Temperature gradients are gradual changes in temperature with distance, measured in degrees per unit of distance. They exist naturally and can be created, but convection can destroy them. Insulation helps maintain them. The Earth has different temperature gradients in each layer. Maintaining constant gradients is important in some industrial processes and for optimal performance in living cells.
A temperature gradient is the gradual change in temperature with distance. The slope of the gradient is consistent within a material. A gradient is established whenever two materials at different temperatures are in physical contact with each other. The units of measure for temperature gradients are degrees per unit of distance, such as °F per inch or °C per meter.
Many temperature gradients exist naturally, while others are created. The largest temperature gradient on Earth is the Earth itself. The temperature of the Earth’s core is estimated at about 9,000°F (5,000°C); is 6,650°F (3,700°C) at the core-mantle boundary, while the temperature of the crust is about 200°F (93°C). Each layer has a differently steep temperature gradient, depending on the thermal conductivity of the layer.
There is no temperature gradient between the Earth and the sun because there is no atmosphere between them. Heat capacity is the ability of a material to retain heat. A vacuum has zero heat capacity.
Convection destroys a thermal gradient. In heating a saucepan, the liquid closest to the burner becomes the hottest. When stirred, the hot liquid mixes with the cooler liquid, the heat is distributed evenly, and the temperature gradient is canceled.
If not stirred, convective heat transfer will cause hot liquid to rise and cold to fall, and some circulation will occur, although it will not be as effective as active stirring. Over time, the conduction forces that transfer heat from the bottom will establish equilibrium with the convective forces that circulate the water. If the heat source is low, circulation will be slow, there may be a steep temperature gradient, and the sauce may burn to the bottom. If the heat is high, the sauce will boil, convection heat transfer will be high, and the temperature gradient will be close to zero.
Insulation is used to delay heat transfer by placing material of low thermal conductivity near the heat source. Insulation helps maintain the thermal gradient between the insulated object and ambient conditions. Coffee will stay hotter in a froth cup than in an aluminum cup because froth conducts heat less easily. Similarly, the coffee drinker can burn some fingers by lifting the aluminum cup because the temperature gradient is close to zero and the temperature of the outside of the cup is almost the same as the inside of the cup.
To be stable, a thermal gradient must have a constant heat source and an available heat sink. Maintaining constant gradients is rarely important except when conducting chemical reactions. Many industrial processes require careful heat control. The living cell must also maintain careful heat control for optimal performance. While scientists understand how the human body as a whole maintains a temperature gradient between its core and the outside world, the options available to individual cells are less clear.
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