(1) conduction
In an object with uneven heating, the process of transferring heat energy from the hotter end to the colder end through molecular thermal motion is called conduction. This exchange will continue until the temperature of the whole object is the same.
Heat conduction is the main way of solid heat transfer. In fluids such as gas or liquid, the heat conduction process usually occurs at the same time as convection. )
② Convection
When the temperature of a liquid or gas changes, its density also changes. Liquid or gas with low temperature has high density and flows downward due to gravity; Liquid or gas with high temperature has low density and rises to form convection.
Natural convection due to the density change of liquid or gas itself is called natural convection; If the speed of liquid or gas is accelerated by external force, it is called forced convection.
Because all parts of the fluid are in contact with each other, besides the thermal convection caused by the overall movement of the fluid, it is also accompanied by the thermal conduction caused by the movement of microscopic particles. )
(3) Radiation
In the absence of contact between objects, the process that a high-temperature object directly transfers heat energy to a low-temperature object in the form of electromagnetic waves is called radiation.
An object radiates electromagnetic waves due to its temperature, which is absorbed by the object and becomes heat energy. Generally, thermal radiation mainly propagates through visible light and infrared light with longer wavelength. )
For example, heat energy transmitted from the sun to the earth passes through space in the form of thermal radiation.