Heat: Definition, Mode and Effect of Heat Transfer

We all understand that fire is hot, and the ice is cold, but how do we get the sensation of hotness or coldness? What do we mean by Heat? What is the amount of heat of a body? How does the heat get a transfer from one body to the other? What is the effect of the transfer of heat? Which is more dangerous, water at boiling temperature or steam at boiling temperature?

Heat is one of the most important types of energy for life on Earth to survive. According to thermodynamics, heat is transferred from one body to another due to temperature differences. Cooking, ironing, recreation, transportation and other activities all require heat energy. This type of energy is also important in nature. Let us find out the answers to these questions in the given article.

What is Heat?

Heat is the form of energy that gives us the sensation of hotness or coldness. If the heat is transferred out of the body, we feel cold, and if the heat is transferred to the body, we feel hot. We can understand it more clearly with the following example.
During winters our surrounding is colder than our body’s temperature, and the heat is transferred out from our body to the surroundings. Due to this loss of heat, we feel cold. During summer the surrounding is hotter than our body’s temperature, and thus the heat flows from the surrounding to our body and thus due to this heat gained we feel hot.
We cannot determine the amount of the heat of a substance or the total heat content of the substance, but we can only determine the heat lost or heat gained, i.e., we can only measure the amount of heat that gets transferred from one body to the other.

Mode of Transfer of Heat

We now know that we can only measure the amount of heat that gets transferred, but how does the heat gets transferred?
Heat can be transferred through three different modes,

1. Radiation
2. Conduction
3. Convection

Learn About Heat Transfer

Conduction

The metal part gets hotter when put on a stove. This is because the heat is transferred through the bulk of the metal, which means the heat travels through the medium. This mode of heat transfer is known as conduction.
Conduction:  Conduction is a process of transfer of heat in which the heat is transferred through the bulk of matter or medium. This process is similar to that of conduction of electric current through a conductor.

Convection

The bulk of the water gets the heat by the movement of the molecules of water itself. This method of transfer of heat is known as convection.
Convection: Convection is the process of heat transfer in the case of fluids where the molecules are to move. Fluids in motion carry the heat as they move, with them in this method.
This mode of heat transfer only exists in fluids.

Radiation

The heat transfer from the sun to the earth without the help of any medium happens through radiation.
Radiation: Radiation is the process through which the heat is transferred without the requirement of any medium is known as radiation. In radiation, the heat is transferred as electromagnetic waves.

Effect of Heat Transfer

The transfer of heat can bring out various changes depending on the situation. For example:
Transfer of heat can increase or decrease the temperature or internal energy of the body. It can change the state of the substance from solid to liquid or liquid to gas or vice versa, depending on the direction of heat flow. Also, when heat is supplied to matter, they expand, changing their dimensions.
Apart from these common effects, heat can change various physical properties like malleability and ductility, the surface tension of fluids, resistivity, viscosity, etc.

Change in Temperature or Internal Energy

If heat is transferred from the substance to the surrounding, the temperature of the substance decreases, but if the heat gets transferred to the substance from the surrounding, then the temperature of the substance increases. Internal energy directly depends on the temperature; therefore, as the temperature changes, the internal energy also changes.
The magnitude of change in temperature depends on the specific heat capacity of the substances.
Specific heat capacity is an intrinsic property of a material. That is, it is independent of the amount of substance. It is defined as the amount of heat required to change the temperature of \(1\;{\rm{Kg}}\) of material by \(1\) Kelvin.
Relation between the heat transferred and the temperature change is given by,
\(Q=m S \Delta T\)
Where,
\(m\) is the mass of the substance.
\(S\) is the specific heat capacity of the substance.
\(\Delta T\) is the change in temperature

Change of State

When a substance is at its boiling point and if we heat it further, the temperature of the substance doesn’t change; instead, the state of the substance changes. 
Example:
If we heat a pan with water, then the temperature of the water increases till it reaches the boiling point; thereafter, if we still provide more heat, the water starts to convert to steam, and the temperature of the water remains constant. This happens at the temperature where the substance changes its state.
The amount of heat required to change \(1\;{\rm{Kg}}\) of substance from liquid to gaseous state is known as Latent heat of vaporisation.
The amount of heat required to change \(1\;{\rm{Kg}}\) of substance from solid to liquid is known as Latent heat of fusion.
Sometimes the substance directly gets converted into a gas from the solid state. For example, carbon dioxide directly gets converted into the gas form from solid-state. This phenomenon is known as sublimation, and heat required in changing the state is known as latent heat of sublimation.
The heat required to the change of state of a substance is given by,
\(Q=m L\)
Where,
\(m\) is the mass of the substance whose state gets converted.
\(L\) is the latent heat of the substance.

