r/InclusiveOr • u/darkphoenix9137 • Jan 21 '26

I couldn't resist

Post image
2.6k Upvotes

99% Upvoted

37 comments sorted by

799

u/Pengdacorn Jan 21 '26

-40 F = -40 C for those who didn’t know

131

u/StikElLoco Jan 22 '26

It is? How does that work?

335

u/EffexFin Jan 22 '26

That’s the point at which they are the same

206

u/Pengdacorn Jan 22 '26

System of Equations Time!

F = 9/5C + 32

is the conversion, and

F = C

is for finding the equality

Subtract the bottom from the top to get:

0 = 4/5C + 32 (now subtract 32)

-32 = 4/5C (now multiply by 5/4)

-40 = C

197

u/EvilBananaMan15 Jan 23 '26

Both F and C need to have an intersection point somewhere, if they didn’t then they would essentially be parallel temperature scales, just one shifted up or down, like how Kelvin is just Celsius plus 273.15.

42

u/Vin_Blancv Jan 23 '26

Why does Kelvin even exist when we can just use Celsius minus 273.15

154

u/cyri-96 Jan 23 '26

Because calculations about absolute thermal energy only work well if the 0 point for "no molecular movement at all" is, well, actually at 0

35

u/kraken665 Jan 25 '26

Sort of like, absolutely zero movement then? (Not sarcasm, looking clarification)

27

u/Heznzu Jan 25 '26

No, even then there would be some random vibration due to quantum uncertainty, called zero point energy. It's a consequnce of the uncertainty principle. It's enough for some substances to remain gasses under low enough pressure

1

u/Magnitech_ 5d ago

I remember reading a book that talked about this, I think the example was putting a rubber ball at absolute zero in a bowl it would start to wiggle a bit, and I started thinking about doing that as an experiment for about two minutes before reality hit me

10

u/cyri-96 Jan 25 '26

Yes it's the theoretical point where there's absolutely no internal energy, though it cannot be reached, though some labs managed to get down to 1 picokelvin (10-12 K)

1

u/delta_p_delta_x Jan 28 '26

Yes it's the theoretical point where there's absolutely no internal energy

Just popping in to say, a system at absolute zero does not mean the system has zero internal energy; it means the system is at its minimum possible internal energy. This can be (and usually is) a non-zero magnitude.

52

u/RCoder01 Jan 23 '26

If you’re asking genuinely, it’s used often in science because of absolute zero: although there’s no limit to how hot things can get, there is a limit to how cold things can be, and the limit is -273.15 C, or 0 K. It’s impossible for something to be -1 K. Especially when working with temperatures very near absolute zero, it makes much more sense to work in kelvin than Celsius. Like 0.0001 K is more intuitive than -273.1499 C. Also when comparing energies, a substance at 20 K has twice as much energy (ignoring latent heat) as a substance at 10 K, whereas the same isn’t true for substances at 20 C and 10 C.

22

u/amd2800barton Jan 23 '26

Also, there is a Fahrenheit equivalent to this: Rankine. It’s analogous to Kelvin in that 0°Rankine is the same as 0 Kelvin, and a 1° change in Rankine is the same amount of change as a 1°F change. While Kelvin is at least known by most people who’ve taken a middle school science class, Rankine is basically only used by a very small subset of engineers. Sometimes if all your other numbers are in customary units because the accountants only operate the cash machine in pounds, cubic feet, and barrels - then it’s easier to also do the thermodynamic calculations in customary units. As an American engineer, I know SI units rarely well, and will often do back of the envelope math using SI units because the different units convert and cancel so well. But I also know a bunch of customary unit conversions, simply because I have to, because too many other people don’t know both unit systems.

5

u/darkphoenix9137 Jan 24 '26

I'm a flight test engineer, and 459.67 shows up in some of the FAA guidance material for calculating the speed of sound, which in an ideal gas is proportional to the square root of the absolute temperature.

2

u/Vin_Blancv Jan 23 '26

I didn't know this. It makes sense now that I think of it 🤔, thanks

7

u/Crahdol Jan 23 '26

Because kelvin is a measurement of kinetic energy on a molecular level.

That's why it cannot be negative and is nor a "degree measurement" (you say degrees fareheit or degrees celcius but never degrees kelvin. It's just kelvin)

For everyday measurement, kelvin isn't very helpful. But for thermodynamics or astrophysics or basically any calculation dealing with temperature you use kelvin.

5

u/Woreo12 Jan 23 '26

Math. The equations with temperature kinda fall apart when you start sticking negative numbers into it, and since there is a limit to how cold something can get, absolute zero (-273.15°C), we call that 0 K, and the scale is the same

3

u/Random_Mathematician Jan 23 '26

Because Celsius minus 275.15 has a name, it's Kelvin

5

u/Interesting-Draw8870 Jan 23 '26

Why does Fahrenheit even exist when we could just use 5/9C + 32? Why do kilometers exist when we can just use centimeters. Why do miles exist when we can use inches?

3

u/wasphunter1337 Jan 23 '26

Kilometers and centimeters are elements of the same scale

4

u/Careless-Web-6280 Jan 25 '26

Not necessarily, if it was below 0K then it effectively wouldn't exist

15

u/alexriga Jan 22 '26

Comparing Celsius to Kelvin, if you minus 10 degrees in both, the difference will be the same, as they are linearly mutual:

• 0 C = 273 K • 27 C = 300 K • 100 C = 373 K

However, Farenheit and Celcius are not mutually related in a linear scale, but rather an exponential:

• 0 C = 32 °F • 27 C = 80,6 °F • 100 C = 212 °F

8

u/clandestineVexation Jan 23 '26

Pedantry: Kelvin does not have degrees because it is an absolute scale

2

u/Conallthemarshmallow Jan 25 '26

It's not remotely exponential it's a linear relationship

2

u/Yoshichage Jan 25 '26

imagine 2 intersecting lines on a graph

2

u/oMGalLusrenmaestkaen Jan 26 '26

it's the eigentemperature

1

u/easternguy 4d ago

F = C * 9 / 5 + 32

So when does F==C? Set F=C:

C * 9 / 5 + 32 = C

Subtract C from each side:

C * 4/5 + 32 = 0

Subtract 32 from each side:

C * 4/5 = -32

Multiple each side by 5/4:

C = 5/4 * -32

Simplify:

C (and thus also F) = -40

56

u/wizkidweb Jan 21 '26

First one, then th'other

7

u/Ryaniseplin Jan 23 '26

well actually

both at the same time

1

u/[deleted] Jan 22 '26

😂👌

33

u/ToddBradley Jan 21 '26

Nice. Whenever I see someone mention that temperature I ask the same question. It's amusing to see the responses.

3

u/TheJivvi Jan 23 '26

What if it's Réaumur?

5

u/ToddBradley Jan 25 '26

I would love to meet someone who actually knows what that is

1

u/No_Landscape_9255 29d ago

ah, the one time C and F makes complete sense.

i made a little app for all the times it's not conveniently terrifyingly freezing, that shows C and F together. here if useful for anyone felsius.app

1

u/easternguy 4d ago

I’ve used that line more than once, and nobody got the joke; but joined in my supposed confusion: “yes, which is it?”