4. Modes of Heat Transfer

Heat can travel from one place to another, or from one body to another, by three distinct mechanisms: conduction, convection, and radiation. Conduction and convection require a material medium to transfer heat, whereas radiation does not — it can travel through a vacuum, which is how heat from the Sun reaches the Earth across empty space.

4.1 Conduction

Conduction is the mode of heat transfer in which heat passes from one particle of a substance to the adjacent particle, without any actual movement of the particles themselves from their mean positions. It occurs mainly in solids. Two mechanisms account for conduction: in non-metals, heat is transferred as vibrational energy is passed from molecule to molecule through collisions — a molecule that vibrates more vigorously due to higher temperature transfers some of this vibrational energy to its cooler neighbour. In metals, heat is conducted mainly by free (delocalised) electrons, which move readily through the metallic lattice, carry kinetic energy from the hotter region to the cooler region, and collide with atoms there, transferring energy — this is also why metals are typically good conductors of both heat and electricity. Substances are classified by how readily they conduct heat: good conductors (such as silver, copper, aluminium, and iron) allow heat to pass through them easily and are used, for instance, in cooking vessels and radiator fins, whereas bad conductors or insulators (such as wood, glass, rubber, plastic, wool, air, and asbestos) resist the flow of heat and are used for handles of cooking utensils, thermal insulation of buildings, and winter clothing.

Thermal Conductivity

Thermal conductivity (denoted k) is a quantitative measure of a material's ability to conduct heat; it is defined as the quantity of heat conducted per second across unit area of a slab of the material of unit thickness, for a unit temperature difference between its faces, and is measured in watts per metre per kelvin (W/m·K). Materials with a high thermal conductivity (silver ≈ 429, copper ≈ 401, aluminium ≈ 237, in W/m·K) are excellent conductors, while materials with very low thermal conductivity (air ≈ 0.024, wood ≈ 0.1–0.2, glass wool ≈ 0.04) are excellent insulators. It is worth noting that among common metals, silver is the best conductor of heat, followed closely by copper; among all common materials, air trapped in still pockets is one of the poorest conductors, which is exactly why materials like wool, cotton, and fibreglass insulation — all of which trap large amounts of still air within their fibres — are effective insulators.

4.2 Convection

Convection is the mode of heat transfer in which heat is carried from one place to another by the actual bulk movement of the heated particles of a fluid (a liquid or a gas). It occurs because when a fluid is heated, it expands, becomes less dense, and rises, while the surrounding cooler, denser fluid sinks to take its place — setting up a continuous circulating current called a convection current. Convection cannot occur in solids because the particles of a solid are fixed in position and cannot move from place to place in bulk.

Convection may be natural (free) convection, where the fluid motion is driven purely by density differences arising from heating (as in a pot of boiling water, land and sea breezes, or the rising of hot air from a radiator), or forced convection, where the fluid is made to move by an external agency such as a fan, pump, or blower (as in a hair dryer, a car radiator with a coolant pump, or a forced-air heating system). Everyday examples of natural convection include: the heating of a room by a room heater placed near the floor (warm air rises, cool air sinks to be heated in turn, setting up a circulating current that warms the whole room); land and sea breezes (during the day, land heats up faster than the sea, so air over land rises and cooler air from the sea rushes in to replace it, creating a sea breeze; at night the reverse happens, producing a land breeze); and the functioning of a chimney, which relies on the upward convection current of hot smoke and gases to draw fresh air into a fire.

4.3 Radiation

Radiation is the mode of heat transfer in which heat travels from one place to another directly, in the form of electromagnetic waves (mainly infrared radiation), without requiring any intervening medium — it can travel even through a perfect vacuum. This is the only mode by which heat from the Sun reaches the Earth across roughly 150 million kilometres of largely empty space. Every object, simply by virtue of having a temperature above absolute zero, continuously emits thermal radiation; the hotter the object, the greater the amount of radiation emitted and the shorter the wavelength at which the emission peaks (this relationship is captured in physics by Stefan-Boltzmann's law and Wien's displacement law respectively, though detailed derivations are beyond SSC/RRB syllabus requirements). Dark, dull, and rough surfaces are good absorbers and good emitters of radiant heat, whereas light-coloured, shiny, and polished surfaces are poor absorbers/emitters and good reflectors of radiant heat. This is why: people wear light-coloured clothes in summer (to reflect away solar radiation and stay cooler) and dark clothes in winter (to absorb more radiant heat); cooking utensils are often blackened on the outside bottom (to absorb heat from the flame efficiently) but kept shiny on the sides; and vehicles and buildings in hot climates are often painted white or light colours.

4.4 Comparative Summary of the Three Modes

4.5 Everyday Applications of Heat Transfer Principles

• Thermos (vacuum) flask: designed to minimise heat transfer by all three modes simultaneously. A double-walled glass or steel vessel has a vacuum between the walls (preventing conduction and convection, since there is no medium), the walls facing the vacuum are silvered (to reflect radiant heat back in or out, minimising radiation loss), the flask is supported on an insulating (poor-conducting) cork or plastic base and stopper (minimising conduction to the outside), and the outer covering is typically a poor conductor as well.
• Woollen clothes keep us warm in winter not because wool itself generates heat, but because wool traps air (a poor conductor) between its fibres, preventing the body's heat from conducting away and reducing convective loss.
• Cooking utensils are typically made of metals (good conductors) so that heat from the flame is efficiently conducted to the food, while their handles are made of wood, bakelite, or other insulating materials so they can be safely held.
• A refrigerator keeps food cool by continuously extracting heat from inside the cabinet and rejecting it outside using a working fluid called a refrigerant, which is repeatedly compressed and expanded in a closed cycle. The refrigerant evaporates inside cooling coils (absorbing latent heat from the food compartment, which cools it) and is then compressed by an electric compressor, causing it to become a hot high-pressure gas; this hot gas passes through condenser coils (usually at the back of the fridge) where it releases heat to the surrounding room and condenses back into a liquid; the liquid then passes through an expansion valve, drops in pressure and temperature, and re-enters the cooling coils to repeat the cycle. Insulated walls of the refrigerator minimise heat entering the cold cabinet from the room by conduction.
• A pressure cooker uses the principle that the boiling point of a liquid increases with increasing pressure. The tightly sealed lid does not let steam escape freely, so pressure builds up inside as water boils, and this elevated pressure raises the boiling point of water well above 100°C (commonly to around 120°C at typical operating pressures). Cooking at this higher temperature, in the presence of steam that penetrates food efficiently, cooks food considerably faster than in an open vessel. A weighted safety valve on the lid maintains the pressure at a safe, roughly constant value by releasing excess steam once a threshold pressure is reached, and also acts as a critical safety device against dangerous over-pressurisation.