These original Kirt's Cogitations™ may be reproduced
(no more than 5, please) provided proper credit is given to me, Kirt Blattenberger.
here to return to the Table of Contents.
Cog·i·ta·tion [koj-i-tey'-shun] – noun: Concerted
reflection; meditation; contemplation.
Kirt [kert] – proper noun: RF Cafe webmaster.
How Many Know -40°F = -40°C ?
Some of what you and I consider common knowledge is
largely unrealized by most people. Call me a geek, but I take pleasure in pointing out to people that the Fahrenheit
and Centigrade scales are equal at -40°, and I especially enjoy working out the simple proof for them. Most people
appreciated the effort and are amazed, claiming to have never seen that before. When I read the following in
Smithsonian magazine, "Winter temperatures here, some 250 miles northeast of St. Petersburg, sometimes plunge
minus 40 degrees Fahrenheit," I wondered whether the author knew that -40°F = -40°C. Maybe he just didn't want
to confuse his readers by omitting the redundant superfluous 'F' or 'C,' and it couldn't be 'K' because there are
no negative Kelvin degrees. It could also be that he knew but figured most people do not, so he chose his favorite
temperature unit. The other possibility is that he simply did not know, but it really doesn't matter.
every once in a while people land on the RF Cafe website who do not have a background in science or engineering,
I'll take this opportunity to present the aforementioned proof:
The conversions back and forth between Fahrenheit
and Celsius are
1. F = (C * 1.8) + 32
2. C = (F - 32) / 1.8
As a quick
test, almost everyone knows that 0°C = 32°F, so
C = (32 - 32) / 1.8 = 0 / 1.8 = 0
In the other direction,
F = (C * 1.8) + 32 = (0 * 1.8) + 32 = 0 + 32 = 32
Additionally, 100°C = 212°F, so
C = (212 - 32)
/ 1.8 = 180 / 1.8 = 100
Now for the proof. Substitute equation 1. into equation 2. to find where C = F,
C = (C * 1.8) + 32 = 1.8C + 32
Subtract 1.8C from both sides of the equation,
C - 1.8C = 32
-0.8C = 32
Divide both sides by -0.8,
C = 32 / (-0.8)
C = -40
So, C = F at -40
works the same for substituting equation 2. into equation 1.,
F = (F - 32) / 1.8
Multiply both sides
of the equation by 1.8,
1.8F = F - 32
1.8F - F = -32
0.8F = -32
F = -32 / 0.8
F = -40
So, F = C at -40
The entire exercise takes less than a minute with pencil and paper. Enlighten
a friend or relative the next time an opportunity arises.