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Basic Physics Lesson-9 : Heat and Temperature

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Summer is hotter and winter is colder. What does it mean? The temperatures in summer are relatively quite higher than that of the winters. Have you thought the reasons for that? What makes the change in temperature in a particular place? Why hills are cooler than plains? We would be studying and getting answers to all these questions in this lesson on heat and temperature.

Memory lane

What do we understand by heat

Heat is a form of energy. When we keep a pot filled with water under sunlight it warms up. What happens is that sun rays transfer their energy to the water making it warmer. When we burn wood, coal, or any such material it burns in presence of atmospheric oxygen and in that reaction creates heat which we can feel being near it. Heat from the burning materials radiate to us and we feel its presence. Heat is a valuable form of energy and can be used for a variety of purposes. We can cook food with it, we can enjoy it in winters and can heat up our homes, and it can be used to melt many solid materials and many other such applications. In winter season we wear heavy and thick clothes so that our body heat is not radiated out and we also feel warm inside that protection. So our clothes act like a shield for heat transfer.

Heat is also required in industries for various tasks. For boiling many liquids, for melting the solids, for creating high temperatures facilitating certain chemical reactions, and many more such usage.

How does heat flows from one place to another?

As we perceived that heat is a form of energy, the next thing which comes in mind is that how it moves from one place to another. In severe winters we like to be near the fire to enjoy the warmth we get from it. The heat from the fire source comes to us through the radiation of this energy from the fire source to our body. This is the way that we also get the heat energy from sun which is of course is the source of all warmth and life on Earth. Existence of life on Earth without sun energy can not be imagined.

Let us now see what are the different modes in which heat energy can travel from one place to another. There are three ways in which it can happen and these are -

  • Radiation.
  • Convection.
  • Conduction.

The simplest example of radiation is the heat energy coming from Sun to Earth in form of light rays. In other ways we can say that light waves reaching Earth are heating it up. They are heating up everything including the atmosphere through which they are passing and reaching the Earth surface. We can use a solar cooker to cook food using this energy. It is surprising to note as how much powerful source of energy the Sun is as an infinitesimal part of it is only reaching Earth and is able to elevate the low night temperatures to scorching day temperatures. From a rough estimate only about one billionth part of Sun's energy reaches Earth but it is so effective for our life here.

When we heat a liquid then the molecules of liquid take this energy and transfer it to other molecules. As all the molecules in a liquid are in random movement (known as Brownian motion in Physics), this sets a heat current known as convection current in the liquid medium. Heat flows in the liquids in this convection mode. Even in mediums like air or gas heat creates convection currents and heats up the medium.

The third is conduction and it happens when heat transfer takes place through a solid or a semi solid or any intermediate material. In this mode the heat propagates through the elementary molecular structure of the material. For example if we heat an iron rod at one end, slowly the rod would get warmer and we can touch the other end to feel the heat. Conduction of heat is a property of the material and some of them conduct nicely while others are not a good conductor of heat. When heat can move through a material quickly and in sufficient quantity then we say that the material is a good thermal conductor. If it moves through it very slowly and gets thermal resistance then we say that it is a poor thermal conductor of heat. And finally if it does not flow through it then we say that the material is a thermal insulator. That is why we require thermally insulated jackets in winters. Even wollen pullovers provide almost similar protection to us.

Loss of heat

Hot bodies lose their heat through radiation, convection and conduction. As we learned that heat flows from higher temperature body to lower temperature one, it can be understood that the temperature of the body at higher temperature will go down and the body which is rceiving this heat would warm up when they are kept together. This would continue till there is an equilibrium. In many cases it might happen that both the bodies come to the same resultant temperature and then the heat flow ceases. In such cases we say that first body lost heat while the second one gained.

What do we understand by temperature

When we heat a material then we have many times observed that its temperature rises and some of the materials may change their state also from solid to liquid and then from liquid to gas. During the process of heating it acquires this heat energy and then its temperature starts rising. So temperature is an entity which reflects as how much heat is there contained in a body when it is at an equilibrium with its surroundings. If the body loses heat to its surroundings then its temperature starts decreasing and if it gains heat energy from the surroundings then its temperature rises.

To understand the concept of temperature let us start with a block of ice or snow. We take its temperature as 0 (zero) degree Centigrade (known as celsius degree also) which is the common unit of temperature. There are other units also and we would come to that shortly. Let us just melt this snow or ice block and it will convert into the water having temperature of 0 degrees only until we further heat it and increase its temperature. It is interesting to note here that we have given some heat energy to the ice block to melt it but it changed its state from solid to liquid and its temperature remained 0 only. This heat taken by the ice block to transform its state is called latent heat. Now let us heat this cool water of zero degree slowly and increase its temperature till it finally starts boiling. That is the point which is taken as 100 degree Centigrade and thus we have a temperature scale with us where we can compare the temperature of other bodies falling in between these two points. This is the common centigrade scale for temperatures used however other popular units are also there.

