What is Heat in Physics

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What is Heat of Fusion?

Heat is one of the essential energy forms on Earth for the survival of different lives. Heat transfer takes place from one body to the other because of the difference in temperature according to thermodynamics. We use heat energy in our day to day activities such as cooking, transportation, ironing, recreation and much more. Heat energy also plays a crucial role when it comes to nature. The occurrence of rain, wind, change in the seasons, etc. is all dependent on the gradient that is created because of the uneven heating of various regions. In this article, we will discuss what is meant by heat, what is Latent heat and what is used to measure heat.


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What is Heat in Science

When the temperature of a body increases, the vibrations of the atoms or molecules tend to increase. These vibrations then transfer from one part of the body to the other. The measure of the energy with which these atoms or molecules vibrate in the given system is known as the heat stored in that particular object.


According to the definition of heat, it is referred to as the flow of energy from a warmer object to a cooler object. The direction of the heat flow occurs in the substance having a higher temperature to the substance having a lower temperature. This happens because the molecules that vibrate faster transfer their energy to the molecules that vibrate slower. This vibrational energy is also called heat content. The heat content in a body makes that either hot or cold. The higher is the heat content, the hotter is the body.


A substance can even absorb heat without undergoing an increase in the temperature when it changes from one physical state to the other. During the process of melting, the substance changes from the solid-state to the liquid state. During the process of sublimation, solid tend to get converted to vapours. Heat is an energy form which can also be converted to work. Hence, the amount of energy is expressed in the same units as that of work. Energy is either expressed in terms of joules, calories, kilowatt-hours or foot-pounds.


Heat can also be converted to different other forms of energy. For example, the heat energy gets converted to the mechanical energy in a motorized vehicle. Similarly, in the electric bulbs that we use in our day to day life, the electrical energy gets converted to the light energy.


Let us consider an experiment as follows.

We will take three beakers, the first one having cold water, the second one containing hot water and the third one having water which is at the normal temperature. Now, if we dip one finger in the first beaker and then immediately into the third one, we would notice that the water in the third beaker is comparatively hotter than the previous one. If we dip our finger in the second beaker and then in the third one, we would notice that the water in the third beaker would be comparatively colder than the second one. We can therefore conclude that we cannot depend on our sense of touch for judging the level of hotness or coldness of an object. Hence, this is the reason the degree of the hotness or coldness of an object is measured in temperature.


Temperature refers to the measure of the hotness or coldness of the body. The different temperature scales are Celsius (C) scale, Fahrenheit (F) scale, or Kelvin scale (K).


Classification of Heat

Heat is classified as hot and cold.

  1. Hot

The objects having a higher heat content are referred to as hot objects (the hotness or coldness of a given object is a relative term and hence, is always measured in regards to a reference object). Some examples of the hot objects around us are the sun, hot pans, fire, lava from the volcanic eruptions, air from the hairdryer, etc.

  1. Cold

The objects having lower heat content are referred to as cold objects. The examples of the colder objects around us that we see are ice, cold drinks, the air from an air conditioner, etc.

FAQ (Frequently Asked Questions)

Q1. What is the Heat of Reaction?

Ans. The heat of reaction refers to the amount of heat added or subtracted in a chemical reaction for keeping all the substances present in it at the same temperature. If the pressure in a vessel that consists of the reacting system is at a constant value, the heat of reaction measured tends to represent the change occurring in the thermodynamic quantity. This is known as enthalpy or the heat content, which accompanies the process. This refers to the difference between the enthalpy of all the substances that are present at the end of the reaction and the enthalpy of the substances that are present at the beginning of the reaction.


Hence, the heat of reaction that is determined at the constant pressure is also referred to as the enthalpy of the reaction and is denoted by the symbol ΔH. In case the heat of the reaction is positive, the reaction is called an endothermic reaction, and if the heat of the reaction is negative, it is called an exothermic reaction.

Q2. What is Heat Measured in?

Ans. Since heat is a form of energy, it is denoted as joules (J) according to the International System of Units (SI). However, in many different applied fields of engineering, calories and British Thermal Unit are often used. The standard unit for the heat transfer rate is called watt (W), which refers to one-joule per second. The heat that is released by a system into the surroundings is a negative quantity by convection, i.e., Q < 0. However, when the system tends to absorb heat from its surroundings, it is a positive quantity i.e., Q > 0. Hence, the heat transfer rate, or the heat flow per unit time is denoted by the symbol Q.