"Always from those who has more to those who have less."

Spontaneously, heat always spreads from warmer regions to colder ones.

Heat Transfer

Two systems isolated at different temperatures will exchange heat until their temperatures became the same (thermal equilibrium). These heat exchanges can occur in three different ways: Conduction, Convection and Irradiation (radiation).

The heat transfer rate \(\Phi \) between systems is the ratio of the amount of heat \(Q \) transferred through the area \(S \) in a time interval \(\Delta t)\). Mathematically: $$\Phi = \frac{Q}{\Delta t}.$$

Conduction

For heat conducting, the thermal energy is transmitted from particle to particle (atoms, molecules or electrons) by direct collision of them. In the vacuum, the heat cannot be transferred by conduction.

Heat transfer between solids is usually done by conduction.

Conduction heat transfer. The molecules that are at higher temperatures have a higher thermal vibration than those with lower temperatures, and the collision between the molecules causes the heat to be transmitted from the more agitated molecules (hotter) to the less agitated (colder).

In the case of thermal conduction, the following law is important:

Fourier Law
The thermal energy \(Q \) transmitted through a rectangular object in a certain time interval \(\Delta t \) is: $$\frac{\Delta Q}{\Delta t} = -\frac{k A(T_2 – T_1)}{\Delta x},$$ where \(\Delta x\) is the length of the object in the direction of heat flow, \(A \) is the cross-sectional area through which heat flows, and \(T_1\) and \(T_2 \) are cold and hot temperatures at boundaries, respectively, and \(K\) is the thermal conductivity constant.

The table shows the thermal conductivity of some materials.
Thermal conductivity
Materials Condutivity (W/m.K)
Stainless Steel 14
Copper 401
Dry air 0.026
Polyurethane Foam 0.024
Mineral wool 0.043
Fiberglass 0.048
White pine 0.11
Glass 1.0

Convection

"During convection the energy is transmitted by the flow of gas or liquid".

During heat transfer by convection, the hottest molecules are moved from one place to another. Being the thermal energy carried with them. For convection it is necessary that the molecules have mobility, that is, that the system is a fluid (liquid or gas).

The process of heat transfer by convection is very complex, so there is no simple and general equation to describe it.

Convection currents. When we heat a fluid, the part that is closest to the flame heats up faster and thus becomes "lighter", that is, its density decreases. As the liquid density at the bottom of the vessel is smaller, it tends to rise, and the liquid on top, "heavier" (higher density), tends to descend. With this process the convection currents arise.

All the winds in the atmosphere are large convective currents.

Radiation

"Even in vacuum, heat spreads."

Solar irradiation . It is through the radiation that the energy of the sun reaches the Earth, our main source of energy.

Radiation is the process of transmitting energy through electromagnetic waves, such as lights, radio waves, TV, "x-ray" etc. This type of energy transfer can also occur in the vacuum.

All objects that are above absolute zero radiate energy.

A body, being in thermal equilibrium with its surroundings, radiates and absorbs energy at the same rate, and therefore its temperature remains constant.

Definitions and important laws:

Black Body
It is a body that absorbs all the radiation that falls on it and has an emissivity equal to 1.
Stefan-Boltzmann Law
The radiation power, \(P_r = \frac{\Delta Q}{\Delta t}\), of a surface area \(A \), with a temperature \(T \) and emissivity \(\varepsilon\), can be expressed by the relation: $$P_r = A \varepsilon \sigma T^4,$$ where \(\sigma \) is called the Stefan-Boltzmann constant \((\sigma=5,67.10^{-8} W/m^2 k^4)\) and \(\varepsilon\) is the emissivity of the surface. Generally, the emissivity of a light surface is greater than a dark one. Its values are obtained in laboratory and are tabulated.
Newton's Cooling Law
The rate of cooling (or heating) of a body is proportional to the temperature difference between the body and its surroundings. That is, given the body temperature \(T(t)\) and the temperature of the environment \(M(t)\) at time \(t\), we then have: $$ \frac{\Delta T}{\Delta t} = k [M(t) - T(t)], $$ where \(k\) is a proportionality constant.
Observations
  • Microwave ovens heat and / or cook food by irradiation.
  • The energy required for the photosynthesis of the plants is obtained by irradiation.