Free Tutorial in Basic Physics - Heat and Temperature

There are many basic concepts in Physics, understanding of which helps the students to gain knowledge in this subject. Learning about heat energy and its transmission from one place to other is one of such areas and this article tries to explain it in detail.


In summertime all of us yearn to go to hill stations to enjoy the cool and pleasant climate there. Have you ever thought about why hills are cooler than the plains? In the places where severe winters are there, people use heating devices or like to sit near the fire. What makes the temperature of a place high or low? We would be discussing and learning all these aspects of heat and temperature in this article.

What is heat

In Physics we define heat as a form of energy. When we go near a fire we feel hot. When we go in the sunlight we feel hot. It means that the heat energy from that source of heat is reaching us and falling on our body and we start feeling warmer. When we come out of an AC room we feel hot.

Burning wood or coal creates fire and that is a source of heat for us and we can use it for a variety of purposes from cooking food to heating water for taking a hot water bath. If we keep a thick sheet of asbestos sheet or cement sheet between the fire and our body then the heat will not reach us and will be blocked by the sheet and we would not feel it.

What is temperature

Temperature is a measure of heat only and gives us an idea of relative values of heat in different places. When we go to a cool place we wear woollens or thick cotton dresses which help us in containing the heat of our body as thick clothes will not allow the heat to go outside and we would feel warm. If we do not wear sufficient clothes in a cool place then our body will radiate more heat outside and we will start feeling cold. This difference between our body and the outside environment can be understood better in terms of temperature and when we say that the temperature outside is low it means that our body would radiate heat to the outside and at the same time if we say that the outside temperature is high then it simply means that our body is receiving heat from outside. It also gives us an idea that the heat will flow from a region of high temperature to low temperature. If we open the door of an AC room for a long time that as the temperature outside is more the heat from outside will enter the room and soon the room would start warming up. So differences in temperature and flow of heat from one region to another are very strongly linked directly.

There are some interesting aspects about the temperature and it would be worthwhile to learn them before going ahead. Let us take an example of an ice cube which is said to be at zero degrees centigrade. If we slightly heat this it would become water and interestingly the temperature of the water is also zero degrees centigrade. So whatever heat we gave it, was used to transform it from the ice to water form. Now if we further heat it then its temperature would rise and slowly it would warm up and we can further heat it to the boiling point of water which is 100-degree centigrade. At that point, water starts converting into steam and if we continue heating then all the water would soon evaporate as steam. It is important to note here that heat is required to change the form of the materials as well as raise their temperatures.

How does heat move from one place to another

Heat is a form of energy and it moves from one place to another in the following three ways -
1. Radiation.
2. Conduction.
3. Convection.

The most common example of heat moving from one place to another under the radiation mode is the energy coming from Sun to Earth. It is interesting to note that Sun is so far from us but the energy in the shape of light waves reaches Earth and is sufficient to keep the Earth warmer to allow life to survive on this unique planet in the universe. Another point worth noting is that Sun radiates its heat or sends its light to all the directions in the three dimensions and Earth is so far and receives a minuscule (very very little) of that energy which is surprisingly sufficient for its life and habitat. Imagine how much total energy Sun would be radiating in all directions.

The second mode of transfer of heat energy is conduction. When we heat a metal rod at one end then after some time due to conduction of heat from one point to another the other parts of the metal rod also get heated up. It will depend upon the conductive properties of the medium as how much time it will take in happening that or how much heat will propagate in that way. For example when we burn a long wood piece then we can easily touch its other part because wood is not a good conductor of heat though it burns well. That is why the holders of metallic utensils are made of plastic or ebonite material which does not conduct heat and we can hold the utensils through them. Another example of conduction is the heat coming from the interior of Earth to the surface. We all know that the Earth's interior is full of lava and magma which is very hot and is in a molten state. Due to its high temperature, the heat starts flowing from its inner core to outwards but the flow is very less as the solid rocks making the Earth crust are not very conductive. Still, some amount of heat always comes out from the Earth and that is an important factor in maintaining the surface temperatures. When we go up in the atmosphere then the temperature starts decreasing and becomes very low when we reach at greater heights in the atmosphere. That is the reason why the hills are cooler than the plains.

The third mode of heat flow is convection which generally happens in liquids. When we heat a liquid kept in a pan then due to the heat there is a movement of particles in the liquid and then heat transfers from one place to another in that liquid and these are called convective currents maintaining the heating up of the liquid uniformly.

