Physics trivia

# Five things you need to know about heat

Heat is a form of energy in transit that arises exclusively because of the difference in temperature between two bodies.

1) Heat is a form of energy

When there is a difference in temperature between two bodies, energy transfers from the hotter body to the colder body until the two reach thermal equilibrium. This transit of energy is called heat. The physical unit of heat in the International System of Units is the Joule (J), however, it is common to measure the amount of heat in calories (cal).

## 2) Heat or temperature?

Heat and temperature are different concepts, but they are commonly confused. The first is a form of energy, and the second is a measure of the degree of agitation of particles in a body and is therefore related to the speed of these particles. The unit of temperature in the International System of Units is the Kelvin (K), however, there are several different temperature scales, such as Celsius (ºC) and Fahrenheit (ºF).

## 3rd) Troubled history of the concept of heat

Heat has been a concept widely debated by philosophers and scientists throughout history. Initially, some Greek philosophers, such as Aristotle and Empedocles , believed that it was one of the constituent elements of matter, being known as the fire element . Much later came the idea of ​​phlogiston , the substance that was the principle of fire. According to its creator, the physician George Stahl , when a body was heated, it received phlogiston and, when it was cooled, it lost it. A few years later, the chemist Lavoisieroverthrew the idea of ​​phlogiston by showing that combustion was, in fact, a chemical reaction with oxygen and introduced the concept of an imponderable (massless) element that was responsible for heating bodies, called caloric .

The modern definition of heat came only with Benjamin Thompson , also known as Lord Kelvin , who managed to establish that heat is a form of energy . The numerical equivalence between heat and energy, however, was only quantitatively determined by Julius Mayer and was later improved by James Prescott Joulle in 1843.

Heat can be transferred mainly in three different ways: conduction , convection and radiation . Microscopically, the vibration of atoms in a body can be transmitted to adjacent atoms if there is contact between two surfaces. This phenomenon is more common among solids and is called conduction . Convection occurs by heating fluids, such as atmospheric gases or even water. With increasing temperature, the density of these fluids decreases, favoring the buoyant force . about them. As a result, these heated portions of the fluids tend to rise , transferring heat to the surroundings until they reach thermal equilibrium When this happens, the fluid drops, and the process can be repeated until there are no more temperature differences.

Radiation , in turn, does not need a physical medium to happen, as it occurs through the propagation of electromagnetic waves The amount of heat emitted by electromagnetic waves is proportional to the fourth power of body temperature ( Q α T 4 ), according to the Steffan-Boltzmann Law .

## 5th) Energy not used

The biggest source of energy on planet Earth is the Sun. The energy emitted by the Sun in the form of electromagnetic waves is on the order of 4.10 26 W (watts). Because of the great distance, a tiny portion of this energy reaches us: the solar energy radiated on the Earth’s surface is about 1400 W/m 2 (Watts per square meter). Each year, the energy that reaches us from the Sun is about 8000 times greater than the energy consumed by the United States of America.