# Definition of heat in physics : meaninm and heat transmission

We define heat in physics as: "The contribution of energy transformed as a result of a chemical or nuclear reaction and transferred between two systems, or between two parts of the same system."

This amount of energy is not attributable to work or conversion between two different types of energy.

According to the definition, heat is a form of transferred energy and not a form of contained energy, such as internal energy. Therefore, heat and work are forms of energy that cannot be associated with the system's state.

The unit of heat is the joule (J) in the International System. However, it is still often used as the unit of measure for calories. Sometimes purely technical units are also used: like kWh or the British Thermal Unit (BTU).

We define a calorie as the amount of heat required to raise the temperature of one gram of water by one degree Celsius.

## What is specific heat?

A substance's specific heat (or heat capacity) is defined as the amount of heat required to raise or lower the temperature of a unit mass by one kelvin.

In the international system, the unit of measurement of specific heat is J / (K · kg).

## Difference Between Latent and Sensible Heat

If the heat transfer changes the state of aggregation, it is called latent heat.

On the other hand, if the transfer results in a decrease in the temperature difference, we speak of sensible heat.

The heat of reaction is also used when heat is consumed or generated by a chemical reaction.

### Example

For example, the temperature rise of water from 20°C to 50°C under standard conditions is determined by the fact that sensible heat is provided.

If the water is at boiling temperature, even if the heat continues to be supplied, the temperature does not increase until the phase change to gas is complete. In this case, the energy is stored as latent heat.

## Relationship between internal energy, heat, and temperature

Heat is not a property associated with a thermodynamic equilibrium configuration. In the presence of a temperature gradient, heat flows from higher temperatures to lower according to the second law of thermodynamics until it reaches a point of thermal equilibrium is reached.

The amount of heat exchanged depends on the particular path followed by the transformation to get from the initial state to the final state. In other words, heat is not a state function.

Instead, internal and thermal energy is a state function associated with the thermodynamic state of the system.

### Energy in transit

On the other hand, heat is not a thermodynamic property. Therefore, heat can be defined as "energy in transit," not as "energy possessed by a body."

Heat is exchanged between two bodies (or two parts of the same body) and is not possessed by a single body (as with internal energy). In particular, energy flows due to a temperature difference between the system under study.

## Heat transmission

The transfer of heat energy between systems can be done in three ways:

1. Propagation by thermal conduction: In a single body or between bodies in contact, energy is transmitted. This transfer is produced by impacts caused by the movement of the atoms and molecules of both materials in touch. In each impact, part of the kinetic energy of each particle passes to the other material. As a result, heat always flows from high temperatures to lower temperatures.

2. Convective Propagation: In a fluid in motion, parts of the fluid can heat or cool as they come into contact with outer surfaces. It is actually a transfer of energy.

3. Radiation propagation: Heat transmission can occur over a distance (also in a vacuum) through electromagnetic waves. For example, solar energy travels from the Sun to Earth by radiation.

## What causes sensations of hot and cold?

The sensation of "heat" or "cold" you feel when touching an object is determined by its temperature and the thermal conductivity of the material made, in addition to other factors.

Although it is possible to compare the relative temperatures of two bodies with touch (with some caution), it is impossible to give a complete evaluation.

We can use calorimeters are used to calculate the heat transfer.

Temperature is an index of the average kinetic energy of the body particles under examination. Heat is the energy that a body at a higher temperature transfers to a body at a lower temperature.

The cold and heat sensation is due to the temperature difference between the hand and the object and the speed with which the object can transfer heat to the hand (or another object at different temperatures).

However, by providing heat to a body, not only does the temperature rise, there is a sharper sensation of heat, but there are directly measurable variations in some physical properties.

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Data de publicació: August 24, 2018
Última revisió: July 27, 2022