Mechanisms of heat transfer

The temperature difference is always the cause of a heat flux regardless of the transfer mechanism.

The greater the temperature difference, the greater the strive for temperature equalisation.
This is essentially the same as saying that a high heat flux and a high temperature gradient require each other.

Temperature differences stimulate the equalisation effort, in which a heat quantity (thermal energy) is exchanged.

A distinction is made between the three mechanisms of heat transfer.

Heat conduction:

Heat conduction occurs in substances that do not have a uniform temperature. Similarly, if substances with different temperatures come into contact with each other.
All aggregate states allow this transfer mechanism.

On the right in the image is the section (A-A) through a heated saucepan. The hot stove (1) transfers the heat through the bottom of the saucepan (2) to the water (3). The heat flow is shown by the red arrows.

Convection:

Convection is heat transport with a flowing medium. This may be a gas or a liquid (both are fluids). A distinction is made between free convection in which the fluid rises due to density differences due to heating and forced convection in which the fluid flows as a result of a technical device.

(A) Cooling tower - free convection:
How water evaporates and heats the air. Steam and air rise due to density differences.

(B) Fan of a computer processor - forced convection:
The heat of the computer processor (3) is fed into cooling fins (2) (conduction). The fan(1) provides a high air flow to ensure that the cooling is effective.

Heat radiation:

Energy is dissipated and absorbed at each surface as electromagnetic radiation. Any type of radiation takes energy with it. The exchange increases or reduces the heat quantity of a body.

The best example of heat radiation is the sun. Technical applications are shown on the left:

(A) Halogen spotlight with focus:
The radiation of the halogen lamp (1) is linked in the focus (2) and heats this point. One application of this is "light soldering". In light soldering the radiation energy is used to melt the solder at the focal point.

(B) Heating mantle:
A flame causes the surface (3) in the bottom part to glow. This emits heat, which warms up the environment. The hood (4) over the glowing part reflects down that component that is emitted upwards.

Thermal energy is stored in any substance with a temperature above absolute zero. Since the equalisation effort of the heat is associated with the prevailing temperature, the amount of thermal energy that can be exchanged is not unlimited.

The temperature difference determines how much heat can be exchanged.