Another popular unit for temperature is Fahrenheit and what we represent as 0 degree Centigrade is represented in this unit system is as 32 degree Fahrenheit. Similarly what we represent as the boiling water temperature as 100 degree Centigrade, is represented as 212 degree Fahrenheit. Being linear quantities it is easy to convert from Fahrenheit to Centigrade and vice versa. The formula for this conversion is very simple and one can derive it also using basic algebra. If F is the equivalent Fahrenheit temperature for C Centigrade then the formula can be written as -

(F-32)/9 = C/5

Human body temperature is conventionally measured in Fahrenheit and many of us are well aware of it that it is around 98.4 degree Fahrenheit. Using the above formula we can convert it to degree Centigrade. It would come to 36.8 degree Centigrade. Due to biological and chemical processes in our body it keeps its temperature constant irrespective of outside temperature but any dead material would not be able to do so and its temperature would change as per the surrounding temperature.

Heat and temperature

What is absolute zero temperature?

If we go to polar regions on Earth then the temperature there is much below zero degree Centigrade and is measured in minus degrees, say minus 40 degree centigrade. Now an interesting question rise here as what can be the lowest temperature attainable.

Scientists have found that if we decrease the temperature of any material then there is a limit to that as a point comes when we can not cool it down further and this point is known as absolute zero. Its value is determined as -273 degree centigrade. This gives rise to another temperature scale known as Kelvin scale. So, when we say zero degree Kelvin it means -273 degree centigrade. Going further in this scale we get 0 degree centigrade equal to 273 degree Kelvin and 100 degree centigrade as 373 degree Kelvin. In low temperature Physics it is common to use Kelvin scale.

What is temperature gradient?

Heat moves from higher temperature to lower temperature in a medium or directly also through radiation and if we measure the temperature across various points in the path of this heat travel then we would find a gradual change of temperature from high to low point. This is technically known as temperature gradient. Let us understand it with a practical example of temperature gradient inside Earth. Due to the heat coming from Sun, Earth attains a temperature in the day time and in the night the temperature goes down. This cycle continues. Have you ever thought what other things are there in the Earth which affect its temperature. The answer is that deep inside the Earth there is molten lava on which the upper solid layers of Earth literally rest or scientifically speaking float. This is the same lava which sometimes erupts from a volcano. Now the temperature of Earth deep down is very high and due to that some heat tries to flow out of it towards Earth surface passing through the thick Earth crust. The Earth surface is relatively at a lower temperature so this heat flow is a continues process.

Earth core temperature is very high and estimated in the range around 5000 to 7000 degree centigrade. It is really very hot something like more than molten iron! Earth has many more intermediate layers above the core and finally the solid crust is there. The distance from centre of Earth to surface is about 3200 km and seeing this huge distance and the thermal conductivity of the rocks beneath it is easy to visualise the low temperatures on Earth surface in spite of some heat coming from core to outside. This is the temperature difference which gives rise to a temperature gradient when we dig a deep well (like we drill in oil and gas exploration) and measure the temperature inside it at various depths. This is known as geothermal gradient and on an average has a value of 25 to 30 degree centigrade per 1000 metres. From this it is imperative that for drilling deep Wells in petroleum upstream industry, we should have high temperature drilling machinery.

Concept of specific heat

Different materials respond to heat differently. Same heat energy will increase the temperature of different materials by different degrees. In Physics we define specific heat as the ratio of heat required for a body to increase its temperature by one degree centigrade to that required to increase one degree for the same mass of water. Now, water by definition has specific heat equal to 1. It is the heat required to increase the temperature of 1 gram of water by 1 degree centigrade. Incidentally this amount of heat is known as 1 calorie in units of heat.

If we give heat energy Q to a body of mass m having a specific heat of s and its temperature is raised by Δt then mathematically we can represent the relationship as follows -

Q = m x s x Δt


Q = msΔt

Quick exercise:

Q. 100 gram of hot water at 80 degree Centigrade is mixed with 2 litres (2000 gram) of water at room temperature of 22 degree Centigrade. Calculate the temperature of the mixture assuming that no heat is lost in the surroundings.

A. Let the final temperature after mixing is t.

Now the 80 degree water has come down to temperature t and hence the heat it has given out would be -

100 x 1 x (80-t) _ _ _ _ _ _ _ _ _ _ (A)

Also the 22 degree water has come up to temperature t and thus the heat taken by it would be -

2000 x 1 x (t-22) _ _ _ _ _ _ _ _ _ _(B)

If there is no heat loss in the surroundings then these two things A and B should be equal to each other. So -

100 x 1 x (80-t) = 2000 x 1 x (t-22)

Solving it for t, we get t = 24.7 degree Centigrade. Which means that a small amount of hot water can not increase the temperature of a bigger volume of water much and that is why we get only a nominal increase of 2.7 degree in this case.

Units of heat

CGS Unit:

In the conventional old system of units that is CGS (centimetre, gram, second), the unit of heat is expressed as calorie (cal) and 1 cal is the heat energy needed to increase the temperature of 1 gram of clean water by one degree Centigrade (celsius). The bigger unit is kilocalorie (kcal) where 1 kcal = 1000 cal.

