Heat Transfer

The transfer of heat is normally from a high temperature object to a lower temperature object. Heat transfer changes the internal energy of both systems involved according to the First Law of Thermodynamics.

Heat Conduction

Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones. For heat transfer between two plane surfaces, such as heat loss through the wall of a house, the rate of conduction heat transfer is:

Calculation 计算 = heat transferred in time = 在时间内传递的热量 = thermal conductivity of the barrier = 隔层的热导率 area 面积 = temperature 温度 = thickness of barrier 势垒厚度

Heat Convection

Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (see Ideal Gas Law). Hot water is likewise less dense than cold water and rises, causing convection currents which transport energy.

Convection can also lead to circulation in a liquid, as in the heating of a pot of water over a flame. Heated water expands and becomes more buoyant. Cooler, more dense water near the surface descends and patterns of circulation can be formed, though they will not be as regular as suggested in the drawing. 对流也可以导致液体中的循环，例如在火焰上方加热一锅水时的情况。受热的水会膨胀并变得更具浮力。较冷、密度更大的水在表面下沉，从而形成循环模式，尽管这些模式不会像图中所示那样规则。

Convection cells are visible in the heated cooking oil in the pot at left. Heating the oil produces changes in the index of refraction of the oil, making the cell boundaries visible. Circulation patterns form, and presumably the wall-like structures visible are the boundaries between the circulation patterns. 对左侧锅中加热的烹饪油来说，可以观察到对流细胞。加热油会引起油的折射率发生变化，使细胞边界变得可见。形成了循环模式， presumably，那些可见的壁状结构就是这些循环模式之间的边界。

Convection is thought to play a major role in transporting energy from the center of the Sun to the surface, and in movements of the hot magma beneath the surface of the earth. The visible surface of the Sun (the photosphere) has a granular appearance with a typical dimension of a granule being 1000 kilometers. The image at right is from the NASA Solar Physics website and is credited to G. Scharmer and the Swedish Vacuum Solar Telescope. The granules are described as convection cells which transport heat from the interior of the Sun to the surface. 对流被认为在将太阳内部的热量输送到表面以及地表下炽热岩浆的运动中起着重要作用。太阳的可见表面（光球层）呈现出颗粒状的外观，每个颗粒的典型尺寸为1000公里。右图来自NASA太阳物理网站，由G. Scharmer和瑞典真空太阳望远镜提供。这些颗粒被描述为对流细胞，它们将热量从太阳内部输送到表面。

In ordinary heat transfer on the Earth, it is difficult to quantify the effects of convection since it inherently depends upon small nonuniformities in an otherwise fairly homogeneous medium. In modeling things like the cooling of the human body, we usually just lump it in with conduction.