Heat transfer occurs through one of three processes or a combination of the three. Conduction is the flow of heat through a body from an area of higher temperature to an area of lower temperature. Molecules in the hot region increase their vibrational energy as they are heated. As they collide with molecules with lower vibrational energy (cooler ones), some of the vibrational energy is passed to the cooler ones. Their energy is increased and heat is passed on.

Heat transfer by convection is the movement of heat by motion of a fluid. This motion can he the result of some force, such as a pump circulating heated water, and is referred to as forced convection. If the motion is the result of differences in density (thermal or compositional), the convection is referred to as buoyancy-driven, density-driven, or natural convection.

Radiation is the emission of energy from the surface of a body. Energy is emitted in the form of electromagnetic waves or photons (packets of light). The character (wavelength, energy of photons, etc.) of the radiation depends on the temperature, surface area, and characteristics of the body emitting the energy. Electromagnetic waves travel with the speed of light through empty space and are absorbed (and/or reflected) by objects they fall on, thus transferring heat. An excellent example of radiative heating is the sun's heat that we experience on Earth.

Mathematics Standards † • Mathematical Connections Science Standards † • Earth and Space Science † • Physical Science † • Unifying Concepts and Processes The mass of a body divided by its volume is its average density.

When two or more liquids are immiscible they do not mix chemically.

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Something that is concave is curved inward like the inner surface of a sphere. Something that is convex is curved like the outer surface of a sphere. A variety of concave and convex lenses and mirrors are used in the design of eyeglasses, magnifying glasses, cameras, microscopes, and telescopes. In the example in the text, water cupping upward produces a concave surface; mercury cupping downward produces a convex surface.