Background 1: Celsius and Fahrenheit
The Celsius temperature scale is defined as a linear scale such
that water freezes at 0 degrees and boils at 100 degrees (both at
sea level). Similarly, the Fahrenheit temperature scale is defined
linearly such that water freezes at 32 degrees and boils at 212
degrees (both at sea level).
Notice it takes 180 degrees Fahrenheit to go from the freezing
point to the boiling point compared to only 100 for Celsius and
the freezing point is already at 32 suggesting a conversion
formula of: F = 180/100 C + 32 or simply
(1) F = 9/5 C + 32.
If we solve for C by first subtracting 32 from both sides and
multiplying by 5/9, we find
(2) C = 5/9(F - 32)
converts Fahrenheit to Celsius. As we will later learn (1) is a
slope intercept representation of a line while (2) is a point
slope representation.
Background 2: Absolute Zero and Kelvin/Rankine
Absolute zero is the lowest possible temperature, where particles
have the minimum possible energy and atomic motion (vibrations)
essentially stops. This is very important to scientists and
happens at C = -273.15 degrees Celsius or equivalently F = (C ×
9/5) + 32 = (-273.15 × 9/5) + 32 = -459.67.
Rather than continuously deal with large negative numbers
scientists have recentered the Celsius and Fahrenheit scales,
called the Kelvin (K) and Rankine (R) scales, respectively by
setting K = C + 273.15 and R = F + 459.67. The scaling ratio from
Celsius to Fahrenheit remains the same so R = 9/5 K. Negative
Kelvin or Rankine temperatures do not have physical meaning.