Experience and experiments quickly showed that all substances need different amounts of heat to raise their temperatures by one degree Celsius. Stand by a blacksmith as he or she pounds on a red hot bar of iron, the sparks will fly. Those shards of metal are very, very hot, (their temperature is high). But if one of them lands you your bare skin, apart from the shock, it is unlikely that you will be burned. On the other hand, let a drop of hot water fall on your skin at a temperature much below that of the hot iron, and you will probably be scalded. Despite the difference in temperature, water is holding much more heat than the iron.

It takes one calorie of heat to raise the temperature of one gram of water by one degree, but the same calorie of heat will raise the temperature of one gram of aluminum 4.5 degrees, one gram of iron by 9 degrees, one gram of copper by 11 degrees, one gram of silver 18 degrees and one gram of lead by 33 degrees. This clearly shows the distinction between heat and temperature.

In setting the 'calorie standard' therefore it is important to define the substance to which the heat is being added. It also gives us another property of substances called the specific heat, i.e. the amount of heat (or energy) needed to raise the temperature of one gram of that substance by one degree Celsius.

Even water is not consistent in this regard. The amount of heat (or energy) needed to raise the temperature of water by one degree, varies according to where on the Celsius temperature scale you start. One gram of water at 1 degree Celsius needs more energy to raise its temperature to 2 degrees Celsius than it takes to raise the temperature of one gram of water at 92 degrees Celsius to 93 degrees Celsius. The standard unit - the calorie - must, therefore, also include in its definition, the range of temperature used.

This question is referring, specifically, to the “15 degree calorie” which has a value of 4.1855J where 1J is the SI unit for a unit of energy.

A 15 degree calorie is defined as:

The amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure.

[1]

The definition is as it is, precisely because at different temperatures and pressures the amount of energy required to heat 1 g of water by 1 degree changes (it, in fact, reduces) per degree of increased temperature.

This is an empirical fact that was determined by experiment. Given the empirical truth of this fact, it is necessary to specify the reference temperature when discussing the number of calories required to heat a mass of water since the number varies according to how hot the water is.

A thermochemical calorie is exactly 4.184J which is .0015J less than a “15 degree calorie.”. The value of a thermochemical calorie is specified by direct reference to the SI units and not in relation to a mass of water. This ratio is absolute and does not depend on a medium, such as water, whose properties vary according to temperature.

Footnotes

Calorie is simply energy required to increase the temp. Of water by 1°c at standard atmosphere which is approximately 4.185 but there are may definations of calorific value

1)the amount of energy equal to exactly 4.184 joules = thermo chemical calorie

2)the amount of energy required to warm one gram of air-free water from 3.5 to 4.5 °C at standard atmospheric pressure.=4°c calorie

3)the amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure. Experimental values of this calorie ranged from 4.1852 to 4.1858 J. = 15°c calorie.

So, it's standard value which is obtained from experiments.there are several other definitions.

No, I don’t think that any other number would be just as good. I admit up front that I don’t know a better answer for certain; but I can make a guess that you can verify.

What you didn’t say (exactly) is that the definition of the calorie is the amount of heat transferred when a gram of water changes temperature by 1deg. K. Note, a change in temperature in degrees C is the same as a change of temperature in degrees K; but the latter is consistent with all other uses of T in thermodynamics applications. It’s not just some kind of coincidence that one calorie, defined otherwise, happens to be the heat change between 14.5 & 15.5 K.

Because defining the unit of heat is a concept central to the thermodynamics of heat exchange, it’s important that it shall be easy to measure it precisely. The problem with doing this measurement is that the heat capacity of any substance varies with temperature. At super-cryogenic temperatures, heat capacity varies only slightly with temperature, but it is not precisely constant. Consequently, the magnitude of the change in heat accompanying a 1K change of temperature in water will vary with temperature. In other words, the very definition of the calorie would depend on the temperature at which it is measured. Besides this conceptual imprecision, there is the practical problem of how one goes about measuring a calorie precisely. To wit, an error in measuring the temperature at which the heat change is measured will propagate through to an error in measuring the heat change that depends on it.