Change in Dimension

Due to heat transfer, the dimension of the object can also change. This phenomenon is known as thermal expansion.
According to the law of thermal expansion, the increase in the length of an object is directly proportional to the change in temperature and the length of the object.
\(\Delta l \propto l \Delta T\)
\(\Rightarrow \Delta l=\alpha l \Delta T\)
Where,
\(\alpha\) is the coefficient of thermal expansion
Thermal expansion can be categorised into three categories.

1. Linear expansion

\(\Delta l=\alpha l \Delta T\)

2. Superficial expansion

\(\Delta A=\beta A \Delta T\)

3. Volume expansion

\(\Delta V=\gamma V \Delta T\)

Application of Heat

Thermal power plants convert the heat energy into electrical energy

Heat energy is used to run steam engines

Heat energy is used to run diesel engines

Sample Problems

Q.1. Find the amount of heat required to raise the temperature of \(2 \mathrm{~kg}\) of water by \(5\) Kelvin. The specific heat of water is \(4.186 \mathrm{~J} \mathrm{~kg}^{-1} \mathrm{~K}^{-1}\)
Sol
Given,
Mass of the water is \(2 \mathrm{~kg}\)
Change in temperature is \(5\) Kelvin.
We know that the heat required to change the temperature is given by,
\(Q=m S \Delta T\)
Where,
\(m\) is the mass of the substance.
\(S\) is the specific heat capacity of the substance.
\(\Delta T\) is the change in temperature.
Putting in the values we get,
\(Q=m S \Delta T\)
\(\Rightarrow Q=2 \times 4.186 \times 5\)
\(\Rightarrow Q=41.86 \mathrm{~J}\)

Q.2. Find the amount of heat required to change \(10 \mathrm{~kg}\) of water into steam. The latent heat of vaporisation of water is \(2250 \mathrm{~kJ} \mathrm{~kg}^{-1}\)
Sol,
Given,
The mass of the water is \(10 \mathrm{~kg}\)
We know that the amount of heat required to change water into steam is given by,
\(Q=m L\)
Where,
\(m\) is the mass of the substance whose state gets converted.
\(L\) is the latent heat of the substance.
Putting in the values, we get,
\(Q=10 \times 2250 \times 10^{3}\)
\(\Rightarrow Q=2.25 \times 10^{7} \mathrm{~J}.\)

Summary

In this article, we learned about heat. We discussed the different modes of transfer of heat, that is, conduction, convection, and radiation. We also came to know that convection can occur only in fluids where the medium particles are to move, and a medium is not required for the transfer of heat by radiation. We also discussed the effect of the transfer of heat. Apart from these, many properties such as resistivity, viscosity depend on the temperature, which can change due to the transfer of heat.

PRACTICE QUESTIONS RELATED TO HEAT

Frequently Asked Questions

Q.1. What is heat?
Ans: Heat is a form of energy that gives us the sensation of hotness or coldness.

Q.2. Does heat transfer require a medium?
Ans: Heat transfer can happen through different modes. One of them is radiation, and radiation does not require a medium. The heat from the sun reaches the earth via radiation as heat travel as electromagnetic radiation.

Q.3. What is specific heat capacity?
Ans: The specific heat capacity of a substance is the heat required to change the temperature of \(1 \mathrm{~kg}\) of that substance by \(1\) kelvin.

Q.4. What is latent heat?
Ans: The latent heat of a substance is the amount of heat required to change the state of a \(1 \mathrm{~kg}\) of a substance at the boiling orzing point.

Q.5. What is sublimation?
Ans: Sublimation is a process in which a substance directly gets converted into a gaseous state from a solid state.

We hope you find this article on ‘Heat’ helpful. In case of any queries, you can reach back to us in the comments section, and we will try to solve them.