Concepts of Heat, Temperature and Thermometers

In this article, we are going to understand the concepts of heat, temperature, and the various scales and instruments that we use to measure them.

But first things first. What is heat, and how is it different from temperature?

Table of Contents

• Heat and Temperature
• Calorimetry
• Temperature Scales
• Thermometer

Heat and Temperature

Every body and object in this universe is made up of atoms and molecules. These atoms and molecules are in constant motion. And we know that any object in motion has some kinetic energy. Heat of a body or an object is nothing but this kinetic energy.

Temperature is the degree of hotness or coldness of a body. So, an object that has more heat will show higher temperature, and vice-versa.

If object A has a higher temperature that object B, then:

• Object A is hotter than object B.
• Object B is colder than object A.

If two objects/systems having different temperatures (and by extension heat) are kept in contact with each other, than heat will flow from the system having higher temperature to the one having lower temperature.

This heat transfer (or flow of energy) will continue till both the systems attain the same temperature. In this process, the hotter system will cool down, while the colder system will get heated up.

Units of Heat and Temperature

The SI unit of heat energy is joule (J), while SI unit of temperature is kelvin (K). However, we also have Celsius (°C) and Fahrenheit (°F) as widely used units of temperature.

Note

You may have noticed that the SI unit of heat is the same as that of work. In fact, work and heat are two equivalent forms of energy.

Another unit used in case of heat is Calorie. It is the amount of heat required to raise the temperature of 1 g of water through 1°C (from 10°C to 11°C). It is represented by Cal, wherein 1 Cal = 4.18 J. In fact, the science of heat measurement, Calorimetry, drives its name from Calorie.

Now, let’s understand the science of measurement of heat and temperature.

Calorimetry

Calorimetry is a branch of thermal physics in which we measure heat using the principle of heat loss/gain.

Principle of Calorimetry is based on the Principle of Conservation of Energy. As per Principle of Calorimetry, if two bodies having different temperatures (and so different heat) are kept in contact, and no heat is lost to the surroundings, then:

Amount of Heat Lost by hotter body = Amount of Heat Gained by colder body

Amount of heat lost or gained, ∆Q = ms∆θ
Where, m is the mass of the body, ∆θ is the rise or fall in temperature, and s is specific heat capacity of the body (which is a constant).

Calorimeter

It’s an apparatus (generally, a cylindrical vessel made of copper) used for the purpose of calorimetry, i.e. we use a calorimeter for heat measurement.

Temperature Scales

To define any standard temperature scale, two fixed reference points are needed. In general, these two fixed points are:

• The Ice point or freezing point of water.
• The Steam point or boiling point of water

These temperatures are measured at standard pressure.

There are three famous temperature scales that we use to measure and compare temperatures.

• Celsius temperature scale
• Fahrenheit temperature scale
• Kelvin temperature scale

Let’s learn about them in more detail.

Celsius temperature scale

Celsius scale was designed by Anders Celsius in 1710.

On the Celsius scale, there are 100 equal intervals between the two reference points – Melting point of Ice and Boiling point of Water.

• Melting point of ice at standard atmosphere pressure is 0°C.
• Boiling point of water at standard atmosphere pressure is 100°C.

Fahrenheit temperature scale

Fahrenheit scale was designed by Gabriel Fahrenheit in 1717.

On the Fahrenheit scale, there are 180 equal intervals between the two reference points – Melting point of Ice and Boiling point of Water.

• Melting point of ice at standard atmosphere pressure is 32°F.
• Boiling point of water at standard atmosphere pressure is 212°F.

Note

The normal temperature of a human body is 37°C or 98.4°F.

Kelvin temperature scale

This scale was invented by British scientist Lord Kelvin.

This scale has no negative value. That’s because the lowest possible temperature – 273.15 °C is taken as the zero point on this scale, that is 0 K (which is called Absolute Zero Temperature). So, the concept of Absolute Zero is the foundation of this scale.

Absolute zero

Theoretically there is no limit on maximum temperature, but there is a limit (or restriction) on the minimum temperature. But why’s that?

As we reduce the temperature of a matter, the atomic/molecular motion of that matter/body keeps on reducing. This molecular motion ceases completely at absolute zero temperature. No molecular motion means no heat. As the body cannot lose any more heat, its temperature cannot fall further.

For an ideal gas this absolute minimum temperature (i.e. absolute zero) is – 273.15 °C or 0 K.

Also, the size of the unit for Kelvin temperature is the same as Celsius degree.

So, Temperature at Kelvin scale, T = C + 273.15

Reaumur scale

It’s another temperature scale, which was invented by R. A. Reaumur in the year 1730. In it the melting point of ice is regarded as 0°R.

Conversion between Celsius and Fahrenheit temperature scales

Following are the formulae that you can use to convert between the different temperature scales.