And so, it behooves scientists to define the calorie over a temperature range in which the heat capacity is least sensitive to temperature. And that would be within a temperature interval that contains a relative maximum in the heat capacity, i.e. where [math]\frac{\Delta C}{\Delta T} \overset {\sim }{=} 0[/math]. This is also true at T=0, where C is at a minimum (0), but we can’t do measurements there. There must be a reference to the dependence of C on T somewhere. Find it, and see how C depends on T over a range of temperatures.

I think you are asking why did they select THAT range of temperature, not why you need to specify any specific temperature range in defining the standard for a calorie. Of course, the heat capacity of water, or any material, changes with temperature, so to define a unit of energy based upon the temperature change of a substance has to reference a specific temperature range. But in the case of the calorie definition why not use so-called Standard Temperature (of STP) of 0C? The problem is, besides the fact that this is the freezing point, that water has anomalous thermal expansion coefficient behavior close to 0C, curved at low temperature and actually negative between 0 and 4C. If you look at the expansion curve of water it becomes a linear function with temperature at a little below the range of 14.5–15.5. One does not want a standard based upon a material’s behavior to be confounded by the anomalous non-linear behavior of that substance.

Because there’s no point in using a temperature scale based on absolute zero in everyday life. Kelvin is very useful in science, but its frame of reference in the ordinary lives of humans is nonexistent. The average temperature on Earth is about 288 K. The human body is about 310 K. Water boils at about 373 K.

Or, would it make more sense to use a temperature scale that uses 0–100 as the typical range of temperature anywhere in the civilized world (Fahrenheit) or one that uses 0–100 as the range between the freezing and boiling points of water (Celsius), both of which are sensible, understandable scales that relate well to how human beings actually live?

Before there was a proper understanding of temperature, “degrees of hotness” were very crude and subjective making it difficult for early scientists to compare results. Early thermometers had as few as 6 divisions. Newton used one with 12. Even worse, degree of hotness of 6 on Newton’s thermometer would not necessarily be the same as 6 on a thermometer in (say) Germany.

However, because of the demand, particularly by those studying weather, inventors and manufacturers worked to get better ones. Enter Daniel Gabriel Fahrenheit.

Fahrenheit’s thermometers were a game changer. Because of the quality of his manufacture, his thermometers were consistent (i.e. you would expect two of them to give the same result, which was less the case with others). Because he produced glass tubes with smaller (but very even) bores, the same volume change would give greater length. It was therefore possible to mark them in smaller divisions of temperature.

He succeeded in making thermometers of reasonable length and dividing them into 96 divisions between human body temperature (his wife) and a mixture of ice and salt. Remember that early thermometers had been in 12 divisions. Fahrenheit was first able to divide these into 24, then 48 then 96.

This meant that they could be used for measuring air and water temperature throughout the year in most of Europe. (Remember the key market was people investigating weather, along with barometers. This still remains a key convenience of the Fahrenheit scale.)

Thus degrees Fahrenheit became the effective scientific standard for temperature. He died in 1736.

The scale was later modified slightly (see below) so that the boiling point of water was 212 °F (at a standard reading on the new barometers) and melting point of ice was 32 °F to give exactly 180 ° between these two convenient points. This enabled the thermometers to be more widely used in science (expanding the market) and moved away from human body temperature which is less fixed.

In 1739 René A. F. de Réaumur proposed a scale in which the melting point of ice was zero and the boiling point of water was 80. This suggestion of fixed points was a good one. The scale was done by marking into 5 divisions, then bisecting these etc. (Dividing into 100 would have been much harder.) For reasons of patriotism, this was then preferred by French scientists.

The choice of fixed points was a good one. In 1741 Anders Celsius proposed a scale in which the boiling point of water was 0 and the melting point of ice was 100 (yes, really) but more sensible people reversed this to make the Centigrade scale, given the symbol °C. This was later renamed in honour of Celsius, possibly because it meant that the symbol could stay the same.

In 1954, better understanding and equipment defined an international scale of temperature, named after Lord Kelvin. For convenience the °K was made the same as the °C, but its zero point was absolute zero as defined by thermodynamics.