Heat loss and heat gain

When we have a body at high temperature say a hot metal piece having a temperature say 80 degrees centigrade and we simply keep it on the marble floor then slowly the metal piece will lose its heat through radiation as well as some conduction to the marble floor where it is kept. The result will be that the metal piece will slowly lose its heat to its surroundings and finally after some time it will come down to the room temperature which is generally around 20 to 28 degrees centigrade depending upon the time of the year. When that happens we say that the metal piece has cooled down and has come to room temperature. During this process, the metal piece lost a lot of heat. We could use that heat for heating something else like say we put that hot metal piece in normal water say at 25 degrees centigrade and by the time the metal piece loses its heat the water gets warmer and both of them reach a common temperature (say 36-degree centigrade) which is above the normal water temperature. Now if we keep this just like that for some time then both of them will start losing their heat to their surroundings and finally come to the normal room temperature. So heat loss from a body depends mainly on the factor that is the difference between its temperature and the surrounding environment temperature.

Students often ask a question here if Sun is radiating so much heat then why its temperature is not going down. To understand this we should know what is the source of energy and light generation in the Sun. The source of this great natural resource is nuclear reactions in Sun and as the Sun is a massive body there is so much material available in it for those reactions that it is going to provide us with this light for billions of years to come. It looks amazing but is a scientific fact based on scientific observations. Scientists have calculated that Sun would be giving us light for about 7.5 billion years!

Units of temperature and heat

There are mainly two units of temperature which are common and popular throughout the world. One is Centigrade and the other is Fahrenheit. The temperature of ice is 0 degrees Centigrade while boiling water is 100 degrees Centigrade. If we use the Fahrenheit unit then the ice temperature will be 32 degrees Fahrenheit and the boiling water temperature will be 212 degrees Fahrenheit.

It is interesting to note that we measure our body temperature in Fahrenheit though nowadays thermometers with Centigrade scale are also available in the market. Conventionally we are habitual of the Fahrenheit scale for measuring the body temperature and the normal human body temperature is about 98.4 degrees Fahrenheit. It is equal to 36.8 degrees Centigrade.

There is a simple quick conversion formula for converting temperatures between Fahrenheit and Centigrade scales and the formula is -
(F-32)/9 = C/5
F is the value of temperature in the Fahrenheit scale and C is the value in the Centigrade scale.
Let us use the above formula to find out the temperature of a patient in Centigrade if we are given that his temperature in Fahrenheit scale is 103 degrees.
(103-32)/9 = C/5, solving this simple equation we get C=39.4 degrees Centigrade. It is obvious that as we are accustomed to the Fahrenheit scale for measuring fever, the Centigrade scale would be inconvenient for us.

Heat is generally measured in calories but there are other units also for it. Internationally SI units are used for heat but CGS and BTU units are also in use. In the SI system, the heat is having units as Joule. It is defined as the work done by the 1 Newton force which acts in a 1-metre distance. In the CGS system, the unit of heat is calorie and 1 calorie is defined as the energy required to increase the temperature of 1 gram of water by one degree Centigrade. In BTU (British Thermal Unit) system the unit of heat is Btu and 1 Btu is defined as the amount of heat required to raise the temperature of one pound of water through 1 degree Fahrenheit.

Some important conversions between these different units are as follows -
1 calorie = 4.184 Joule
1 calorie = 3.968 x 10^-3 BTU
1 Joule = 0.239 calorie
1 Joule = 9.478 x 10^-4 BTU
1 Btu = 1055.06 Joule
1 Btu = 252 calorie

Some examples related to heat and temperature

  • We all use thermos flask in our houses. It is generally made up of double-walled glass or special metal alloy with a lid and seal washer at one end. This arrangement helps in checking the heat of the hot water or tea kept in the flask to go out and that is the reason that the liquid inside remains hot for a longer time.
  • In winters we wear woollen clothes, cap, muffler etc to retain the heat of the body and avoid transfer of heat to the outside low-temperature environment.
  • In summer we use air-conditioning to decrease the temperature inside a room so that we feel comfortable in it.


Heat is a form of energy and it moves from one place to other depending upon the temperature difference between the two. Heat always flows from a higher temperature area to a lower temperature area. If we mix very hot boiling water with lukewarm water the result would be in between as the very hot water will transfer the heat to lukewarm water and the mixture would come to a temperature in between making the whole thing as simply warm water.

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