Temperature

Making a relative scale is simple. Create a device which reacts to changes in temperature. Pick one temperature and assign it a number. Next, pick a second temperature and assign it a number. Divide the space between the numbers into regularly sized increments. Now you have a temperature scale.

Celsius is a relative scale. The temperature at which water freezes is defined as 0 °C. The temperature at which water boils is defined as 100 °C. Every temperature is expressed relative to this scale.

Ether boils at 34.6 °C. This value only has meaning because it is a comparison to the temperature at which water freezes and boils.

Fahrenheit is also a relative scale. Science legend has is that 0 °F was the coldest temperature observed and 100 °F was the hottest temperature observed, weather wise, in Dr. Fahrenheit's town.

Kelvin is NOT a relative scale. The Kelvin scale is ABSOLUTE. While studying the relationships between the temperature, the pressure, and the volume of a gas Charles, Boyle and others noted that -273.15 °C must be the lowest temperature attainable. -273.15 °C, or 0 K, is Absolute Zero.

So, the size of the degree is still arbitrary-it is the same size as a degree C. However, 0 K is the lowest possible temperature; it was not arbitrarily assigned.

Another look at Absolute and Relative Scales

Temperature Conversions

One conversion is easy. Celsius to Kelvin.

0 °C = 273.15 K

So, just add 273.15 to a temperature given in °C to find the temperature in K.

25 °C = 298.15 K

20 °C = 273.15 K

Or to convert from Kelvin to Celsius subtract 273.15 from a temperature given in K to find the temperature in °C.

196 K = -77 °C

Fahrenheit to Celsius is more complicated. The size of the degree is different, and the zero point is defined differently. You can use a formula:

Or you could analyze the temperature scales.

On the celsius scale there are (100 - 0) = 100 degrees between freezing and boiling.

On the Fahrenheit scale there are (212 - 32) = 180 degrees between freezing and boiling.

So,

100 °C units = 180 °F units or 1 °C unit = 1.8 °F unit
But temperatures cannot be interconverted simply by using this conversion factor. The scales start at different numbers. As an example lets convert the freezing point for benzene from 5.0 °C to °F.

5 °C is 5 celcius degrees above the freezing point of water.

5 °C units x 1.8 °F/1 °C = 9 °F units

Using the conversion tells us that on the Fahrenheit scale the freezing point of benzene is 9 °F above the freezing point of water; since water freezes at 32 °F, the freezing point of benzene is 41 °F.

During the great oil crisis of the seventies we were all told that we should be cold all winter long; that is, we should keep our thermostats set to 68 °F. What is the temperature in °C?

68 °F is (68 - 32) = 36 °F units above the freezing point of water.

36 °F units x 1 °C/1.8 °F = 20 °C units above the freezing point of water. So, 68 °F is 20 °C.

Convert using boiling point instead of freezing point

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