When two substances at different temperatures come into contact, heat transfer occurs between them. The heat transfers from the hotter substance (higher temperature) to the cooler substance (lower temperature) until thermal equilibrium is reached. This process is governed by the second law of thermodynamics, which states that heat naturally flows from a hotter body to a cooler one.
This heat transfer can happen through three main mechanisms:
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Conduction: This is the primary method of heat transfer when two solids are in contact. At the molecular level, higher-energy (hotter) molecules collide with lower-energy (cooler) molecules, transferring kinetic energy and raising the temperature of the cooler substance while lowering the temperature of the hotter one.
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Convection: If the substances involved are fluids (liquids or gases), convection can occur. This involves the bulk movement of the fluid, leading to the transfer of heat due to the movement of warmer and cooler areas within the fluid.
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Radiation: While less common for two substances in direct contact, heat can also be transferred through electromagnetic radiation. This is the least efficient mechanism but can still play a role, especially if there are gaps or if the substances are at very different temperatures.
Ultimately, the process continues until both substances reach the same temperature, at which point they are in thermal equilibrium.
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