\(\frac{C - 0}{100 - 0} = \frac{F - 32}{212 - 32} = \frac{R - 0}{80 - 0}\)

Or \(\frac{C}{5} = \frac{F - 32}{9} = \frac{R}{4}\)

Celsius and Fahrenheit thermometer scales read the same at the temperature of -40°. That is, -40°C = -40°F

Thermometer

Thermometer is an instrument that we use to measure the temperature of a body. These thermometers are calibrated so as to assign a numerical value to each temperature within a certain range.

Now, there are many types of thermometers based on their internal mechanism, and/or the purpose for which they are used. Let’s have a look at a few of them.

Liquid thermometer

Two of the most commonly used liquids in these liquid-in-glass thermometers are mercury and alcohol.

Mercury: The freezing and boiling points of mercury are –39°C and 357°C respectively. That’s why, mercury thermometer is often calibrated to measure temperatures from 30°C to 350°C. A few of the advantages of using mercury are:

• Mercury is clearly visible in capillary tubes.
• Mercury has fairly uniform expansion with increase of temperature.

Alcohol: The freezing point of alcohol is – 115°C. That’s why, alcohol thermometer is often calibrated to measure temperatures below – 40°C.

Clinical thermometer

Clinical thermometer is the thermometer used to measure the temperature of the human body. As temperature of human body may vary in short intervals of time, and has very short range, we often use mercury in it, and calibrate it to Fahreheit scale.

Its lower fixed point is often kept at 95°F (35°C) and upper fixed point at 110°F (43° C).

Gas thermometer

Hydrogen is the most common gas used in these types of thermometers. In fact, constant volume hydrogen gas thermometer is considered the standard gas thermometer, and is used to fabricate other gaseous thermometers.

• Using Hydrogen thermometer, we can measure temperatures up to 500°C.
• Using Nitrogen thermometer, we can measure temperatures up to 1500°C.

Platinum resistance thermometer

Principle: As the temperature is increased, the resistance of a metal increases quite uniformly along with it. So, if we can measure the resistance of a metal, we can know the temperature.

Advantages of Platinum resistance thermometer:

• Platinum resistance thermometer is very accurate. In fact, it is so accurate, consistent, and precise that it is often used to standardize other thermometers.
• In thermometers based on gas and liquid, we face the issue of freezing and boiling temperatures, and because they expend with increase in temperature, the size of the thermometer needs to be bigger too. However, Platinum resistance thermometers do not suffer from these issues. So, we can use them to measure temperatures in a wide range (–200°C to 1200°C).

Disadvantage of Platinum resistance thermometer:

• Platinum takes time to attain the temperature of the liquid in which it is immersed. That’s because platinum has a large thermal capacity. So, we have to wait a while to see the temperature measurements due to this time lag. So, this thermometer is pretty useless in situations where we need to measure rapidly varying temperatures.

Electronic Thermometer

It is also based on the same principle, i.e. the fact that the resistance of a good conductor depends on its temperature. It employs an electric circuit that uses thermo-resistor or a thermistor, whose resistance changes with temperature. This thermometer measures the resistance and converts it into temperature, which is displayed on a digital panel.

Thermo-couple thermometer

Principle: This thermometer is based on Seebek effect (discovered by Seebek in 1932). If two dissimilar metals are joined together to form a closed circuit (hence called thermo-couple), and a temperature difference is established between their junctions, an e.m.f. is developed (called thermoelectric e.m.f.). This causes an electric current to flow through the circuit. The magnitude of thermoelectric e.m.f. and hence the current depends on:

• the nature of the two metals joined together.
• the temperature difference of their junctions.

Two junctions constructed by different metals are placed at two temperatures – one cold and the other hot. This produces thermoelectric e.m.f. and current, using which we can measure the temperature at the desired junction.

Total radiation pyrometer

Principle: This thermometer is based on the principle of Stefan’s law. As per Stefan’s law, heat radiation emitted per second per unit area is proportional to the fourth power of the absolute temperature.

Advantages of Total radiation pyrometer:

• Through this thermometer we can measure temperature of a body without touching it. That’s because it measures the temperature by estimating the radiation emitted by a body. So, using it we can measure temperature of even far-flung bodies, such as stars, sun etc.
• It can measure temperatures of very high order, e.g. that of stars, sun etc.

Disadvantage of Total radiation pyrometer:

• As this thermometer measures temperature by detecting the radiation emitted by a body, it does not work well if a body doesn’t emit a suitable amount of radiation that can be detected. As cold bodies emit very less radiation, it cannot measure the temperature of cold bodies. It can only measure temperatures of more than 800°C.

Cryogenics and Pyrometry

• Cryogenics: It involves measurement of temperature close to 0 K (i.e. absolute zero).
• Pyrometry: It involves measurement of very high temperatures.