SI Unit:

In SI system, the unit of heat is Joule. In fact in SI system the unit of energy itself is Joule (defined as work done by a force of 1 Newton which acts in 1 metre distance) and same is used to denote heat energy also. Actually 4.184 Joule of energy is required to raise the temperature of 1 gram of water from 0 to 1 degree Centigrade. Which means that 1 calorie = 4.184 Joule.

BTU Unit:

There is also one imperial unit of heat which is known as BTU (British Thermal Unit represented as Btu also). This is the amount of heat required to raise the temperature of one pound of water through 1 degree Fahrenheit.


1 cal = 4.184 Joule

1 cal = 3.968 x 10-3 BTU

1 Joule = 0.239 cal

1 Joule = 9.478 x 10-4 BTU

1 BTU = 1055.06 Joule

1 BTU = 252 cal

Some examples related to heat

Thermos flask is one common example of keeping heat preserved for long times where it is achieved by isolating it from the outside environment. So using a glass bulb or double walled metal container with non conductor material in between the double walls does the trick. The opening lid of container is also designed in a special way by providing air tight gaskets or 'o' rings to help stopping the leakage of heat from the thermos flask. Good quality thermos flask can retain the hot water or other liquids hot for more than 24 hours which is quite remarkable.



Heat is a form of energy and very important for the existence of life on our planet Earth. Heat moves from high amounts of it to low and the movement speed and value depends on the way it propagates from one point to other and how the particular medium allows it to flow. A conductor of heat like a metal would allow heat to move through it quickly while a less conducting or thermal insulator will impede its flow significantly.


1. https://www.britannica.com/science/heat

2. https://www.physicsclassroom.com/Class/thermalP/u18l1d.cfm

3. https://www.topperlearning.com/frank-solutions/icse-class-10-physics/frank-modern-certificate-physics-part-ii/heat-exercises-and-mcq

4. https://www.engineeringtoolbox.com/heat-units-d_664.html

5. https://byjus.com/physics/unit-of-heat/

6. https://en.m.wikipedia.org/wiki/Geothermal_gradient#:~:text=Geothermal%20gradient%20is%20the%20rate,in%20most%20of%20the%20world.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Umesh Chandra Bhatt


Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on November 04, 2020:

Dale, thanks a lot.

Dale Anderson from The High Seas on November 03, 2020:

I read one of your other physics hub articles and enjoyed that one too. Please keep these coming.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 14, 2020:

SKMUNSHI, thanks for your elaborate comments.

SKMUNSHI on September 14, 2020:

This article on introduction to heat tranfer is nicely written and will be of immense help to readers to get introduced to to this important subject on energy tranfer.The author has introduced all important cocepts on the subject in a systematic way and introduced the urge to kow more on the subject .Great!

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on September 10, 2020:

Anurag, thanks a lot.

Anurag on September 10, 2020:

A very interesting and conceptual article cum lesson. You have precisely covered the important and basic aspects of Thermodynamics!! Its quite refreshing!! Hoping to see more lessons from you in upcoming article!!

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on August 02, 2020:

Flourish, thanks for your visit. Appreciate.

FlourishAnyway from USA on August 01, 2020:

Students should find this very useful.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on July 31, 2020:

Anurag, thanks a lot.

Anurag on July 31, 2020:

Very insightful lesson.

Thorough coverage of basic thermodynamics done.

Found really useful for clearing and getting revision of basic thermodynamics.

Thanks for sharing.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on July 29, 2020:

Linda, thanks for your encouraging comment.

Linda Crampton from British Columbia, Canada on July 29, 2020:

Thanks for sharing all the physics facts, Umesh. They will be very useful for students.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on July 29, 2020:

Mark, thanks a lot.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on July 29, 2020:

Denise, thanks for your visit.

When we go up in the atmosphere it is thinning up and heat absorbed by it also reduces and so its temperature is less. Eventually it reaches very low and this creates a temperature gradient and hills sre cooler.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on July 29, 2020:

Liz, I feel so humble with you encouraging comment. Thanks.

Mark Tulin from Ventura, California on July 29, 2020:

Thank you for the physics lesson. Fascinating.

Denise McGill from Fresno CA on July 29, 2020:

It is quite the science lesson. Are there any real practical uses for the everyday? Like why is it cooling in the hills or the mountains than the valleys? They are closer to the sun and should be hotter, right? Is it because the air is thinner there? Just wondering.



Liz Westwood from UK on July 29, 2020:

You have a gift for explaining physics in a much more easily comprehensible form than my teachers in the past.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on July 29, 2020:

Ann, thanks a lot for your nice comment. So kind of you.

Ann Carr from SW England on July 29, 2020:

At school, we had lessons in Chemistry, Physics and Biology, all a complete mystery and with boring teachers. Physics was the only one which held a little interest for me, mainly as my father could help me with the homework! I failed my Physics "O" level. All my other subjects were good or very good. I think some inspired teaching would have helped a lot.

Now I understand a lot more as an adult and your explanation is comprehensive. Thanks.