We define thermal energy as part of the internal energy of a system. The internal energy is proportional to its absolute temperature and increases or decreases by energy transfer.
Increasing the thermal energy of a body, its molecules move faster.
Energy is measured in Joules (J) according to the SI of measures. However, when it comes to heat energy, calories (cal) are also often used.
Examples of thermal energy
In boiling water, we are transferring heat into the water. Its internal energy rises.
The chimneys emit heat by radiation when the fire is burning.
Solar energy is transferred by radiation to the Earth.
The human body can convert nutrients to energy which part is used to keep it hot.
A light bulb converts electricity into light and heat. The bulb light emits the heat by radiation.
How is thermal energy transferred between objects?
Energy is transmitted in three different ways: conduction, convection, and radiation.
Radiation heat transfer
Radiative energy does not need a transmission medium. It is carried out through waves.
Infrared radiation (IR) is a type of energy invisible to the human eyes, but we can sense it as heat.
The Sun and a light bulb are examples of heat that are transmitted by radiation:
Conduction heat transfer
Conduction is the transfer of heat. Thermal energy is transmitted through the motion of the adjacent molecules of two bodies that come into contact with a surface area. Thermal conductivity makes that the heat and temperature transfer occurs faster or slower.
When one object is in physical contact with other higher temperature object, conduction occurs. Some energy passed from the hot item to the colder one. An insulating material is that do not conduct heat.
Convection heat transfer
Convection transfers heat energy through liquids and gases. The differences in temperature within the atmosphere generate air circulations that redistribute the heat in its mass. Cooler air tends to down and to absorb heat from objects.
What is the difference between thermal energy and heat?
The difference between thermal energy and heat is that thermal energy is not being transferred, but remains part of the system’s internal energy (kinetic energy of its internal particles); Instead, heat is energy in the transfer.
Heat energy is transferred from a hot-system to another cold system according to the 2nd law of thermodynamics.
An example of the difference is a burning candle. A candle generates thermal energy. While it is burning, it dissipates its thermal energy into the environment (transfer energy). As soon as the thermal energy crosses the sail boundary, it is no longer thermal energy; it becomes heat, energy in transit.