To rationalize the unit of temperature along with others, the °K was later renamed the kelvin, with the symbol K. ( Units named after people have a capital letter for the symbol, but are written with a small letter to distinguish them from the person.) Thus a temperature difference of 10 °C is exactly the same as a temperature difference of 10 K, but a temperature of 10 °C is 283.15 K.

The Celsius scale remains an allowed one within the SI rules, and is now defined in terms of the kelvin. The Fahrenheit scale has also been redefined in terms of the kelvin and the thermodynamic temperature, and is mainly used in the USA. They are both more convenient for everyday use. (“Your child has a slight fever of 311.” “Temperatures may drop as low as 271 K tonight, with ground frost.”)

Both fahrenheit and celcius are degree scales. This means that the notional unit is too big, and divided into lesser units.

In both römer and celcius, zero is set to a reproducable cold, and one to a reproducable hot. The hot point is the boiling of water. The number of degrees is taken as a fashionable number, ie 60 or 100.

Celcius fixed the cold point at the freezing of pure water, whereas römer used a freezing of a water/salt mix.

Fahrenheit’s mercury in glass had a finer resolution. and divided römer’s degree into four, used a better cold, and fixed the top end of the scale, because römer’s boiling point was too high. So we have 0 at a tempure used in freezers, and the upper point moves down from 60 to 53 degrees, that is 212 F. The other fixed points of römer’s scale are preserved at four times in the fahrenheit scale.

The tricky thing here is to set a scale that has 0 at absolute zero, and both freezing and boiling at multiples of 10. Ideally, ordinary temperatures should be in the same hundred. The gorems i use run wuth freezing point at 410, and boiling at 560, running to abs zero at 0, and water sublimating from ice to steam at 970.

Stoves do not have thermostats. The scale determines the power, not the temperature. Here the scale is typically 1–6, sometimes 1–12. I once measured the powers of my stove. I got following powers for the large plate, nominally 2 kW

A strange gap between 3 and 4, the powers I almost exclusively use.

The calorie was a former metric unit, which was based on convenient materials. You could easily calibrate a calorimeter (a laboratory device used to measure the heat of reaction and physical changes). As mentioned elsewhere, it was the energy required to raise the temperature of 1 gram of water by one degree Celsius. It has the symbol cal.

However, as it was a bit small for some purposes, people dealing with food used what they called the big calorie, which should really have been called the kilocalorie, since it is 1000 cal = Cal (with a capital C). However it is very commonly (I would say mostly) given with a small c. This is the unit mentioned in the question, heating one kilogram of water by one degree.

Because the numbers quoted for diet purposes became well-known, it has persisted. Some food labelling quotes both joules and kilocalories for the energy content of products.

The joule was adopted for the SI system because it did not depend on the properties of an arbitrary substance (water, its specific heat) but could be defined based on the fundamental quantities of mass, length and time, so that the SI is what is called a coherent system.

You need to be precise with the wording to understand this. It is not a temperature of one degree Celsius that is equal to a temperature of one kelvin.

It is a temperature difference of one Celsius degree that equals a temperature difference of one kelvin. That’s true simply because one Celsius degree is defined as being equal to a temperature difference one kelvin.

One degree Celsius is a temperature. It is equal to 274.15 kelvin.

A Celsius degree is the size of an interval on the Celsius scale. People are often not so careful wih the wording, of course.

The “kelvin” is a unit. It is not correct to say “degrees kelvin” or “a kelvin degree”. That makes it harder to distingush between the two meanings than with the Celsius scale. However, the meaning is usually clear from the context.

0 degrees Celsius is equal to 32 degrees Fahrenheit: 0 °C = 32 °F. Zero on one scale is not really hot on the other. 0 °F = -18 °C

The temperature T in degrees Fahrenheit (°F) is equal to the temperature T in degrees Celsius (°C) times 9/5 plus 32: T(°F) = (T(°C) × 9/5) + 32 or T(°F) = (T(°C) × 1.8) + 32

To go from Celsius to Fahrenheit: T(°C) =(T(°F) - 32)/1.8

T(°C) = T(°F) at -40 degrees

In Celsius, zero was originally defined as water's freezing point.

In Fahrenheit, zero was defined as the temperature of an equal-part ice and salt solution of brine?... Eh, I never really got it.

