definition of heat

Heat is the process through which thermal energy is transferred between bodies. In this section we will study

What is the heat

How heat is related to the change in temperature through:
The heat capacity
The specific heat
The fundamental equation of terminology
How heat relates to changes in the state of aggregation
What happens when two bodies at different temperatures come into contact
The mechanisms of heat propagation
Heat concept

The particles of the bodies are not at rest but are in constant agitation. As a consequence of this agitation, the bodies have a certain thermal energy. Temperature is an indicator of the thermal energy that bodies have. In a general way we can say that, at a higher temperature, more energy of this type. Well, bodies and systems can exchange thermal energy. This heat energy exchanged is called heat. Sometimes heat is also called the process of energy transfer.

Heat is the energy exchanged between a body and its environment due to being at different temperatures. Heat, like work, is energy in transit, so it can also be understood as a method to transfer energy.

Is heat the same as temperature?

definition of heat No, heat and temperature are not the same thing. It is very possible that this idea seems strange to you because every day we usually confuse it. However, we propose the following experiment.

Fill two pots of water, one with half or one third of water the other
Put both on a flame of equal intensity. Record the time it takes for each of them to start boiling
Measure the temperature of each one at the moment they start to boil
If you do, you will see that:

The one that has less water begins to boil before, or what is the same, has needed less energy to reach the boiling point.
If you measure the temperature when you start to boil in both cases, both temperatures are equal (around 100 ºC).
This is because the fire transfers energy to the pot and this in turn to the water. We know that transmitted energy as heat. In the one that has less water, less heat has been used to reach the same temperature. Therefore, temperature and heat are not the same thing.

Heat transfer in a pan

Heat capacity
When you put oil in a pan and light the fire, the oil will begin to increase its temperature. In general, when we supply heat to a body it can increase its temperature.

The heat capacity of a body is the relationship between the heat supplied to the body and its temperature increase. It can be calculated through the expression:


C: Heat capacity. It is the amount of heat that the body has to exchange with its environment so that its temperature varies a Kelvin. Its unit of measurement in the International System is July for kelvín (J / K), although the calorie is also frequently used per degree centigrade (cal / ºC)
Q: Heat exchanged. Amount of thermal energy exchanged with the environment. Its unit of measurement in the International System is July (J), although calorie (cal) is also frequently used. 1 cal = 4,184 J
ΔT: Temperature variation. It is determined by the difference between the initial and final temperature ΔT = Tf -Ti. Its unit of measure in the International System is kelvín (K), although Celsius or celsius (ºC) is also commonly used
You can understand heat capacity as the difficulty with which a body increases its temperature when we supply a certain amount of heat. Thus, the higher the heat capacity, the lower the temperature increase for a given quantity of heat supplied. Thermal insulators have a high heat capacity.

What does the heat capacity depend on?

The heat capacity of a body depends on two factors:

The substance by which the body is formed: Do not increase its temperature in the same way a gram of water as a gram of oil or a gram of iron, even when placed on a fire of equal intensity: Iron would be the first to increase its temperature, followed by oil and finally water
The amount of body mass: As you have seen in the previous experiment, does not increase its temperature in the same way a gram and a kilogram of water, even when placed on a fire of equal intensity: one gram of water will vary its temperature more quickly than a kilogram of this same substance
These two factors allow us to define the specific heat of a substance.

Propagation of heat

There are 3 mechanisms by which bodies exchange their thermal energy: conduction, convection and radiation. We are going to study them.

In the conduction two bodies that are directly in contact (or parts of a same body at different temperature) exchange energy without there being material transport. It is based on the direct transfer of kinetic energy at the molecular scale. Each substance has an associated value of thermal conductivity that indicates how quickly it is able to transfer energy. Its unit of measure in the International System is watt per meter and per degree kelvin (W · m-1 · K-1). The substances can be:

Good thermal conductors: They have a high conductivity
Bad thermal conductors: They have a low conductivity
In the previous example of thermal equilibrium, the two boxes exchange heat per conduction.

Heat transfer mechanism: driving

It is produced in liquids and gases. In this case if there is material transport. The heat propagates due to currents called convection currents that occur between different parts of the substance. Those at higher temperatures (because they are in contact with the heat source) decrease their density with respect to the colder ones. The less dense (hotter) parts move to the upper layers and the denser (colder) to the lower layers. Over time, the process is repeated as the upper layers cool and the lower layers cool, thus giving rise to a convection current. This phenomenon occurs, for example, when we heat a water in a pot.

Heat transfer mechanism: convection


It is produced by the propagation of electromagnetic energy through space. The presence of matter is not necessary. For example, when the Sun heats the Earth, its energy travels a great distance in empty space. The sun is a clear example of a body that emits electromagnetic radiation due to its temperature. However, all bodies, due to being at a certain temperature, emit thermal radiation. This radiation is electromagnetic in nature and, in the case of very hot bodies, such as the sun, is within the range of visible light. For other objects the radiation is invisible to our eyes.

Definition of heat in chemistry


The heat and temperature are not synonymous, we can say that they are strictly related since the temperature can be determined by the amount of heat accumulated, heat is a physical phenomenon that raises the temperature and dilates a body, the heat it has is the sum of the kinetic energy of all its molecules. So to speak, heat is responsible for the movements of molecules regardless of whether they belong to a gas, a liquid or a solid, when the heat increases, then the energy of that body will increase.


Heat is the amount of kinetic energy, it is an expression of the movement of the molecules that make up a body. When heat enters a body, heating occurs and when it goes out, cooling. Even the coldest objects have some heat because their atoms are moving.
The type of energy that is put into play in heat phenomena is called thermal energy ( The energetic character of heat carries with it the possibility of transforming it into mechanical work. The steam engines that had such a spectacular development at the end of the 18th century and the beginning of the 19th century were a good example of this. Since then the notions of heat and energy were united and the progress of physics allowed, in the middle of the last century, find a detailed explanation for the nature of this new form of energy, which is manifested in heat phenomena.


The temperature is the measure of the heat of a body (and not the amount of heat it contains or can yield).
The temperature is measured in units called degrees, by means of thermometers, this means that to measure the temperature we use one of the magnitudes that undergoes variations linearly as the temperature is altered.
Temperature is the average of the kinetic energy of the molecules of a body.


Radiation presents a fundamental difference with respect to conduction and convection: substances that exchange heat do not have to be in contact, but can be separated by a vacuum.
Convection and conduction processes can only occur when there is a material medium through which heat can be transferred, while radiation can occur in a vacuum.
If you have a hot body inside a glass bell without air, and a thermometer is placed on the outside of the bell, you will see a rise in temperature, which indicates that there is a heat transmission through the vacuum what is between the hot body and the outside.

What is a Thermometer?

A thermometer is an instrument that measures the temperature of a system in a quantitative way. An easy way to do this is to find a substance that has a property that changes regularly with temperature.

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