Heat is defined as the transfer of thermal energy between systems or objects due to a temperature difference. This energy always flows from a region of higher temperature to a region of lower temperature, following the second law of thermodynamics. Understanding this movement of energy is fundamental to nearly every physical process, from the warming of the atmosphere to the cooking of a meal.
Temperature, in contrast, is a measure of the average kinetic energy of the particles—atoms or molecules—within a substance. A higher temperature indicates that the particles are moving or vibrating faster. While temperature reflects the intensity of this molecular movement within an object, heat describes the transfer of the resulting thermal energy between objects.
Heat Transfer Through Direct Contact: Conduction
Conduction is the transfer of heat that occurs through direct physical contact between materials, without any large-scale movement of the material itself. This process is most significant in solids, where atoms are fixed in a lattice structure and are closely packed. Energy is transferred when rapidly vibrating atoms in a warmer region collide with their slower-moving neighbors, passing kinetic energy along the material.
Metals are effective conductors because they possess free-moving electrons that rapidly transport thermal energy throughout the material. In contrast, materials like wood or air are poor conductors, making them useful as thermal insulators. For example, when a metal spoon is placed into a hot beverage, the end in the liquid transfers energy to the handle through this chain of molecular vibrations.
Heat Transfer Through Fluid Movement: Convection
Convection involves the transfer of heat through the movement of heated fluids, including both liquids and gases. This process is driven by changes in fluid density that result from heating and cooling. When a fluid is heated, it expands and becomes less dense than the surrounding, cooler fluid.
The less dense, warmer fluid rises due to buoyancy, while the cooler, denser fluid sinks toward the heat source. This continuous cycle creates a physical circulation known as a convection current. A common example is heating a room with a radiator, where warm air rises toward the ceiling and pushes cooler air down to be heated, establishing a continuous flow.
Heat Transfer Through Electromagnetic Waves: Radiation
Radiation is the only form of heat transfer that does not require a medium to propagate, allowing energy to travel through the vacuum of space. This transfer occurs through the emission of electromagnetic waves, typically in the infrared portion of the spectrum for objects near room temperature. All matter with a temperature above absolute zero emits thermal radiation as a result of the random thermal motion of its charged particles.
When this radiation is absorbed by another object, the energy of the waves is converted back into thermal energy, causing the object’s temperature to rise. The most familiar example is the Sun warming the Earth, as its energy travels millions of miles through space. Similarly, the sensation of warmth felt near a campfire or a hot stovetop element is due to the infrared radiation being emitted and absorbed.
Everyday Applications of Heat Flow
In daily life, the three modes of heat transfer rarely occur in isolation and frequently operate in tandem. Cooking on a stovetop, for instance, involves all three mechanisms: heat moves from the burner to the pan through conduction, transfers to the food, and the glowing element emits thermal radiation absorbed by the pot.
Engineers leverage these principles to design systems that either maximize or minimize energy transfer. A thermos bottle is designed to minimize all three forms of heat transfer. Its vacuum layer blocks conduction and convection, while its mirrored inner surface reflects infrared radiation back into the liquid, preventing heat loss.
Building insulation manages heat flow by trapping air within materials like fiberglass or foam. This trapped air acts as a poor conductor, slowing the transfer of heat across the walls and minimizing conduction. The insulation also prevents the formation of large air currents within the walls, thereby blocking convection.