(Both scales are now defined much more precisely, but that's where they came from.)

Both scales are useful and widespread, but their zero does not mean an absence of heat, and they can go negative. Absolute zero, as we call it, is -273.15 °C, or -459.67 °F.

There is a scale, Kelvin, where 0 is the absolute zero. Why don't we use it, then, you ask? Because it's much less useful in everyday human experience. Typical Celsius values range from -5 (your freezer) to 35 (a hot day); typical Fahrenheit stays comfortably between 0 to 100, sometimes spilling over in extreme temperatures. In Kelvin, the everyday values would go from 270 to 310.

The concept of degree tends to be used in a context that is more semiquantitative. For example, what is the difference in skill level between a second-degree black belt and a third-degree black belt in martial arts? Is it one and one-half times as skillful? How does one quantify skill level? Certainly a third-degree black belt is more proficient than a second-degree black belt, but it is hard to be more specific.

The Celsius and Fahrenheit scales are of a similar nature because the 0-point is arbitrary. In fact, the 0-points for these two scales are not at the same temperature. These temperature scales are more quantitative than assessing skill levels of black belts, but there is still confusion. In both scales we can definitely say that if two temperature measurements are taken and one is x ° and the other is y °, then the greater of x and y corresponds to the warmer temperature. We get some idea of the degree of difference by calculating y − x, but such a difference is of bigger impact for the Celsius scale than for the Fahrenheit scale, because Celsius degrees are 1.8 times as big as Fahrenheit degrees. The most telling issue, though, is the ratio y/x. If that ratio is 2, that does not mean y is twice as hot as x, all because of the arbitrariness of the 0-point. It is even worse because the ratio between two temperatures is different when express as Celsius temperatures versus as Fahrenheit temperatures. And then, what should one do for the ratio if x is 0 °? How about when x is negative?

The Kelvin scale on the other hand has its 0-point at absolute zero, which means that the temperature is proportional to the internal kinetic energy of molecules. Therefore, 200 K is twice as hot as 100 K. The Kelvin scale is fully quantitative, where both differences and ratios are meaningful.

As a result, the term degree is applied in the context of of Celsius and Fahrenheit, but not for the Kelvin scale. The unit of measurement for the Kelvin scale is called simply the “kelvin” with symbol K; the unit of measurement for the Celsius scale is called the “degree Celsius” (and the word “degree” is required) with symbol °C (and the ° is required) per the official definition of the metric system. Originally, the unit for the Kelvin scale was called the “degree Kelvin” with symbol “°K” so that all scales were treated alike. The use of the word “degree” in the unit name and the character ° in the unit symbol were abrogated officially in 1967 and continued use of the old symbols was no longer permitted as of 1980.

The US National Institute of Standards and Technology is responsible for the US implementation of the metric system as well as maintenance of the US customary units. The US counterparts to the Celsius and Kelvin scales are the Fahrenheit and the Rankine scales, respectively. The unit of measurement for the Fahrenheit scale is the “degree Fahrenheit” with symbol “°F”. Because the Rankine scale is thermodynamic like the Kelvin scale so that ratios are meaningful, the NIST has recommended dropping the word “degree” and character “°” from the unit name and symbol, respectively, for the Rankine scale, though traditionally they have been included.

A serious problem with dropping the ° from the unit symbol for the Celsius and Fahrenheit scales is that C and F are the symbols for the SI units of electric charge (coulomb) and capacitance (farad), respectively. In formal technical writing the “degree” is required for the Celsius and Fahrenheit scales with no room for sloppiness or laziness; the “degree” is forbidden for the Kelvin scale and is disrecommended for the Rankine scale.

It depends on what you mean. If you’re just talking about the units themselves, and not anything to do with the two measurement scales, then 1 degree Celsius is a bigger temperature unit than 1 degree Fahrenheit. In fact

1 Celsius degree = 9/5 Fahrenheit degrees = 1.8 Fahrenheit degrees.

BUT, if you’re talking about an actual temperature measurement of +1 degree on the Celsius scale, and asking what temperature this corresponds to on the Fahrenheit scale, then you have to take into account that the the zero points of the two scales are not the same. The zero point of the Celsius scale is the freezing point of water, so that a temperature of +1 deg. C is 1 Celsius degree above freezing. But the freezing point of water on the Fahrenheit scale is 32 deg. F. You have to add this offset of 32 deg. F to the Celsius value after converting it to Fahrenheit degrees. Therefore:

1 deg. C = 9/5 deg. F + 32 deg F = 33.8 deg. F

Some crossed wires here.

