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Knowing how to convert Kelvin to Celsius is just as important as knowing how to do the reverse conversion. Many physicochemical properties of matter are reported at different temperatures, and these, in general, are expressed in Kelvin. However, most people find it easier to handle temperature in Celsius, so converting Kelvin to Celsius is something many need to do on a daily basis.
This conversion is very simple since both scales only differ in the position of zero. However, it is easier to understand and, above all, to remember, if you understand where both scales come from and how they are used.
Celsius scale (°C)
It is a temperature scale invented in 1742 by Anders Celsius, a Swedish astronomer. This scale is established based on the freezing and boiling point of water at sea level. The freezing point of water is the reference point to which the temperature of 0 °C was assigned, while the boiling temperature was assigned the temperature of 100 °C, which is why it was originally called the centigrade scale. temperature.
The Celsius scale is a relative temperature scale. This means that it does not measure the absolute temperature of a body, but only expresses when the temperature is higher or lower than a reference temperature, in this case the melting or freezing point of water.
In other words, all temperatures that are measured in degrees Celsius indicate how much hotter or colder a body is relative to the melting point of water. Since there are temperatures lower than said melting point, then there can be negative temperatures on this scale. The second reference temperature is used to determine the “size” of the degree Celsius.
The absolute temperature scale and the Kelvin (K)
Unlike the centigrade or Celsius scale, the Kelvin scale is an absolute scale of temperature , not dependent on any reference point. Zero on the Kelvin scale actually indicates the absence of thermal energy in a body. It was created by William Thompson (also known by his peerage, Lord Kelvin, hence his name) in 1848, who assigned each unit on his scale the same size as the degree on the Celsius scale. This makes converting from Kelvin to Celsius much easier, since the only difference between the two scales is the location of absolute zero, which corresponds to -273.15 °C.
It should be noted that the unit on the Kelvin scale is called Kelvin and its symbol is K ; It is not called a “degree Kelvin” nor is its symbol “°K”. This is precisely because it is an absolute and not a relative scale. For example, 273 K is read two hundred and seventy-three Kelvin and not “two hundred and seventy-three degrees Kelvin.”
Formula to convert Kelvin to degrees Celsius
To transform Kelvin to degrees Celsius, it is only necessary to take the temperature in Kelvin and subtract 273.15. It is commonly written:
When using this formula, °C represents the temperature in degrees Celsius (the one you want to find) and not the unit of degrees Celsius. The result of the operation gives directly in °C. To avoid confusion between the variables that appear in the equation and the temperature units, it is preferable not to place the units when making the conversion, but only the numerical value. After carrying out the operation, the symbol of degrees Celsius (°C) is placed.
Example:
Suppose that in a data table it is reported that the pyrolysis of wood begins at 580 K and you want to know what that corresponds to in degrees Celsius. The first step is to substitute 580 for K in the formula (without units to avoid confusion). Then the operation is performed, and finally °C is added:
NOTE: In the example above, the final result is rounded to ones (307K instead of 306.85K) due to significant figures rules. These establish that, in addition and subtraction, the result is rounded to the position of the least common digit of all the numbers, in this case, the units.
A more correct form of the formula to convert Kelvin to Celsius
The above formula, despite being easy to use, often causes confusion because units cannot be included unless they are at the end. To avoid this, it is preferable to use a more correct form of the same equation. In many books on physics and thermodynamics, absolute temperature is represented by the letter T, while relative temperature in degrees Celsius is represented by the Greek letter θ ( theta , which is t in the Greek alphabet). Thus, the previous formula is:
In this case, K and °C do represent the units Kelvin and degrees Celsius respectively, while T and θ represent the temperatures with their respective units. To obtain the temperature in degrees Celsius (that is, θ), it is only necessary to solve for θ by multiplying both sides of the equation by °C.
Example:
Suppose you want to transform the boiling point of oxygen, which is 90.18 K, to degrees Celsius or centigrade. In this case, we have that T = 90.18 K and θ is requested. So, one only needs to plug T into the formula and solve for θ:
So, solving for θ, which is the temperature in degrees Celsius (the one we want), we get:
Reverse conversion, from Celsius to Kelvin
To convert from Celsius to Kelvin, you just need to solve either of the two equations, which means adding 273.15 to both sides of the formula:
Or, more correctly,
How to convert Kelvin to other units
The absolute temperature in Kelvin can be converted to any other temperature scale, be it relative or absolute. For example, you can also convert Kelvin to Fahrenheit (which is a relative scale) or you can convert Kelvin to Rankine (which is an absolute scale).
verification systems
Whenever a unit conversion is carried out, it is important to verify that the result makes sense, since it is always possible to be wrong, even when using formulas as simple as those shown. In the case of conversions from Kelvin to Celsius and vice versa, in one case you have to subtract 273.15 and in the other you have to add, and it can be very easy to forget when to do one or the other. To ensure that the conversion is always carried out correctly, the following two tips can be taken into account:
- The temperature in Kelvin must always be greater than the temperature in degrees Celsius . For example, if when transforming 400 K to Celsius the result gives 673 °C, the transformation is wrong, since the temperature in Kelvin can never be less than the temperature in °C.
- Temperature in Kelvin can never be negative , since it is an absolute scale and the minimum value is zero (no units, just zero). On the other hand, all temperatures less than 273.15 K, when transformed to °C, will give negative values.
