August 9, 2019 21

How Do We Tell Temperature?

How Do We Tell Temperature?

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!

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