Fahrenheit based his temperature scale on 100 degrees between zero at the freezing point of a brine solution and 100 degrees at what he thought was blood temperature in humans. On this scale freezing of water was 32 degrees and 212 degrees for the boiling point of water.

Celsius proposed a scale of 100 degrees between zero at boiling point and 100 degrees at freezing. This gives rise to computational difficulties.

The centigrade scale uses the Celsius degree but with zero at freezing and 100 degrees at boiling.

However Celsius is the SI name for the centigrade scale and in common use.

It can depends on context and how relaxed the conversation is. If I'm asked what the temperature of an icewater bath is, it is 0 Celsius and/or 32 Fahrenheit. There are 100 degrees between freezing and boiling in Celsius, and 180 degrees in Fahrenheit. If I state a system, then saying 'degree' is not necessary, since saying 'Celsius' or Fahrenheit implies what the number is relative to. I/e I know that 45 Fahrenheit is (45 minus 32) one-hundred-eightieths of the way up from the freezing point of water to the boiling point. On the other hand, if I say 'it's dropped 12 degrees in the last hour', since there are multiple systems available, your listener needs more information (C or F) to put your statement in context.

In a nutshell - C or F without 'degrees' is fine anytime. Adding 'degrees' adds no information, but is also not going to cause problems. Saying 'degrees' without C or F (or K) is not enough information unless you're in a situation where people only know one system, or all know what system is in use for the context.

It's a matter of familiarity. The other scales have been in use since the 18th century. Centigrade replaced Fahrenheit during metrification, in the 20th century, and nobody's particularly keen on making another change, which would be an enormous investment.

Both the Celsius and Fahrenheit scales are focused on human-scale temperatures. The Celsius' 0 and 100 are very easy to explain, and both are common temperatures. Fahrenheit seems much more arbitrary, and the reasons are historical (and even wrong), but it's at least familiar. It's used primarily in the US, which has avoided even metrification as too much effort for too little gain.

Kelvin would make a lot of physics and chemistry easier to understand, but unless you're an engineer or a scientist, it doesn't really matter to you where the zero point is. It's more helpful to have it be around the point of something that actually happens in your daily life, for which action needs to be taken: water freezes, snow falls, and surfaces become slippery.

Most likely because it is more “natural” than the Fahrenheit scale, since it is framed by two easy-to-reproduce temperatures: those of the freezing and boiling points of water. (Fahrenheit used the temperature of the coldest salt solution that he could produce, and the (approximate) human body temperature, as his lower and upper points.)

Also, since the size of the Celsius scale (formerly “centigrade scale”, since its determining temperatures differ by 100 degrees) is a neat multiple of 10, it fit well with the metric system (modernly called the SI) which is framed in powers of 10.

So, since the metric system became almost universally established, the Celsius scale did too.

Basically, the Fahrenheit and Celsius scales were both invented to measure temperatures important to everyday humans, but the Kelvin scale was invented to measure absolute temperature for scientists.

Let’s go over the history of the scales.

• The first was the Fahrenheit scale, created in 1724 by Daniel Gabriel Fahrenheit.
• He set it based off three values.
• An ice—water—salt mixture was to be considered 0 °F.
• Water’s freezing point was to be considered 32 °F.
• Human body temperature was to be considered 96 °F.
• Why those? He wanted to use powers of two to make it easier to divide the scale into smaller increments.
• The difference between the freezing point of water and human body temperature was exactly 64 °F, two to the power of six.
• So he only had to divide the interval in half, six times, to determine individual degrees.
• The scale was eventually readjusted due to better scientific calculations, leading to the modern day human body temperature of 98.6 °F.
• The second was the Celsius scale, created in 1742 by Anders Celsius.
• He set it based off two primary values, both measured at standard sea—level temperature and pressure.
• Water would freeze at 100 °C.
• Water would boil at 0 °C.
• Carl Linneaus reversed the scale later, to be consistent with the fact that adding energy to something will often raise its temperature.
• This scale was called the Centigrade scale for a long time because there was exactly a 100° separation between freezing and boiling.
• In the 1840s, scientists realised that temperature was really thermal motion of a substance, so the lowest possible temperature would be when that motion stops.
• So William Thomson, later known as Lord Kelvin, decided to create a scale with the same degree size as Celsius, but with the zero point set at absolute zero.
• It was thus effectively the Celsius scale, but shifted by 273.15.

