Heat energy can be transferred from one location to another by one of three methods: conduction, convection, and radiation.
The metal handle of an iron skillet placed on a heated burner gets hot by conduction. Conduction occurs when the heat travels through the heated solid. The transfer rate (H) is the ratio of the amount of heat per time transferred from one location in an object to another H = Q/Δ t, where H has units of watts or J/s, when Q is in joules, and Δ t is in seconds. The temperature between two parts of the conducting medium—the pan bottom and the handle—must be different for conduction to take place. The formula for heat conduction from one side to another of a slab with thickness L and cross‐sectional area A is given by
where the heat flows from T 2 to T 1 and T 2 > T 1, as shown in Figure 3.
Heat flows from regions of higher temperature to lower.
The constant (k), called thermal conductivity, is found in tables listing properties of materials. The fact that different materials have different k values explains why the metal shelf of a refrigerator feels colder than the food on it even if both are at thermal equilibrium. The conductivity constant is relatively large for metals, and the metal feels colder because the heat is conducted away from the hand more quickly by metal than by other materials.
Heat transported by the movement of a heated substance is a result of convection. The most common example of convection is the warmed mass of air rising from a heater or fire.
The third mechanism for heat transfer is radiation in the form of electromagnetic waves. Radiant energy from the sun warms the earth. The rate at which an object emits radiant energy is proportional to the fourth power of its absolute temperature. The Stefan‐Boltzmann law, which describes the relationship, is written P = σ AeT 4, where P is the power radiated in watts, σ is a constant equal to 5.6696 × 10 −s W/m 2K 4, A is the surface area of the object in m 2, T is the absolute temperature, and e is the emissivity constant, which varies from 0 to 1 depending upon the properties of the surface.
The thermos bottle, or Dewar flask, is an object that minimizes heat transfer by conduction, convection, and radiation. The flask is constructed of double‐walled Pyrex glass with silvered inner walls. The space between the walls is evacuated to reduce heat transfer by conduction and convection. The silvered walls reflect most of the radiant heat to cut heat transfer by radiation. The container is effectively used to store either cold or hot liquids for long periods of time.