figure
out what to wear when you get there.
Want to know more?
Tell us if you want a video about triple point or any other physical chemistry wonders and
while you’re at it, check out these temperature-sensitive Reactions episodes.
Thanks for watching!