Using absolute zero as the reference point perfectly links temperature to thermal energy, so that is why scientists use the Kelvin scale. Doubling the thermal energy of an object will cause its Kelvin temperature to double.

But, since the vast majority of ordinary humans never deal with temperatures much lower than —40°, it didn’t make sense to have the main temperature scale for ordinary humans be one with a base of absolute zero.

In addition, absolute zero is a difficult thing to measure and has never technically been achieved (although scientists have come really close).

By contrast, the zero points on both the Celsius and Fahrenheit scales are easily repeatable in nature, so the scales can be calibrated that way.

“Why does the conversion between Celsius and Fahrenheit involve multiplication/division and addition/subtraction?”

There are two main differences between the Fahrenheit and Celsius scales: they start in different places, and the degrees are different sizes.

If one scale started, say, at 0, and the other at 20, then we could convert from one to the other just by adding 20 (or subtracting 20 in the other direction). But it's not that simple.

If both scales started at 0, and one counted degrees that were twice as big as degrees in the other scale, then we could convert from one to the other just by doubling (or halving in the other direction). But it's not that simple either.

First: what Celsius calls 0°, Fahrenheit calls 32°. (That's the freezing point of water.) If that were the only difference, we could add 32, as suggested above. But Celsius degrees are also bigger than Fahrenheit degrees; one degree Celsius is 1.8 degrees Fahrenheit. (In other words, going from 0°C to 1°C is the same as going from 32°F to 33.8°F.)

So we need to multiply and add.

In math, we call this a linear function. Remember y=mx+b in algebra? That's the equation of a line. If you tell me what m (the slope) and b (the y-intercept) are, I can draw you the line. It will be all the X and Y values that make the equation true. (Or, for any X you want, we can find the matching Y that makes the equation work, and vice versa.)

Well then, here's another linear equation: F = 1.8C+32. Give me a Celsius number, call it C, and I can find the corresponding Fahrenheit number for you. It'll be the one that makes the equation work.

By the way: why are Fahrenheit and Celsius so different? Because Fahrenheit was first, he made a reliable thermometer, and he labeled it according to the way the thermometer worked. Celsius was a different approach — take water, the freezing and boiling of which is very important to human beings. Call the freezing point 0, and the boiling point 100, and work out everything else from there.

In other words, Celsius was carefully designed to be useful and convenient to human beings. It was a refinement of Fahrenheit, which was kind of arbitrary.

If you are talking about everyday usage, such as weather reports or room temperatures, it makes much sense to use Celsius as the range of 100 degrees from the freezing to boiling point of water is a convenient piece of scaling as it encompasses a lot of everyday needs. Things like room thermostats would require three figure scales for no great purpose should the Kelvin scale be adopted.

The Fahrenheit scale is rather more arbitrary in its scaling points, so it’s not so obvious why it would be used. That said, if people are happy to use that scale then that’s up to them.

In scientific circles, Kelvin is much more common, at least in the physical , not life sciences (where Celsius is more convenient). Kelvin is the natural scale in those physical sciences as the relevant laws of thermodynamics are references to absolute zero. Astronomers will deal with temperatures in Kelvin, and that even carries over to some everyday uses such as the “temperature” of light sources. The colour temperature of light bulbs is most often expressed in Kelvin.

nb. there’s something of an irony in the “colour temperature” labelling of light sources. What are referred to as “warm white” lights represent a lower temperature than what are referred to as “cool white” lights. It’s an object lesson in the difference between common usage phrases and strict scientific definitions.